Linux 3.12.39

[linux/fpc-iii.git] / kernel / cgroup.c

/*
 *  Generic process-grouping system.
 *
 *  Based originally on the cpuset system, extracted by Paul Menage
 *  Copyright (C) 2006 Google, Inc
 *
 *  Notifications support
 *  Copyright (C) 2009 Nokia Corporation
 *  Author: Kirill A. Shutemov
 *
 *  Copyright notices from the original cpuset code:
 *  --------------------------------------------------
 *  Copyright (C) 2003 BULL SA.
 *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
 *
 *  Portions derived from Patrick Mochel's sysfs code.
 *  sysfs is Copyright (c) 2001-3 Patrick Mochel
 *
 *  2003-10-10 Written by Simon Derr.
 *  2003-10-22 Updates by Stephen Hemminger.
 *  2004 May-July Rework by Paul Jackson.
 *  ---------------------------------------------------
 *
 *  This file is subject to the terms and conditions of the GNU General Public
 *  License.  See the file COPYING in the main directory of the Linux
 *  distribution for more details.
 */

#include <linux/cgroup.h>
#include <linux/cred.h>
#include <linux/ctype.h>
#include <linux/errno.h>
#include <linux/init_task.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/mm.h>
#include <linux/mutex.h>
#include <linux/mount.h>
#include <linux/pagemap.h>
#include <linux/proc_fs.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/backing-dev.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/magic.h>
#include <linux/spinlock.h>
#include <linux/string.h>
#include <linux/sort.h>
#include <linux/kmod.h>
#include <linux/module.h>
#include <linux/delayacct.h>
#include <linux/cgroupstats.h>
#include <linux/hashtable.h>
#include <linux/namei.h>
#include <linux/pid_namespace.h>
#include <linux/idr.h>
#include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
#include <linux/eventfd.h>
#include <linux/poll.h>
#include <linux/flex_array.h> /* used in cgroup_attach_task */
#include <linux/kthread.h>
#include <linux/file.h>

#include <linux/atomic.h>

/*
 * cgroup_mutex is the master lock.  Any modification to cgroup or its
 * hierarchy must be performed while holding it.
 *
 * cgroup_root_mutex nests inside cgroup_mutex and should be held to modify
 * cgroupfs_root of any cgroup hierarchy - subsys list, flags,
 * release_agent_path and so on.  Modifying requires both cgroup_mutex and
 * cgroup_root_mutex.  Readers can acquire either of the two.  This is to
 * break the following locking order cycle.
 *
 *  A. cgroup_mutex -> cred_guard_mutex -> s_type->i_mutex_key -> namespace_sem
 *  B. namespace_sem -> cgroup_mutex
 *
 * B happens only through cgroup_show_options() and using cgroup_root_mutex
 * breaks it.
 */
#ifdef CONFIG_PROVE_RCU
DEFINE_MUTEX(cgroup_mutex);
EXPORT_SYMBOL_GPL(cgroup_mutex);        /* only for lockdep */
#else
static DEFINE_MUTEX(cgroup_mutex);
#endif

static DEFINE_MUTEX(cgroup_root_mutex);

/*
 * cgroup destruction makes heavy use of work items and there can be a lot
 * of concurrent destructions.  Use a separate workqueue so that cgroup
 * destruction work items don't end up filling up max_active of system_wq
 * which may lead to deadlock.
 */
static struct workqueue_struct *cgroup_destroy_wq;

/*
 * Generate an array of cgroup subsystem pointers. At boot time, this is
 * populated with the built in subsystems, and modular subsystems are
 * registered after that. The mutable section of this array is protected by
 * cgroup_mutex.
 */
#define SUBSYS(_x) [_x ## _subsys_id] = &_x ## _subsys,
#define IS_SUBSYS_ENABLED(option) IS_BUILTIN(option)
static struct cgroup_subsys *cgroup_subsys[CGROUP_SUBSYS_COUNT] = {
#include <linux/cgroup_subsys.h>
};

/*
 * The dummy hierarchy, reserved for the subsystems that are otherwise
 * unattached - it never has more than a single cgroup, and all tasks are
 * part of that cgroup.
 */
static struct cgroupfs_root cgroup_dummy_root;

/* dummy_top is a shorthand for the dummy hierarchy's top cgroup */
static struct cgroup * const cgroup_dummy_top = &cgroup_dummy_root.top_cgroup;

/*
 * cgroupfs file entry, pointed to from leaf dentry->d_fsdata.
 */
struct cfent {
        struct list_head                node;
        struct dentry                   *dentry;
        struct cftype                   *type;
        struct cgroup_subsys_state      *css;

        /* file xattrs */
        struct simple_xattrs            xattrs;
};

/*
 * CSS ID -- ID per subsys's Cgroup Subsys State(CSS). used only when
 * cgroup_subsys->use_id != 0.
 */
#define CSS_ID_MAX      (65535)
struct css_id {
        /*
         * The css to which this ID points. This pointer is set to valid value
         * after cgroup is populated. If cgroup is removed, this will be NULL.
         * This pointer is expected to be RCU-safe because destroy()
         * is called after synchronize_rcu(). But for safe use, css_tryget()
         * should be used for avoiding race.
         */
        struct cgroup_subsys_state __rcu *css;
        /*
         * ID of this css.
         */
        unsigned short id;
        /*
         * Depth in hierarchy which this ID belongs to.
         */
        unsigned short depth;
        /*
         * ID is freed by RCU. (and lookup routine is RCU safe.)
         */
        struct rcu_head rcu_head;
        /*
         * Hierarchy of CSS ID belongs to.
         */
        unsigned short stack[0]; /* Array of Length (depth+1) */
};

/*
 * cgroup_event represents events which userspace want to receive.
 */
struct cgroup_event {
        /*
         * css which the event belongs to.
         */
        struct cgroup_subsys_state *css;
        /*
         * Control file which the event associated.
         */
        struct cftype *cft;
        /*
         * eventfd to signal userspace about the event.
         */
        struct eventfd_ctx *eventfd;
        /*
         * Each of these stored in a list by the cgroup.
         */
        struct list_head list;
        /*
         * All fields below needed to unregister event when
         * userspace closes eventfd.
         */
        poll_table pt;
        wait_queue_head_t *wqh;
        wait_queue_t wait;
        struct work_struct remove;
};

/* The list of hierarchy roots */

static LIST_HEAD(cgroup_roots);
static int cgroup_root_count;

/*
 * Hierarchy ID allocation and mapping.  It follows the same exclusion
 * rules as other root ops - both cgroup_mutex and cgroup_root_mutex for
 * writes, either for reads.
 */
static DEFINE_IDR(cgroup_hierarchy_idr);

static struct cgroup_name root_cgroup_name = { .name = "/" };

/*
 * Assign a monotonically increasing serial number to cgroups.  It
 * guarantees cgroups with bigger numbers are newer than those with smaller
 * numbers.  Also, as cgroups are always appended to the parent's
 * ->children list, it guarantees that sibling cgroups are always sorted in
 * the ascending serial number order on the list.  Protected by
 * cgroup_mutex.
 */
static u64 cgroup_serial_nr_next = 1;

/* This flag indicates whether tasks in the fork and exit paths should
 * check for fork/exit handlers to call. This avoids us having to do
 * extra work in the fork/exit path if none of the subsystems need to
 * be called.
 */
static int need_forkexit_callback __read_mostly;

static struct cftype cgroup_base_files[];

static void cgroup_destroy_css_killed(struct cgroup *cgrp);
static int cgroup_destroy_locked(struct cgroup *cgrp);
static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
                              bool is_add);
static int cgroup_file_release(struct inode *inode, struct file *file);

/**
 * cgroup_css - obtain a cgroup's css for the specified subsystem
 * @cgrp: the cgroup of interest
 * @ss: the subsystem of interest (%NULL returns the dummy_css)
 *
 * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
 * function must be called either under cgroup_mutex or rcu_read_lock() and
 * the caller is responsible for pinning the returned css if it wants to
 * keep accessing it outside the said locks.  This function may return
 * %NULL if @cgrp doesn't have @subsys_id enabled.
 */
static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
                                              struct cgroup_subsys *ss)
{
        if (ss)
                return rcu_dereference_check(cgrp->subsys[ss->subsys_id],
                                             lockdep_is_held(&cgroup_mutex));
        else
                return &cgrp->dummy_css;
}

/* convenient tests for these bits */
static inline bool cgroup_is_dead(const struct cgroup *cgrp)
{
        return test_bit(CGRP_DEAD, &cgrp->flags);
}

/**
 * cgroup_is_descendant - test ancestry
 * @cgrp: the cgroup to be tested
 * @ancestor: possible ancestor of @cgrp
 *
 * Test whether @cgrp is a descendant of @ancestor.  It also returns %true
 * if @cgrp == @ancestor.  This function is safe to call as long as @cgrp
 * and @ancestor are accessible.
 */
bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
{
        while (cgrp) {
                if (cgrp == ancestor)
                        return true;
                cgrp = cgrp->parent;
        }
        return false;
}
EXPORT_SYMBOL_GPL(cgroup_is_descendant);

static int cgroup_is_releasable(const struct cgroup *cgrp)
{
        const int bits =
                (1 << CGRP_RELEASABLE) |
                (1 << CGRP_NOTIFY_ON_RELEASE);
        return (cgrp->flags & bits) == bits;
}

static int notify_on_release(const struct cgroup *cgrp)
{
        return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
}

/**
 * for_each_subsys - iterate all loaded cgroup subsystems
 * @ss: the iteration cursor
 * @i: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
 *
 * Should be called under cgroup_mutex.
 */
#define for_each_subsys(ss, i)                                          \
        for ((i) = 0; (i) < CGROUP_SUBSYS_COUNT; (i)++)                 \
                if (({ lockdep_assert_held(&cgroup_mutex);              \
                       !((ss) = cgroup_subsys[i]); })) { }              \
                else

/**
 * for_each_builtin_subsys - iterate all built-in cgroup subsystems
 * @ss: the iteration cursor
 * @i: the index of @ss, CGROUP_BUILTIN_SUBSYS_COUNT after reaching the end
 *
 * Bulit-in subsystems are always present and iteration itself doesn't
 * require any synchronization.
 */
#define for_each_builtin_subsys(ss, i)                                  \
        for ((i) = 0; (i) < CGROUP_BUILTIN_SUBSYS_COUNT &&              \
             (((ss) = cgroup_subsys[i]) || true); (i)++)

/* iterate each subsystem attached to a hierarchy */
#define for_each_root_subsys(root, ss)                                  \
        list_for_each_entry((ss), &(root)->subsys_list, sibling)

/* iterate across the active hierarchies */
#define for_each_active_root(root)                                      \
        list_for_each_entry((root), &cgroup_roots, root_list)

static inline struct cgroup *__d_cgrp(struct dentry *dentry)
{
        return dentry->d_fsdata;
}

static inline struct cfent *__d_cfe(struct dentry *dentry)
{
        return dentry->d_fsdata;
}

static inline struct cftype *__d_cft(struct dentry *dentry)
{
        return __d_cfe(dentry)->type;
}

/**
 * cgroup_lock_live_group - take cgroup_mutex and check that cgrp is alive.
 * @cgrp: the cgroup to be checked for liveness
 *
 * On success, returns true; the mutex should be later unlocked.  On
 * failure returns false with no lock held.
 */
static bool cgroup_lock_live_group(struct cgroup *cgrp)
{
        mutex_lock(&cgroup_mutex);
        if (cgroup_is_dead(cgrp)) {
                mutex_unlock(&cgroup_mutex);
                return false;
        }
        return true;
}

/* the list of cgroups eligible for automatic release. Protected by
 * release_list_lock */
static LIST_HEAD(release_list);
static DEFINE_RAW_SPINLOCK(release_list_lock);
static void cgroup_release_agent(struct work_struct *work);
static DECLARE_WORK(release_agent_work, cgroup_release_agent);
static void check_for_release(struct cgroup *cgrp);

/*
 * A cgroup can be associated with multiple css_sets as different tasks may
 * belong to different cgroups on different hierarchies.  In the other
 * direction, a css_set is naturally associated with multiple cgroups.
 * This M:N relationship is represented by the following link structure
 * which exists for each association and allows traversing the associations
 * from both sides.
 */
struct cgrp_cset_link {
        /* the cgroup and css_set this link associates */
        struct cgroup           *cgrp;
        struct css_set          *cset;

        /* list of cgrp_cset_links anchored at cgrp->cset_links */
        struct list_head        cset_link;

        /* list of cgrp_cset_links anchored at css_set->cgrp_links */
        struct list_head        cgrp_link;
};

/* The default css_set - used by init and its children prior to any
 * hierarchies being mounted. It contains a pointer to the root state
 * for each subsystem. Also used to anchor the list of css_sets. Not
 * reference-counted, to improve performance when child cgroups
 * haven't been created.
 */

static struct css_set init_css_set;
static struct cgrp_cset_link init_cgrp_cset_link;

static int cgroup_init_idr(struct cgroup_subsys *ss,
                           struct cgroup_subsys_state *css);

/*
 * css_set_lock protects the list of css_set objects, and the chain of
 * tasks off each css_set.  Nests outside task->alloc_lock due to
 * css_task_iter_start().
 */
static DEFINE_RWLOCK(css_set_lock);
static int css_set_count;

/*
 * hash table for cgroup groups. This improves the performance to find
 * an existing css_set. This hash doesn't (currently) take into
 * account cgroups in empty hierarchies.
 */
#define CSS_SET_HASH_BITS       7
static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);

static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
{
        unsigned long key = 0UL;
        struct cgroup_subsys *ss;
        int i;

        for_each_subsys(ss, i)
                key += (unsigned long)css[i];
        key = (key >> 16) ^ key;

        return key;
}

/*
 * We don't maintain the lists running through each css_set to its task
 * until after the first call to css_task_iter_start().  This reduces the
 * fork()/exit() overhead for people who have cgroups compiled into their
 * kernel but not actually in use.
 */
static int use_task_css_set_links __read_mostly;

static void __put_css_set(struct css_set *cset, int taskexit)
{
        struct cgrp_cset_link *link, *tmp_link;

        /*
         * Ensure that the refcount doesn't hit zero while any readers
         * can see it. Similar to atomic_dec_and_lock(), but for an
         * rwlock
         */
        if (atomic_add_unless(&cset->refcount, -1, 1))
                return;
        write_lock(&css_set_lock);
        if (!atomic_dec_and_test(&cset->refcount)) {
                write_unlock(&css_set_lock);
                return;
        }

        /* This css_set is dead. unlink it and release cgroup refcounts */
        hash_del(&cset->hlist);
        css_set_count--;

        list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
                struct cgroup *cgrp = link->cgrp;

                list_del(&link->cset_link);
                list_del(&link->cgrp_link);

                /* @cgrp can't go away while we're holding css_set_lock */
                if (list_empty(&cgrp->cset_links) && notify_on_release(cgrp)) {
                        if (taskexit)
                                set_bit(CGRP_RELEASABLE, &cgrp->flags);
                        check_for_release(cgrp);
                }

                kfree(link);
        }

        write_unlock(&css_set_lock);
        kfree_rcu(cset, rcu_head);
}

/*
 * refcounted get/put for css_set objects
 */
static inline void get_css_set(struct css_set *cset)
{
        atomic_inc(&cset->refcount);
}

static inline void put_css_set(struct css_set *cset)
{
        __put_css_set(cset, 0);
}

static inline void put_css_set_taskexit(struct css_set *cset)
{
        __put_css_set(cset, 1);
}

/**
 * compare_css_sets - helper function for find_existing_css_set().
 * @cset: candidate css_set being tested
 * @old_cset: existing css_set for a task
 * @new_cgrp: cgroup that's being entered by the task
 * @template: desired set of css pointers in css_set (pre-calculated)
 *
 * Returns true if "cset" matches "old_cset" except for the hierarchy
 * which "new_cgrp" belongs to, for which it should match "new_cgrp".
 */
static bool compare_css_sets(struct css_set *cset,
                             struct css_set *old_cset,
                             struct cgroup *new_cgrp,
                             struct cgroup_subsys_state *template[])
{
        struct list_head *l1, *l2;

        if (memcmp(template, cset->subsys, sizeof(cset->subsys))) {
                /* Not all subsystems matched */
                return false;
        }

        /*
         * Compare cgroup pointers in order to distinguish between
         * different cgroups in heirarchies with no subsystems. We
         * could get by with just this check alone (and skip the
         * memcmp above) but on most setups the memcmp check will
         * avoid the need for this more expensive check on almost all
         * candidates.
         */

        l1 = &cset->cgrp_links;
        l2 = &old_cset->cgrp_links;
        while (1) {
                struct cgrp_cset_link *link1, *link2;
                struct cgroup *cgrp1, *cgrp2;

                l1 = l1->next;
                l2 = l2->next;
                /* See if we reached the end - both lists are equal length. */
                if (l1 == &cset->cgrp_links) {
                        BUG_ON(l2 != &old_cset->cgrp_links);
                        break;
                } else {
                        BUG_ON(l2 == &old_cset->cgrp_links);
                }
                /* Locate the cgroups associated with these links. */
                link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
                link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
                cgrp1 = link1->cgrp;
                cgrp2 = link2->cgrp;
                /* Hierarchies should be linked in the same order. */
                BUG_ON(cgrp1->root != cgrp2->root);

                /*
                 * If this hierarchy is the hierarchy of the cgroup
                 * that's changing, then we need to check that this
                 * css_set points to the new cgroup; if it's any other
                 * hierarchy, then this css_set should point to the
                 * same cgroup as the old css_set.
                 */
                if (cgrp1->root == new_cgrp->root) {
                        if (cgrp1 != new_cgrp)
                                return false;
                } else {
                        if (cgrp1 != cgrp2)
                                return false;
                }
        }
        return true;
}

/**
 * find_existing_css_set - init css array and find the matching css_set
 * @old_cset: the css_set that we're using before the cgroup transition
 * @cgrp: the cgroup that we're moving into
 * @template: out param for the new set of csses, should be clear on entry
 */
static struct css_set *find_existing_css_set(struct css_set *old_cset,
                                        struct cgroup *cgrp,
                                        struct cgroup_subsys_state *template[])
{
        struct cgroupfs_root *root = cgrp->root;
        struct cgroup_subsys *ss;
        struct css_set *cset;
        unsigned long key;
        int i;

        /*
         * Build the set of subsystem state objects that we want to see in the
         * new css_set. while subsystems can change globally, the entries here
         * won't change, so no need for locking.
         */
        for_each_subsys(ss, i) {
                if (root->subsys_mask & (1UL << i)) {
                        /* Subsystem is in this hierarchy. So we want
                         * the subsystem state from the new
                         * cgroup */
                        template[i] = cgroup_css(cgrp, ss);
                } else {
                        /* Subsystem is not in this hierarchy, so we
                         * don't want to change the subsystem state */
                        template[i] = old_cset->subsys[i];
                }
        }

        key = css_set_hash(template);
        hash_for_each_possible(css_set_table, cset, hlist, key) {
                if (!compare_css_sets(cset, old_cset, cgrp, template))
                        continue;

                /* This css_set matches what we need */
                return cset;
        }

        /* No existing cgroup group matched */
        return NULL;
}

static void free_cgrp_cset_links(struct list_head *links_to_free)
{
        struct cgrp_cset_link *link, *tmp_link;

        list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link) {
                list_del(&link->cset_link);
                kfree(link);
        }
}

/**
 * allocate_cgrp_cset_links - allocate cgrp_cset_links
 * @count: the number of links to allocate
 * @tmp_links: list_head the allocated links are put on
 *
 * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
 * through ->cset_link.  Returns 0 on success or -errno.
 */
static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
{
        struct cgrp_cset_link *link;
        int i;

        INIT_LIST_HEAD(tmp_links);

        for (i = 0; i < count; i++) {
                link = kzalloc(sizeof(*link), GFP_KERNEL);
                if (!link) {
                        free_cgrp_cset_links(tmp_links);
                        return -ENOMEM;
                }
                list_add(&link->cset_link, tmp_links);
        }
        return 0;
}

/**
 * link_css_set - a helper function to link a css_set to a cgroup
 * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
 * @cset: the css_set to be linked
 * @cgrp: the destination cgroup
 */
static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
                         struct cgroup *cgrp)
{
        struct cgrp_cset_link *link;

        BUG_ON(list_empty(tmp_links));
        link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
        link->cset = cset;
        link->cgrp = cgrp;
        list_move(&link->cset_link, &cgrp->cset_links);
        /*
         * Always add links to the tail of the list so that the list
         * is sorted by order of hierarchy creation
         */
        list_add_tail(&link->cgrp_link, &cset->cgrp_links);
}

/**
 * find_css_set - return a new css_set with one cgroup updated
 * @old_cset: the baseline css_set
 * @cgrp: the cgroup to be updated
 *
 * Return a new css_set that's equivalent to @old_cset, but with @cgrp
 * substituted into the appropriate hierarchy.
 */
static struct css_set *find_css_set(struct css_set *old_cset,
                                    struct cgroup *cgrp)
{
        struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
        struct css_set *cset;
        struct list_head tmp_links;
        struct cgrp_cset_link *link;
        unsigned long key;

        lockdep_assert_held(&cgroup_mutex);

        /* First see if we already have a cgroup group that matches
         * the desired set */
        read_lock(&css_set_lock);
        cset = find_existing_css_set(old_cset, cgrp, template);
        if (cset)
                get_css_set(cset);
        read_unlock(&css_set_lock);

        if (cset)
                return cset;

        cset = kzalloc(sizeof(*cset), GFP_KERNEL);
        if (!cset)
                return NULL;

        /* Allocate all the cgrp_cset_link objects that we'll need */
        if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
                kfree(cset);
                return NULL;
        }

        atomic_set(&cset->refcount, 1);
        INIT_LIST_HEAD(&cset->cgrp_links);
        INIT_LIST_HEAD(&cset->tasks);
        INIT_HLIST_NODE(&cset->hlist);

        /* Copy the set of subsystem state objects generated in
         * find_existing_css_set() */
        memcpy(cset->subsys, template, sizeof(cset->subsys));

        write_lock(&css_set_lock);
        /* Add reference counts and links from the new css_set. */
        list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
                struct cgroup *c = link->cgrp;

                if (c->root == cgrp->root)
                        c = cgrp;
                link_css_set(&tmp_links, cset, c);
        }

        BUG_ON(!list_empty(&tmp_links));

        css_set_count++;

        /* Add this cgroup group to the hash table */
        key = css_set_hash(cset->subsys);
        hash_add(css_set_table, &cset->hlist, key);

        write_unlock(&css_set_lock);

        return cset;
}

/*
 * Return the cgroup for "task" from the given hierarchy. Must be
 * called with cgroup_mutex held.
 */
static struct cgroup *task_cgroup_from_root(struct task_struct *task,
                                            struct cgroupfs_root *root)
{
        struct css_set *cset;
        struct cgroup *res = NULL;

        BUG_ON(!mutex_is_locked(&cgroup_mutex));
        read_lock(&css_set_lock);
        /*
         * No need to lock the task - since we hold cgroup_mutex the
         * task can't change groups, so the only thing that can happen
         * is that it exits and its css is set back to init_css_set.
         */
        cset = task_css_set(task);
        if (cset == &init_css_set) {
                res = &root->top_cgroup;
        } else {
                struct cgrp_cset_link *link;

                list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
                        struct cgroup *c = link->cgrp;

                        if (c->root == root) {
                                res = c;
                                break;
                        }
                }
        }
        read_unlock(&css_set_lock);
        BUG_ON(!res);
        return res;
}

/*
 * There is one global cgroup mutex. We also require taking
 * task_lock() when dereferencing a task's cgroup subsys pointers.
 * See "The task_lock() exception", at the end of this comment.
 *
 * A task must hold cgroup_mutex to modify cgroups.
 *
 * Any task can increment and decrement the count field without lock.
 * So in general, code holding cgroup_mutex can't rely on the count
 * field not changing.  However, if the count goes to zero, then only
 * cgroup_attach_task() can increment it again.  Because a count of zero
 * means that no tasks are currently attached, therefore there is no
 * way a task attached to that cgroup can fork (the other way to
 * increment the count).  So code holding cgroup_mutex can safely
 * assume that if the count is zero, it will stay zero. Similarly, if
 * a task holds cgroup_mutex on a cgroup with zero count, it
 * knows that the cgroup won't be removed, as cgroup_rmdir()
 * needs that mutex.
 *
 * The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
 * (usually) take cgroup_mutex.  These are the two most performance
 * critical pieces of code here.  The exception occurs on cgroup_exit(),
 * when a task in a notify_on_release cgroup exits.  Then cgroup_mutex
 * is taken, and if the cgroup count is zero, a usermode call made
 * to the release agent with the name of the cgroup (path relative to
 * the root of cgroup file system) as the argument.
 *
 * A cgroup can only be deleted if both its 'count' of using tasks
 * is zero, and its list of 'children' cgroups is empty.  Since all
 * tasks in the system use _some_ cgroup, and since there is always at
 * least one task in the system (init, pid == 1), therefore, top_cgroup
 * always has either children cgroups and/or using tasks.  So we don't
 * need a special hack to ensure that top_cgroup cannot be deleted.
 *
 *      The task_lock() exception
 *
 * The need for this exception arises from the action of
 * cgroup_attach_task(), which overwrites one task's cgroup pointer with
 * another.  It does so using cgroup_mutex, however there are
 * several performance critical places that need to reference
 * task->cgroup without the expense of grabbing a system global
 * mutex.  Therefore except as noted below, when dereferencing or, as
 * in cgroup_attach_task(), modifying a task's cgroup pointer we use
 * task_lock(), which acts on a spinlock (task->alloc_lock) already in
 * the task_struct routinely used for such matters.
 *
 * P.S.  One more locking exception.  RCU is used to guard the
 * update of a tasks cgroup pointer by cgroup_attach_task()
 */

/*
 * A couple of forward declarations required, due to cyclic reference loop:
 * cgroup_mkdir -> cgroup_create -> cgroup_populate_dir ->
 * cgroup_add_file -> cgroup_create_file -> cgroup_dir_inode_operations
 * -> cgroup_mkdir.
 */

static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode);
static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);
static int cgroup_populate_dir(struct cgroup *cgrp, unsigned long subsys_mask);
static const struct inode_operations cgroup_dir_inode_operations;
static const struct file_operations proc_cgroupstats_operations;

static struct backing_dev_info cgroup_backing_dev_info = {
        .name           = "cgroup",
        .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK,
};

static int alloc_css_id(struct cgroup_subsys_state *child_css);

static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
{
        struct inode *inode = new_inode(sb);

        if (inode) {
                inode->i_ino = get_next_ino();
                inode->i_mode = mode;
                inode->i_uid = current_fsuid();
                inode->i_gid = current_fsgid();
                inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
                inode->i_mapping->backing_dev_info = &cgroup_backing_dev_info;
        }
        return inode;
}

static struct cgroup_name *cgroup_alloc_name(struct dentry *dentry)
{
        struct cgroup_name *name;

        name = kmalloc(sizeof(*name) + dentry->d_name.len + 1, GFP_KERNEL);
        if (!name)
                return NULL;
        strcpy(name->name, dentry->d_name.name);
        return name;
}

static void cgroup_free_fn(struct work_struct *work)
{
        struct cgroup *cgrp = container_of(work, struct cgroup, destroy_work);

        mutex_lock(&cgroup_mutex);
        cgrp->root->number_of_cgroups--;
        mutex_unlock(&cgroup_mutex);

        /*
         * We get a ref to the parent's dentry, and put the ref when
         * this cgroup is being freed, so it's guaranteed that the
         * parent won't be destroyed before its children.
         */
        dput(cgrp->parent->dentry);

        /*
         * Drop the active superblock reference that we took when we
         * created the cgroup. This will free cgrp->root, if we are
         * holding the last reference to @sb.
         */
        deactivate_super(cgrp->root->sb);

        /*
         * if we're getting rid of the cgroup, refcount should ensure
         * that there are no pidlists left.
         */
        BUG_ON(!list_empty(&cgrp->pidlists));

        simple_xattrs_free(&cgrp->xattrs);

        kfree(rcu_dereference_raw(cgrp->name));
        kfree(cgrp);
}

static void cgroup_free_rcu(struct rcu_head *head)
{
        struct cgroup *cgrp = container_of(head, struct cgroup, rcu_head);

        INIT_WORK(&cgrp->destroy_work, cgroup_free_fn);
        queue_work(cgroup_destroy_wq, &cgrp->destroy_work);
}

static void cgroup_diput(struct dentry *dentry, struct inode *inode)
{
        /* is dentry a directory ? if so, kfree() associated cgroup */
        if (S_ISDIR(inode->i_mode)) {
                struct cgroup *cgrp = dentry->d_fsdata;

                BUG_ON(!(cgroup_is_dead(cgrp)));
                call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
        } else {
                struct cfent *cfe = __d_cfe(dentry);
                struct cgroup *cgrp = dentry->d_parent->d_fsdata;

                WARN_ONCE(!list_empty(&cfe->node) &&
                          cgrp != &cgrp->root->top_cgroup,
                          "cfe still linked for %s\n", cfe->type->name);
                simple_xattrs_free(&cfe->xattrs);
                kfree(cfe);
        }
        iput(inode);
}

static int cgroup_delete(const struct dentry *d)
{
        return 1;
}

static void remove_dir(struct dentry *d)
{
        struct dentry *parent = dget(d->d_parent);

        d_delete(d);
        simple_rmdir(parent->d_inode, d);
        dput(parent);
}

static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
{
        struct cfent *cfe;

        lockdep_assert_held(&cgrp->dentry->d_inode->i_mutex);
        lockdep_assert_held(&cgroup_mutex);

        /*
         * If we're doing cleanup due to failure of cgroup_create(),
         * the corresponding @cfe may not exist.
         */
        list_for_each_entry(cfe, &cgrp->files, node) {
                struct dentry *d = cfe->dentry;

                if (cft && cfe->type != cft)
                        continue;

                dget(d);
                d_delete(d);
                simple_unlink(cgrp->dentry->d_inode, d);
                list_del_init(&cfe->node);
                dput(d);

                break;
        }
}

/**
 * cgroup_clear_dir - remove subsys files in a cgroup directory
 * @cgrp: target cgroup
 * @subsys_mask: mask of the subsystem ids whose files should be removed
 */
static void cgroup_clear_dir(struct cgroup *cgrp, unsigned long subsys_mask)
{
        struct cgroup_subsys *ss;
        int i;

        for_each_subsys(ss, i) {
                struct cftype_set *set;

                if (!test_bit(i, &subsys_mask))
                        continue;
                list_for_each_entry(set, &ss->cftsets, node)
                        cgroup_addrm_files(cgrp, set->cfts, false);
        }
}

/*
 * NOTE : the dentry must have been dget()'ed
 */
static void cgroup_d_remove_dir(struct dentry *dentry)
{
        struct dentry *parent;

        parent = dentry->d_parent;
        spin_lock(&parent->d_lock);
        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        list_del_init(&dentry->d_child);
        spin_unlock(&dentry->d_lock);
        spin_unlock(&parent->d_lock);
        remove_dir(dentry);
}

/*
 * Call with cgroup_mutex held. Drops reference counts on modules, including
 * any duplicate ones that parse_cgroupfs_options took. If this function
 * returns an error, no reference counts are touched.
 */
static int rebind_subsystems(struct cgroupfs_root *root,
                             unsigned long added_mask, unsigned removed_mask)
{
        struct cgroup *cgrp = &root->top_cgroup;
        struct cgroup_subsys *ss;
        unsigned long pinned = 0;
        int i, ret;

        BUG_ON(!mutex_is_locked(&cgroup_mutex));
        BUG_ON(!mutex_is_locked(&cgroup_root_mutex));

        /* Check that any added subsystems are currently free */
        for_each_subsys(ss, i) {
                if (!(added_mask & (1 << i)))
                        continue;

                /* is the subsystem mounted elsewhere? */
                if (ss->root != &cgroup_dummy_root) {
                        ret = -EBUSY;
                        goto out_put;
                }

                /* pin the module */
                if (!try_module_get(ss->module)) {
                        ret = -ENOENT;
                        goto out_put;
                }
                pinned |= 1 << i;
        }

        /* subsys could be missing if unloaded between parsing and here */
        if (added_mask != pinned) {
                ret = -ENOENT;
                goto out_put;
        }

        ret = cgroup_populate_dir(cgrp, added_mask);
        if (ret)
                goto out_put;

        /*
         * Nothing can fail from this point on.  Remove files for the
         * removed subsystems and rebind each subsystem.
         */
        cgroup_clear_dir(cgrp, removed_mask);

        for_each_subsys(ss, i) {
                unsigned long bit = 1UL << i;

                if (bit & added_mask) {
                        /* We're binding this subsystem to this hierarchy */
                        BUG_ON(cgroup_css(cgrp, ss));
                        BUG_ON(!cgroup_css(cgroup_dummy_top, ss));
                        BUG_ON(cgroup_css(cgroup_dummy_top, ss)->cgroup != cgroup_dummy_top);

                        rcu_assign_pointer(cgrp->subsys[i],
                                           cgroup_css(cgroup_dummy_top, ss));
                        cgroup_css(cgrp, ss)->cgroup = cgrp;

                        list_move(&ss->sibling, &root->subsys_list);
                        ss->root = root;
                        if (ss->bind)
                                ss->bind(cgroup_css(cgrp, ss));

                        /* refcount was already taken, and we're keeping it */
                        root->subsys_mask |= bit;
                } else if (bit & removed_mask) {
                        /* We're removing this subsystem */
                        BUG_ON(cgroup_css(cgrp, ss) != cgroup_css(cgroup_dummy_top, ss));
                        BUG_ON(cgroup_css(cgrp, ss)->cgroup != cgrp);

                        if (ss->bind)
                                ss->bind(cgroup_css(cgroup_dummy_top, ss));

                        cgroup_css(cgroup_dummy_top, ss)->cgroup = cgroup_dummy_top;
                        RCU_INIT_POINTER(cgrp->subsys[i], NULL);

                        cgroup_subsys[i]->root = &cgroup_dummy_root;
                        list_move(&ss->sibling, &cgroup_dummy_root.subsys_list);

                        /* subsystem is now free - drop reference on module */
                        module_put(ss->module);
                        root->subsys_mask &= ~bit;
                }
        }

        /*
         * Mark @root has finished binding subsystems.  @root->subsys_mask
         * now matches the bound subsystems.
         */
        root->flags |= CGRP_ROOT_SUBSYS_BOUND;

        return 0;

out_put:
        for_each_subsys(ss, i)
                if (pinned & (1 << i))
                        module_put(ss->module);
        return ret;
}

static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
{
        struct cgroupfs_root *root = dentry->d_sb->s_fs_info;
        struct cgroup_subsys *ss;

        mutex_lock(&cgroup_root_mutex);
        for_each_root_subsys(root, ss)
                seq_printf(seq, ",%s", ss->name);
        if (root->flags & CGRP_ROOT_SANE_BEHAVIOR)
                seq_puts(seq, ",sane_behavior");
        if (root->flags & CGRP_ROOT_NOPREFIX)
                seq_puts(seq, ",noprefix");
        if (root->flags & CGRP_ROOT_XATTR)
                seq_puts(seq, ",xattr");
        if (strlen(root->release_agent_path))
                seq_printf(seq, ",release_agent=%s", root->release_agent_path);
        if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->top_cgroup.flags))
                seq_puts(seq, ",clone_children");
        if (strlen(root->name))
                seq_printf(seq, ",name=%s", root->name);
        mutex_unlock(&cgroup_root_mutex);
        return 0;
}

struct cgroup_sb_opts {
        unsigned long subsys_mask;
        unsigned long flags;
        char *release_agent;
        bool cpuset_clone_children;
        char *name;
        /* User explicitly requested empty subsystem */
        bool none;

        struct cgroupfs_root *new_root;

};

/*
 * Convert a hierarchy specifier into a bitmask of subsystems and
 * flags. Call with cgroup_mutex held to protect the cgroup_subsys[]
 * array. This function takes refcounts on subsystems to be used, unless it
 * returns error, in which case no refcounts are taken.
 */
static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
{
        char *token, *o = data;
        bool all_ss = false, one_ss = false;
        unsigned long mask = (unsigned long)-1;
        struct cgroup_subsys *ss;
        int i;

        BUG_ON(!mutex_is_locked(&cgroup_mutex));

#ifdef CONFIG_CPUSETS
        mask = ~(1UL << cpuset_subsys_id);
#endif

        memset(opts, 0, sizeof(*opts));

        while ((token = strsep(&o, ",")) != NULL) {
                if (!*token)
                        return -EINVAL;
                if (!strcmp(token, "none")) {
                        /* Explicitly have no subsystems */
                        opts->none = true;
                        continue;
                }
                if (!strcmp(token, "all")) {
                        /* Mutually exclusive option 'all' + subsystem name */
                        if (one_ss)
                                return -EINVAL;
                        all_ss = true;
                        continue;
                }
                if (!strcmp(token, "__DEVEL__sane_behavior")) {
                        opts->flags |= CGRP_ROOT_SANE_BEHAVIOR;
                        continue;
                }
                if (!strcmp(token, "noprefix")) {
                        opts->flags |= CGRP_ROOT_NOPREFIX;
                        continue;
                }
                if (!strcmp(token, "clone_children")) {
                        opts->cpuset_clone_children = true;
                        continue;
                }
                if (!strcmp(token, "xattr")) {
                        opts->flags |= CGRP_ROOT_XATTR;
                        continue;
                }
                if (!strncmp(token, "release_agent=", 14)) {
                        /* Specifying two release agents is forbidden */
                        if (opts->release_agent)
                                return -EINVAL;
                        opts->release_agent =
                                kstrndup(token + 14, PATH_MAX - 1, GFP_KERNEL);
                        if (!opts->release_agent)
                                return -ENOMEM;
                        continue;
                }
                if (!strncmp(token, "name=", 5)) {
                        const char *name = token + 5;
                        /* Can't specify an empty name */
                        if (!strlen(name))
                                return -EINVAL;
                        /* Must match [\w.-]+ */
                        for (i = 0; i < strlen(name); i++) {
                                char c = name[i];
                                if (isalnum(c))
                                        continue;
                                if ((c == '.') || (c == '-') || (c == '_'))
                                        continue;
                                return -EINVAL;
                        }
                        /* Specifying two names is forbidden */
                        if (opts->name)
                                return -EINVAL;
                        opts->name = kstrndup(name,
                                              MAX_CGROUP_ROOT_NAMELEN - 1,
                                              GFP_KERNEL);
                        if (!opts->name)
                                return -ENOMEM;

                        continue;
                }

                for_each_subsys(ss, i) {
                        if (strcmp(token, ss->name))
                                continue;
                        if (ss->disabled)
                                continue;

                        /* Mutually exclusive option 'all' + subsystem name */
                        if (all_ss)
                                return -EINVAL;
                        set_bit(i, &opts->subsys_mask);
                        one_ss = true;

                        break;
                }
                if (i == CGROUP_SUBSYS_COUNT)
                        return -ENOENT;
        }

        /*
         * If the 'all' option was specified select all the subsystems,
         * otherwise if 'none', 'name=' and a subsystem name options
         * were not specified, let's default to 'all'
         */
        if (all_ss || (!one_ss && !opts->none && !opts->name))
                for_each_subsys(ss, i)
                        if (!ss->disabled)
                                set_bit(i, &opts->subsys_mask);

        /* Consistency checks */

        if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
                pr_warning("cgroup: sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");

                if (opts->flags & CGRP_ROOT_NOPREFIX) {
                        pr_err("cgroup: sane_behavior: noprefix is not allowed\n");
                        return -EINVAL;
                }

                if (opts->cpuset_clone_children) {
                        pr_err("cgroup: sane_behavior: clone_children is not allowed\n");
                        return -EINVAL;
                }
        }

        /*
         * Option noprefix was introduced just for backward compatibility
         * with the old cpuset, so we allow noprefix only if mounting just
         * the cpuset subsystem.
         */
        if ((opts->flags & CGRP_ROOT_NOPREFIX) && (opts->subsys_mask & mask))
                return -EINVAL;


        /* Can't specify "none" and some subsystems */
        if (opts->subsys_mask && opts->none)
                return -EINVAL;

        /*
         * We either have to specify by name or by subsystems. (So all
         * empty hierarchies must have a name).
         */
        if (!opts->subsys_mask && !opts->name)
                return -EINVAL;

        return 0;
}

static int cgroup_remount(struct super_block *sb, int *flags, char *data)
{
        int ret = 0;
        struct cgroupfs_root *root = sb->s_fs_info;
        struct cgroup *cgrp = &root->top_cgroup;
        struct cgroup_sb_opts opts;
        unsigned long added_mask, removed_mask;

        if (root->flags & CGRP_ROOT_SANE_BEHAVIOR) {
                pr_err("cgroup: sane_behavior: remount is not allowed\n");
                return -EINVAL;
        }

        mutex_lock(&cgrp->dentry->d_inode->i_mutex);
        mutex_lock(&cgroup_mutex);
        mutex_lock(&cgroup_root_mutex);

        /* See what subsystems are wanted */
        ret = parse_cgroupfs_options(data, &opts);
        if (ret)
                goto out_unlock;

        if (opts.subsys_mask != root->subsys_mask || opts.release_agent)
                pr_warning("cgroup: option changes via remount are deprecated (pid=%d comm=%s)\n",
                           task_tgid_nr(current), current->comm);

        added_mask = opts.subsys_mask & ~root->subsys_mask;
        removed_mask = root->subsys_mask & ~opts.subsys_mask;

        /* Don't allow flags or name to change at remount */
        if (((opts.flags ^ root->flags) & CGRP_ROOT_OPTION_MASK) ||
            (opts.name && strcmp(opts.name, root->name))) {
                pr_err("cgroup: option or name mismatch, new: 0x%lx \"%s\", old: 0x%lx \"%s\"\n",
                       opts.flags & CGRP_ROOT_OPTION_MASK, opts.name ?: "",
                       root->flags & CGRP_ROOT_OPTION_MASK, root->name);
                ret = -EINVAL;
                goto out_unlock;
        }

        /* remounting is not allowed for populated hierarchies */
        if (root->number_of_cgroups > 1) {
                ret = -EBUSY;
                goto out_unlock;
        }

        ret = rebind_subsystems(root, added_mask, removed_mask);
        if (ret)
                goto out_unlock;

        if (opts.release_agent)
                strcpy(root->release_agent_path, opts.release_agent);
 out_unlock:
        kfree(opts.release_agent);
        kfree(opts.name);
        mutex_unlock(&cgroup_root_mutex);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
        return ret;
}

static const struct super_operations cgroup_ops = {
        .statfs = simple_statfs,
        .drop_inode = generic_delete_inode,
        .show_options = cgroup_show_options,
        .remount_fs = cgroup_remount,
};

static void init_cgroup_housekeeping(struct cgroup *cgrp)
{
        INIT_LIST_HEAD(&cgrp->sibling);
        INIT_LIST_HEAD(&cgrp->children);
        INIT_LIST_HEAD(&cgrp->files);
        INIT_LIST_HEAD(&cgrp->cset_links);
        INIT_LIST_HEAD(&cgrp->release_list);
        INIT_LIST_HEAD(&cgrp->pidlists);
        mutex_init(&cgrp->pidlist_mutex);
        cgrp->dummy_css.cgroup = cgrp;
        INIT_LIST_HEAD(&cgrp->event_list);
        spin_lock_init(&cgrp->event_list_lock);
        simple_xattrs_init(&cgrp->xattrs);
}

static void init_cgroup_root(struct cgroupfs_root *root)
{
        struct cgroup *cgrp = &root->top_cgroup;

        INIT_LIST_HEAD(&root->subsys_list);
        INIT_LIST_HEAD(&root->root_list);
        root->number_of_cgroups = 1;
        cgrp->root = root;
        RCU_INIT_POINTER(cgrp->name, &root_cgroup_name);
        init_cgroup_housekeeping(cgrp);
        idr_init(&root->cgroup_idr);
}

static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
{
        int id;

        lockdep_assert_held(&cgroup_mutex);
        lockdep_assert_held(&cgroup_root_mutex);

        id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, start, end,
                              GFP_KERNEL);
        if (id < 0)
                return id;

        root->hierarchy_id = id;
        return 0;
}

static void cgroup_exit_root_id(struct cgroupfs_root *root)
{
        lockdep_assert_held(&cgroup_mutex);
        lockdep_assert_held(&cgroup_root_mutex);

        if (root->hierarchy_id) {
                idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
                root->hierarchy_id = 0;
        }
}

static int cgroup_test_super(struct super_block *sb, void *data)
{
        struct cgroup_sb_opts *opts = data;
        struct cgroupfs_root *root = sb->s_fs_info;

        /* If we asked for a name then it must match */
        if (opts->name && strcmp(opts->name, root->name))
                return 0;

        /*
         * If we asked for subsystems (or explicitly for no
         * subsystems) then they must match
         */
        if ((opts->subsys_mask || opts->none)
            && (opts->subsys_mask != root->subsys_mask))
                return 0;

        return 1;
}

static struct cgroupfs_root *cgroup_root_from_opts(struct cgroup_sb_opts *opts)
{
        struct cgroupfs_root *root;

        if (!opts->subsys_mask && !opts->none)
                return NULL;

        root = kzalloc(sizeof(*root), GFP_KERNEL);
        if (!root)
                return ERR_PTR(-ENOMEM);

        init_cgroup_root(root);

        /*
         * We need to set @root->subsys_mask now so that @root can be
         * matched by cgroup_test_super() before it finishes
         * initialization; otherwise, competing mounts with the same
         * options may try to bind the same subsystems instead of waiting
         * for the first one leading to unexpected mount errors.
         * SUBSYS_BOUND will be set once actual binding is complete.
         */
        root->subsys_mask = opts->subsys_mask;
        root->flags = opts->flags;
        if (opts->release_agent)
                strcpy(root->release_agent_path, opts->release_agent);
        if (opts->name)
                strcpy(root->name, opts->name);
        if (opts->cpuset_clone_children)
                set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->top_cgroup.flags);
        return root;
}

static void cgroup_free_root(struct cgroupfs_root *root)
{
        if (root) {
                /* hierarhcy ID shoulid already have been released */
                WARN_ON_ONCE(root->hierarchy_id);

                idr_destroy(&root->cgroup_idr);
                kfree(root);
        }
}

static int cgroup_set_super(struct super_block *sb, void *data)
{
        int ret;
        struct cgroup_sb_opts *opts = data;

        /* If we don't have a new root, we can't set up a new sb */
        if (!opts->new_root)
                return -EINVAL;

        BUG_ON(!opts->subsys_mask && !opts->none);

        ret = set_anon_super(sb, NULL);
        if (ret)
                return ret;

        sb->s_fs_info = opts->new_root;
        opts->new_root->sb = sb;

        sb->s_blocksize = PAGE_CACHE_SIZE;
        sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
        sb->s_magic = CGROUP_SUPER_MAGIC;
        sb->s_op = &cgroup_ops;

        return 0;
}

static int cgroup_get_rootdir(struct super_block *sb)
{
        static const struct dentry_operations cgroup_dops = {
                .d_iput = cgroup_diput,
                .d_delete = cgroup_delete,
        };

        struct inode *inode =
                cgroup_new_inode(S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR, sb);

        if (!inode)
                return -ENOMEM;

        inode->i_fop = &simple_dir_operations;
        inode->i_op = &cgroup_dir_inode_operations;
        /* directories start off with i_nlink == 2 (for "." entry) */
        inc_nlink(inode);
        sb->s_root = d_make_root(inode);
        if (!sb->s_root)
                return -ENOMEM;
        /* for everything else we want ->d_op set */
        sb->s_d_op = &cgroup_dops;
        return 0;
}

static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                         int flags, const char *unused_dev_name,
                         void *data)
{
        struct cgroup_sb_opts opts;
        struct cgroupfs_root *root;
        int ret = 0;
        struct super_block *sb;
        struct cgroupfs_root *new_root;
        struct list_head tmp_links;
        struct inode *inode;
        const struct cred *cred;

        /* First find the desired set of subsystems */
        mutex_lock(&cgroup_mutex);
        ret = parse_cgroupfs_options(data, &opts);
        mutex_unlock(&cgroup_mutex);
        if (ret)
                goto out_err;

        /*
         * Allocate a new cgroup root. We may not need it if we're
         * reusing an existing hierarchy.
         */
        new_root = cgroup_root_from_opts(&opts);
        if (IS_ERR(new_root)) {
                ret = PTR_ERR(new_root);
                goto out_err;
        }
        opts.new_root = new_root;

        /* Locate an existing or new sb for this hierarchy */
        sb = sget(fs_type, cgroup_test_super, cgroup_set_super, 0, &opts);
        if (IS_ERR(sb)) {
                ret = PTR_ERR(sb);
                cgroup_free_root(opts.new_root);
                goto out_err;
        }

        root = sb->s_fs_info;
        BUG_ON(!root);
        if (root == opts.new_root) {
                /* We used the new root structure, so this is a new hierarchy */
                struct cgroup *root_cgrp = &root->top_cgroup;
                struct cgroupfs_root *existing_root;
                int i;
                struct css_set *cset;

                BUG_ON(sb->s_root != NULL);

                ret = cgroup_get_rootdir(sb);
                if (ret)
                        goto drop_new_super;
                inode = sb->s_root->d_inode;

                mutex_lock(&inode->i_mutex);
                mutex_lock(&cgroup_mutex);
                mutex_lock(&cgroup_root_mutex);

                ret = idr_alloc(&root->cgroup_idr, root_cgrp, 0, 1, GFP_KERNEL);
                if (ret < 0)
                        goto unlock_drop;
                root_cgrp->id = ret;

                /* Check for name clashes with existing mounts */
                ret = -EBUSY;
                if (strlen(root->name))
                        for_each_active_root(existing_root)
                                if (!strcmp(existing_root->name, root->name))
                                        goto unlock_drop;

                /*
                 * We're accessing css_set_count without locking
                 * css_set_lock here, but that's OK - it can only be
                 * increased by someone holding cgroup_lock, and
                 * that's us. The worst that can happen is that we
                 * have some link structures left over
                 */
                ret = allocate_cgrp_cset_links(css_set_count, &tmp_links);
                if (ret)
                        goto unlock_drop;

                /* ID 0 is reserved for dummy root, 1 for unified hierarchy */
                ret = cgroup_init_root_id(root, 2, 0);
                if (ret)
                        goto unlock_drop;

                sb->s_root->d_fsdata = root_cgrp;
                root_cgrp->dentry = sb->s_root;

                /*
                 * We're inside get_sb() and will call lookup_one_len() to
                 * create the root files, which doesn't work if SELinux is
                 * in use.  The following cred dancing somehow works around
                 * it.  See 2ce9738ba ("cgroupfs: use init_cred when
                 * populating new cgroupfs mount") for more details.
                 */
                cred = override_creds(&init_cred);

                ret = cgroup_addrm_files(root_cgrp, cgroup_base_files, true);
                if (ret)
                        goto rm_base_files;

                ret = rebind_subsystems(root, root->subsys_mask, 0);
                if (ret)
                        goto rm_base_files;

                revert_creds(cred);

                /*
                 * There must be no failure case after here, since rebinding
                 * takes care of subsystems' refcounts, which are explicitly
                 * dropped in the failure exit path.
                 */

                list_add(&root->root_list, &cgroup_roots);
                cgroup_root_count++;

                /* Link the top cgroup in this hierarchy into all
                 * the css_set objects */
                write_lock(&css_set_lock);
                hash_for_each(css_set_table, i, cset, hlist)
                        link_css_set(&tmp_links, cset, root_cgrp);
                write_unlock(&css_set_lock);

                free_cgrp_cset_links(&tmp_links);

                BUG_ON(!list_empty(&root_cgrp->children));
                BUG_ON(root->number_of_cgroups != 1);

                mutex_unlock(&cgroup_root_mutex);
                mutex_unlock(&cgroup_mutex);
                mutex_unlock(&inode->i_mutex);
        } else {
                /*
                 * We re-used an existing hierarchy - the new root (if
                 * any) is not needed
                 */
                cgroup_free_root(opts.new_root);

                if ((root->flags ^ opts.flags) & CGRP_ROOT_OPTION_MASK) {
                        if ((root->flags | opts.flags) & CGRP_ROOT_SANE_BEHAVIOR) {
                                pr_err("cgroup: sane_behavior: new mount options should match the existing superblock\n");
                                ret = -EINVAL;
                                goto drop_new_super;
                        } else {
                                pr_warning("cgroup: new mount options do not match the existing superblock, will be ignored\n");
                        }
                }
        }

        kfree(opts.release_agent);
        kfree(opts.name);
        return dget(sb->s_root);

 rm_base_files:
        free_cgrp_cset_links(&tmp_links);
        cgroup_addrm_files(&root->top_cgroup, cgroup_base_files, false);
        revert_creds(cred);
 unlock_drop:
        cgroup_exit_root_id(root);
        mutex_unlock(&cgroup_root_mutex);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&inode->i_mutex);
 drop_new_super:
        deactivate_locked_super(sb);
 out_err:
        kfree(opts.release_agent);
        kfree(opts.name);
        return ERR_PTR(ret);
}

static void cgroup_kill_sb(struct super_block *sb) {
        struct cgroupfs_root *root = sb->s_fs_info;
        struct cgroup *cgrp = &root->top_cgroup;
        struct cgrp_cset_link *link, *tmp_link;
        int ret;

        BUG_ON(!root);

        BUG_ON(root->number_of_cgroups != 1);
        BUG_ON(!list_empty(&cgrp->children));

        mutex_lock(&cgrp->dentry->d_inode->i_mutex);
        mutex_lock(&cgroup_mutex);
        mutex_lock(&cgroup_root_mutex);

        /* Rebind all subsystems back to the default hierarchy */
        if (root->flags & CGRP_ROOT_SUBSYS_BOUND) {
                ret = rebind_subsystems(root, 0, root->subsys_mask);
                /* Shouldn't be able to fail ... */
                BUG_ON(ret);
        }

        /*
         * Release all the links from cset_links to this hierarchy's
         * root cgroup
         */
        write_lock(&css_set_lock);

        list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link) {
                list_del(&link->cset_link);
                list_del(&link->cgrp_link);
                kfree(link);
        }
        write_unlock(&css_set_lock);

        if (!list_empty(&root->root_list)) {
                list_del(&root->root_list);
                cgroup_root_count--;
        }

        cgroup_exit_root_id(root);

        mutex_unlock(&cgroup_root_mutex);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&cgrp->dentry->d_inode->i_mutex);

        simple_xattrs_free(&cgrp->xattrs);

        kill_litter_super(sb);
        cgroup_free_root(root);
}

static struct file_system_type cgroup_fs_type = {
        .name = "cgroup",
        .mount = cgroup_mount,
        .kill_sb = cgroup_kill_sb,
};

static struct kobject *cgroup_kobj;

/**
 * cgroup_path - generate the path of a cgroup
 * @cgrp: the cgroup in question
 * @buf: the buffer to write the path into
 * @buflen: the length of the buffer
 *
 * Writes path of cgroup into buf.  Returns 0 on success, -errno on error.
 *
 * We can't generate cgroup path using dentry->d_name, as accessing
 * dentry->name must be protected by irq-unsafe dentry->d_lock or parent
 * inode's i_mutex, while on the other hand cgroup_path() can be called
 * with some irq-safe spinlocks held.
 */
int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
{
        int ret = -ENAMETOOLONG;
        char *start;

        if (!cgrp->parent) {
                if (strlcpy(buf, "/", buflen) >= buflen)
                        return -ENAMETOOLONG;
                return 0;
        }

        start = buf + buflen - 1;
        *start = '\0';

        rcu_read_lock();
        do {
                const char *name = cgroup_name(cgrp);
                int len;

                len = strlen(name);
                if ((start -= len) < buf)
                        goto out;
                memcpy(start, name, len);

                if (--start < buf)
                        goto out;
                *start = '/';

                cgrp = cgrp->parent;
        } while (cgrp->parent);
        ret = 0;
        memmove(buf, start, buf + buflen - start);
out:
        rcu_read_unlock();
        return ret;
}
EXPORT_SYMBOL_GPL(cgroup_path);

/**
 * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
 * @task: target task
 * @buf: the buffer to write the path into
 * @buflen: the length of the buffer
 *
 * Determine @task's cgroup on the first (the one with the lowest non-zero
 * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
 * function grabs cgroup_mutex and shouldn't be used inside locks used by
 * cgroup controller callbacks.
 *
 * Returns 0 on success, fails with -%ENAMETOOLONG if @buflen is too short.
 */
int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
{
        struct cgroupfs_root *root;
        struct cgroup *cgrp;
        int hierarchy_id = 1, ret = 0;

        if (buflen < 2)
                return -ENAMETOOLONG;

        mutex_lock(&cgroup_mutex);

        root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);

        if (root) {
                cgrp = task_cgroup_from_root(task, root);
                ret = cgroup_path(cgrp, buf, buflen);
        } else {
                /* if no hierarchy exists, everyone is in "/" */
                memcpy(buf, "/", 2);
        }

        mutex_unlock(&cgroup_mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(task_cgroup_path);

/*
 * Control Group taskset
 */
struct task_and_cgroup {
        struct task_struct      *task;
        struct cgroup           *cgrp;
        struct css_set          *cset;
};

struct cgroup_taskset {
        struct task_and_cgroup  single;
        struct flex_array       *tc_array;
        int                     tc_array_len;
        int                     idx;
        struct cgroup           *cur_cgrp;
};

/**
 * cgroup_taskset_first - reset taskset and return the first task
 * @tset: taskset of interest
 *
 * @tset iteration is initialized and the first task is returned.
 */
struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset)
{
        if (tset->tc_array) {
                tset->idx = 0;
                return cgroup_taskset_next(tset);
        } else {
                tset->cur_cgrp = tset->single.cgrp;
                return tset->single.task;
        }
}
EXPORT_SYMBOL_GPL(cgroup_taskset_first);

/**
 * cgroup_taskset_next - iterate to the next task in taskset
 * @tset: taskset of interest
 *
 * Return the next task in @tset.  Iteration must have been initialized
 * with cgroup_taskset_first().
 */
struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset)
{
        struct task_and_cgroup *tc;

        if (!tset->tc_array || tset->idx >= tset->tc_array_len)
                return NULL;

        tc = flex_array_get(tset->tc_array, tset->idx++);
        tset->cur_cgrp = tc->cgrp;
        return tc->task;
}
EXPORT_SYMBOL_GPL(cgroup_taskset_next);

/**
 * cgroup_taskset_cur_css - return the matching css for the current task
 * @tset: taskset of interest
 * @subsys_id: the ID of the target subsystem
 *
 * Return the css for the current (last returned) task of @tset for
 * subsystem specified by @subsys_id.  This function must be preceded by
 * either cgroup_taskset_first() or cgroup_taskset_next().
 */
struct cgroup_subsys_state *cgroup_taskset_cur_css(struct cgroup_taskset *tset,
                                                   int subsys_id)
{
        return cgroup_css(tset->cur_cgrp, cgroup_subsys[subsys_id]);
}
EXPORT_SYMBOL_GPL(cgroup_taskset_cur_css);

/**
 * cgroup_taskset_size - return the number of tasks in taskset
 * @tset: taskset of interest
 */
int cgroup_taskset_size(struct cgroup_taskset *tset)
{
        return tset->tc_array ? tset->tc_array_len : 1;
}
EXPORT_SYMBOL_GPL(cgroup_taskset_size);


/*
 * cgroup_task_migrate - move a task from one cgroup to another.
 *
 * Must be called with cgroup_mutex and threadgroup locked.
 */
static void cgroup_task_migrate(struct cgroup *old_cgrp,
                                struct task_struct *tsk,
                                struct css_set *new_cset)
{
        struct css_set *old_cset;

        /*
         * We are synchronized through threadgroup_lock() against PF_EXITING
         * setting such that we can't race against cgroup_exit() changing the
         * css_set to init_css_set and dropping the old one.
         */
        WARN_ON_ONCE(tsk->flags & PF_EXITING);
        old_cset = task_css_set(tsk);

        task_lock(tsk);
        rcu_assign_pointer(tsk->cgroups, new_cset);
        task_unlock(tsk);

        /* Update the css_set linked lists if we're using them */
        write_lock(&css_set_lock);
        if (!list_empty(&tsk->cg_list))
                list_move(&tsk->cg_list, &new_cset->tasks);
        write_unlock(&css_set_lock);

        /*
         * We just gained a reference on old_cset by taking it from the
         * task. As trading it for new_cset is protected by cgroup_mutex,
         * we're safe to drop it here; it will be freed under RCU.
         */
        set_bit(CGRP_RELEASABLE, &old_cgrp->flags);
        put_css_set(old_cset);
}

/**
 * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
 * @cgrp: the cgroup to attach to
 * @tsk: the task or the leader of the threadgroup to be attached
 * @threadgroup: attach the whole threadgroup?
 *
 * Call holding cgroup_mutex and the group_rwsem of the leader. Will take
 * task_lock of @tsk or each thread in the threadgroup individually in turn.
 */
static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
                              bool threadgroup)
{
        int retval, i, group_size;
        struct cgroup_subsys *ss, *failed_ss = NULL;
        struct cgroupfs_root *root = cgrp->root;
        /* threadgroup list cursor and array */
        struct task_struct *leader = tsk;
        struct task_and_cgroup *tc;
        struct flex_array *group;
        struct cgroup_taskset tset = { };

        /*
         * step 0: in order to do expensive, possibly blocking operations for
         * every thread, we cannot iterate the thread group list, since it needs
         * rcu or tasklist locked. instead, build an array of all threads in the
         * group - group_rwsem prevents new threads from appearing, and if
         * threads exit, this will just be an over-estimate.
         */
        if (threadgroup)
                group_size = get_nr_threads(tsk);
        else
                group_size = 1;
        /* flex_array supports very large thread-groups better than kmalloc. */
        group = flex_array_alloc(sizeof(*tc), group_size, GFP_KERNEL);
        if (!group)
                return -ENOMEM;
        /* pre-allocate to guarantee space while iterating in rcu read-side. */
        retval = flex_array_prealloc(group, 0, group_size, GFP_KERNEL);
        if (retval)
                goto out_free_group_list;

        i = 0;
        /*
         * Prevent freeing of tasks while we take a snapshot. Tasks that are
         * already PF_EXITING could be freed from underneath us unless we
         * take an rcu_read_lock.
         */
        rcu_read_lock();
        do {
                struct task_and_cgroup ent;

                /* @tsk either already exited or can't exit until the end */
                if (tsk->flags & PF_EXITING)
                        goto next;

                /* as per above, nr_threads may decrease, but not increase. */
                BUG_ON(i >= group_size);
                ent.task = tsk;
                ent.cgrp = task_cgroup_from_root(tsk, root);
                /* nothing to do if this task is already in the cgroup */
                if (ent.cgrp == cgrp)
                        goto next;
                /*
                 * saying GFP_ATOMIC has no effect here because we did prealloc
                 * earlier, but it's good form to communicate our expectations.
                 */
                retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
                BUG_ON(retval != 0);
                i++;
        next:
                if (!threadgroup)
                        break;
        } while_each_thread(leader, tsk);
        rcu_read_unlock();
        /* remember the number of threads in the array for later. */
        group_size = i;
        tset.tc_array = group;
        tset.tc_array_len = group_size;

        /* methods shouldn't be called if no task is actually migrating */
        retval = 0;
        if (!group_size)
                goto out_free_group_list;

        /*
         * step 1: check that we can legitimately attach to the cgroup.
         */
        for_each_root_subsys(root, ss) {
                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);

                if (ss->can_attach) {
                        retval = ss->can_attach(css, &tset);
                        if (retval) {
                                failed_ss = ss;
                                goto out_cancel_attach;
                        }
                }
        }

        /*
         * step 2: make sure css_sets exist for all threads to be migrated.
         * we use find_css_set, which allocates a new one if necessary.
         */
        for (i = 0; i < group_size; i++) {
                struct css_set *old_cset;

                tc = flex_array_get(group, i);
                old_cset = task_css_set(tc->task);
                tc->cset = find_css_set(old_cset, cgrp);
                if (!tc->cset) {
                        retval = -ENOMEM;
                        goto out_put_css_set_refs;
                }
        }

        /*
         * step 3: now that we're guaranteed success wrt the css_sets,
         * proceed to move all tasks to the new cgroup.  There are no
         * failure cases after here, so this is the commit point.
         */
        for (i = 0; i < group_size; i++) {
                tc = flex_array_get(group, i);
                cgroup_task_migrate(tc->cgrp, tc->task, tc->cset);
        }
        /* nothing is sensitive to fork() after this point. */

        /*
         * step 4: do subsystem attach callbacks.
         */
        for_each_root_subsys(root, ss) {
                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);

                if (ss->attach)
                        ss->attach(css, &tset);
        }

        /*
         * step 5: success! and cleanup
         */
        retval = 0;
out_put_css_set_refs:
        if (retval) {
                for (i = 0; i < group_size; i++) {
                        tc = flex_array_get(group, i);
                        if (!tc->cset)
                                break;
                        put_css_set(tc->cset);
                }
        }
out_cancel_attach:
        if (retval) {
                for_each_root_subsys(root, ss) {
                        struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);

                        if (ss == failed_ss)
                                break;
                        if (ss->cancel_attach)
                                ss->cancel_attach(css, &tset);
                }
        }
out_free_group_list:
        flex_array_free(group);
        return retval;
}

/*
 * Find the task_struct of the task to attach by vpid and pass it along to the
 * function to attach either it or all tasks in its threadgroup. Will lock
 * cgroup_mutex and threadgroup; may take task_lock of task.
 */
static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
{
        struct task_struct *tsk;
        const struct cred *cred = current_cred(), *tcred;
        int ret;

        if (!cgroup_lock_live_group(cgrp))
                return -ENODEV;

retry_find_task:
        rcu_read_lock();
        if (pid) {
                tsk = find_task_by_vpid(pid);
                if (!tsk) {
                        rcu_read_unlock();
                        ret= -ESRCH;
                        goto out_unlock_cgroup;
                }
                /*
                 * even if we're attaching all tasks in the thread group, we
                 * only need to check permissions on one of them.
                 */
                tcred = __task_cred(tsk);
                if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
                    !uid_eq(cred->euid, tcred->uid) &&
                    !uid_eq(cred->euid, tcred->suid)) {
                        rcu_read_unlock();
                        ret = -EACCES;
                        goto out_unlock_cgroup;
                }
        } else
                tsk = current;

        if (threadgroup)
                tsk = tsk->group_leader;

        /*
         * Workqueue threads may acquire PF_NO_SETAFFINITY and become
         * trapped in a cpuset, or RT worker may be born in a cgroup
         * with no rt_runtime allocated.  Just say no.
         */
        if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
                ret = -EINVAL;
                rcu_read_unlock();
                goto out_unlock_cgroup;
        }

        get_task_struct(tsk);
        rcu_read_unlock();

        threadgroup_lock(tsk);
        if (threadgroup) {
                if (!thread_group_leader(tsk)) {
                        /*
                         * a race with de_thread from another thread's exec()
                         * may strip us of our leadership, if this happens,
                         * there is no choice but to throw this task away and
                         * try again; this is
                         * "double-double-toil-and-trouble-check locking".
                         */
                        threadgroup_unlock(tsk);
                        put_task_struct(tsk);
                        goto retry_find_task;
                }
        }

        ret = cgroup_attach_task(cgrp, tsk, threadgroup);

        threadgroup_unlock(tsk);

        put_task_struct(tsk);
out_unlock_cgroup:
        mutex_unlock(&cgroup_mutex);
        return ret;
}

/**
 * cgroup_attach_task_all - attach task 'tsk' to all cgroups of task 'from'
 * @from: attach to all cgroups of a given task
 * @tsk: the task to be attached
 */
int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
{
        struct cgroupfs_root *root;
        int retval = 0;

        mutex_lock(&cgroup_mutex);
        for_each_active_root(root) {
                struct cgroup *from_cgrp = task_cgroup_from_root(from, root);

                retval = cgroup_attach_task(from_cgrp, tsk, false);
                if (retval)
                        break;
        }
        mutex_unlock(&cgroup_mutex);

        return retval;
}
EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

static int cgroup_tasks_write(struct cgroup_subsys_state *css,
                              struct cftype *cft, u64 pid)
{
        return attach_task_by_pid(css->cgroup, pid, false);
}

static int cgroup_procs_write(struct cgroup_subsys_state *css,
                              struct cftype *cft, u64 tgid)
{
        return attach_task_by_pid(css->cgroup, tgid, true);
}

static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
                                      struct cftype *cft, const char *buffer)
{
        BUILD_BUG_ON(sizeof(css->cgroup->root->release_agent_path) < PATH_MAX);
        if (strlen(buffer) >= PATH_MAX)
                return -EINVAL;
        if (!cgroup_lock_live_group(css->cgroup))
                return -ENODEV;
        mutex_lock(&cgroup_root_mutex);
        strcpy(css->cgroup->root->release_agent_path, buffer);
        mutex_unlock(&cgroup_root_mutex);
        mutex_unlock(&cgroup_mutex);
        return 0;
}

static int cgroup_release_agent_show(struct cgroup_subsys_state *css,
                                     struct cftype *cft, struct seq_file *seq)
{
        struct cgroup *cgrp = css->cgroup;

        if (!cgroup_lock_live_group(cgrp))
                return -ENODEV;
        seq_puts(seq, cgrp->root->release_agent_path);
        seq_putc(seq, '\n');
        mutex_unlock(&cgroup_mutex);
        return 0;
}

static int cgroup_sane_behavior_show(struct cgroup_subsys_state *css,
                                     struct cftype *cft, struct seq_file *seq)
{
        seq_printf(seq, "%d\n", cgroup_sane_behavior(css->cgroup));
        return 0;
}

/* A buffer size big enough for numbers or short strings */
#define CGROUP_LOCAL_BUFFER_SIZE 64

static ssize_t cgroup_write_X64(struct cgroup_subsys_state *css,
                                struct cftype *cft, struct file *file,
                                const char __user *userbuf, size_t nbytes,
                                loff_t *unused_ppos)
{
        char buffer[CGROUP_LOCAL_BUFFER_SIZE];
        int retval = 0;
        char *end;

        if (!nbytes)
                return -EINVAL;
        if (nbytes >= sizeof(buffer))
                return -E2BIG;
        if (copy_from_user(buffer, userbuf, nbytes))
                return -EFAULT;

        buffer[nbytes] = 0;     /* nul-terminate */
        if (cft->write_u64) {
                u64 val = simple_strtoull(strstrip(buffer), &end, 0);
                if (*end)
                        return -EINVAL;
                retval = cft->write_u64(css, cft, val);
        } else {
                s64 val = simple_strtoll(strstrip(buffer), &end, 0);
                if (*end)
                        return -EINVAL;
                retval = cft->write_s64(css, cft, val);
        }
        if (!retval)
                retval = nbytes;
        return retval;
}

static ssize_t cgroup_write_string(struct cgroup_subsys_state *css,
                                   struct cftype *cft, struct file *file,
                                   const char __user *userbuf, size_t nbytes,
                                   loff_t *unused_ppos)
{
        char local_buffer[CGROUP_LOCAL_BUFFER_SIZE];
        int retval = 0;
        size_t max_bytes = cft->max_write_len;
        char *buffer = local_buffer;

        if (!max_bytes)
                max_bytes = sizeof(local_buffer) - 1;
        if (nbytes >= max_bytes)
                return -E2BIG;
        /* Allocate a dynamic buffer if we need one */
        if (nbytes >= sizeof(local_buffer)) {
                buffer = kmalloc(nbytes + 1, GFP_KERNEL);
                if (buffer == NULL)
                        return -ENOMEM;
        }
        if (nbytes && copy_from_user(buffer, userbuf, nbytes)) {
                retval = -EFAULT;
                goto out;
        }

        buffer[nbytes] = 0;     /* nul-terminate */
        retval = cft->write_string(css, cft, strstrip(buffer));
        if (!retval)
                retval = nbytes;
out:
        if (buffer != local_buffer)
                kfree(buffer);
        return retval;
}

static ssize_t cgroup_file_write(struct file *file, const char __user *buf,
                                 size_t nbytes, loff_t *ppos)
{
        struct cfent *cfe = __d_cfe(file->f_dentry);
        struct cftype *cft = __d_cft(file->f_dentry);
        struct cgroup_subsys_state *css = cfe->css;

        if (cft->write)
                return cft->write(css, cft, file, buf, nbytes, ppos);
        if (cft->write_u64 || cft->write_s64)
                return cgroup_write_X64(css, cft, file, buf, nbytes, ppos);
        if (cft->write_string)
                return cgroup_write_string(css, cft, file, buf, nbytes, ppos);
        if (cft->trigger) {
                int ret = cft->trigger(css, (unsigned int)cft->private);
                return ret ? ret : nbytes;
        }
        return -EINVAL;
}

static ssize_t cgroup_read_u64(struct cgroup_subsys_state *css,
                               struct cftype *cft, struct file *file,
                               char __user *buf, size_t nbytes, loff_t *ppos)
{
        char tmp[CGROUP_LOCAL_BUFFER_SIZE];
        u64 val = cft->read_u64(css, cft);
        int len = sprintf(tmp, "%llu\n", (unsigned long long) val);

        return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
}

static ssize_t cgroup_read_s64(struct cgroup_subsys_state *css,
                               struct cftype *cft, struct file *file,
                               char __user *buf, size_t nbytes, loff_t *ppos)
{
        char tmp[CGROUP_LOCAL_BUFFER_SIZE];
        s64 val = cft->read_s64(css, cft);
        int len = sprintf(tmp, "%lld\n", (long long) val);

        return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
}

static ssize_t cgroup_file_read(struct file *file, char __user *buf,
                                size_t nbytes, loff_t *ppos)
{
        struct cfent *cfe = __d_cfe(file->f_dentry);
        struct cftype *cft = __d_cft(file->f_dentry);
        struct cgroup_subsys_state *css = cfe->css;

        if (cft->read)
                return cft->read(css, cft, file, buf, nbytes, ppos);
        if (cft->read_u64)
                return cgroup_read_u64(css, cft, file, buf, nbytes, ppos);
        if (cft->read_s64)
                return cgroup_read_s64(css, cft, file, buf, nbytes, ppos);
        return -EINVAL;
}

/*
 * seqfile ops/methods for returning structured data. Currently just
 * supports string->u64 maps, but can be extended in future.
 */

static int cgroup_map_add(struct cgroup_map_cb *cb, const char *key, u64 value)
{
        struct seq_file *sf = cb->state;
        return seq_printf(sf, "%s %llu\n", key, (unsigned long long)value);
}

static int cgroup_seqfile_show(struct seq_file *m, void *arg)
{
        struct cfent *cfe = m->private;
        struct cftype *cft = cfe->type;
        struct cgroup_subsys_state *css = cfe->css;

        if (cft->read_map) {
                struct cgroup_map_cb cb = {
                        .fill = cgroup_map_add,
                        .state = m,
                };
                return cft->read_map(css, cft, &cb);
        }
        return cft->read_seq_string(css, cft, m);
}

static const struct file_operations cgroup_seqfile_operations = {
        .read = seq_read,
        .write = cgroup_file_write,
        .llseek = seq_lseek,
        .release = cgroup_file_release,
};

static int cgroup_file_open(struct inode *inode, struct file *file)
{
        struct cfent *cfe = __d_cfe(file->f_dentry);
        struct cftype *cft = __d_cft(file->f_dentry);
        struct cgroup *cgrp = __d_cgrp(cfe->dentry->d_parent);
        struct cgroup_subsys_state *css;
        int err;

        err = generic_file_open(inode, file);
        if (err)
                return err;

        /*
         * If the file belongs to a subsystem, pin the css.  Will be
         * unpinned either on open failure or release.  This ensures that
         * @css stays alive for all file operations.
         */
        rcu_read_lock();
        css = cgroup_css(cgrp, cft->ss);
        if (cft->ss && !css_tryget(css))
                css = NULL;
        rcu_read_unlock();

        if (!css)
                return -ENODEV;

        /*
         * @cfe->css is used by read/write/close to determine the
         * associated css.  @file->private_data would be a better place but
         * that's already used by seqfile.  Multiple accessors may use it
         * simultaneously which is okay as the association never changes.
         */
        WARN_ON_ONCE(cfe->css && cfe->css != css);
        cfe->css = css;

        if (cft->read_map || cft->read_seq_string) {
                file->f_op = &cgroup_seqfile_operations;
                err = single_open(file, cgroup_seqfile_show, cfe);
        } else if (cft->open) {
                err = cft->open(inode, file);
        }

        if (css->ss && err)
                css_put(css);
        return err;
}

static int cgroup_file_release(struct inode *inode, struct file *file)
{
        struct cfent *cfe = __d_cfe(file->f_dentry);
        struct cftype *cft = __d_cft(file->f_dentry);
        struct cgroup_subsys_state *css = cfe->css;
        int ret = 0;

        if (cft->release)
                ret = cft->release(inode, file);
        if (css->ss)
                css_put(css);
        if (file->f_op == &cgroup_seqfile_operations)
                single_release(inode, file);
        return ret;
}

/*
 * cgroup_rename - Only allow simple rename of directories in place.
 */
static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
                            struct inode *new_dir, struct dentry *new_dentry)
{
        int ret;
        struct cgroup_name *name, *old_name;
        struct cgroup *cgrp;

        /*
         * It's convinient to use parent dir's i_mutex to protected
         * cgrp->name.
         */
        lockdep_assert_held(&old_dir->i_mutex);

        if (!S_ISDIR(old_dentry->d_inode->i_mode))
                return -ENOTDIR;
        if (new_dentry->d_inode)
                return -EEXIST;
        if (old_dir != new_dir)
                return -EIO;

        cgrp = __d_cgrp(old_dentry);

        /*
         * This isn't a proper migration and its usefulness is very
         * limited.  Disallow if sane_behavior.
         */
        if (cgroup_sane_behavior(cgrp))
                return -EPERM;

        name = cgroup_alloc_name(new_dentry);
        if (!name)
                return -ENOMEM;

        ret = simple_rename(old_dir, old_dentry, new_dir, new_dentry);
        if (ret) {
                kfree(name);
                return ret;
        }

        old_name = rcu_dereference_protected(cgrp->name, true);
        rcu_assign_pointer(cgrp->name, name);

        kfree_rcu(old_name, rcu_head);
        return 0;
}

static struct simple_xattrs *__d_xattrs(struct dentry *dentry)
{
        if (S_ISDIR(dentry->d_inode->i_mode))
                return &__d_cgrp(dentry)->xattrs;
        else
                return &__d_cfe(dentry)->xattrs;
}

static inline int xattr_enabled(struct dentry *dentry)
{
        struct cgroupfs_root *root = dentry->d_sb->s_fs_info;
        return root->flags & CGRP_ROOT_XATTR;
}

static bool is_valid_xattr(const char *name)
{
        if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN) ||
            !strncmp(name, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN))
                return true;
        return false;
}

static int cgroup_setxattr(struct dentry *dentry, const char *name,
                           const void *val, size_t size, int flags)
{
        if (!xattr_enabled(dentry))
                return -EOPNOTSUPP;
        if (!is_valid_xattr(name))
                return -EINVAL;
        return simple_xattr_set(__d_xattrs(dentry), name, val, size, flags);
}

static int cgroup_removexattr(struct dentry *dentry, const char *name)
{
        if (!xattr_enabled(dentry))
                return -EOPNOTSUPP;
        if (!is_valid_xattr(name))
                return -EINVAL;
        return simple_xattr_remove(__d_xattrs(dentry), name);
}

static ssize_t cgroup_getxattr(struct dentry *dentry, const char *name,
                               void *buf, size_t size)
{
        if (!xattr_enabled(dentry))
                return -EOPNOTSUPP;
        if (!is_valid_xattr(name))
                return -EINVAL;
        return simple_xattr_get(__d_xattrs(dentry), name, buf, size);
}

static ssize_t cgroup_listxattr(struct dentry *dentry, char *buf, size_t size)
{
        if (!xattr_enabled(dentry))
                return -EOPNOTSUPP;
        return simple_xattr_list(__d_xattrs(dentry), buf, size);
}

static const struct file_operations cgroup_file_operations = {
        .read = cgroup_file_read,
        .write = cgroup_file_write,
        .llseek = generic_file_llseek,
        .open = cgroup_file_open,
        .release = cgroup_file_release,
};

static const struct inode_operations cgroup_file_inode_operations = {
        .setxattr = cgroup_setxattr,
        .getxattr = cgroup_getxattr,
        .listxattr = cgroup_listxattr,
        .removexattr = cgroup_removexattr,
};

static const struct inode_operations cgroup_dir_inode_operations = {
        .lookup = simple_lookup,
        .mkdir = cgroup_mkdir,
        .rmdir = cgroup_rmdir,
        .rename = cgroup_rename,
        .setxattr = cgroup_setxattr,
        .getxattr = cgroup_getxattr,
        .listxattr = cgroup_listxattr,
        .removexattr = cgroup_removexattr,
};

/*
 * Check if a file is a control file
 */
static inline struct cftype *__file_cft(struct file *file)
{
        if (file_inode(file)->i_fop != &cgroup_file_operations)
                return ERR_PTR(-EINVAL);
        return __d_cft(file->f_dentry);
}

static int cgroup_create_file(struct dentry *dentry, umode_t mode,
                                struct super_block *sb)
{
        struct inode *inode;

        if (!dentry)
                return -ENOENT;
        if (dentry->d_inode)
                return -EEXIST;

        inode = cgroup_new_inode(mode, sb);
        if (!inode)
                return -ENOMEM;

        if (S_ISDIR(mode)) {
                inode->i_op = &cgroup_dir_inode_operations;
                inode->i_fop = &simple_dir_operations;

                /* start off with i_nlink == 2 (for "." entry) */
                inc_nlink(inode);
                inc_nlink(dentry->d_parent->d_inode);

                /*
                 * Control reaches here with cgroup_mutex held.
                 * @inode->i_mutex should nest outside cgroup_mutex but we
                 * want to populate it immediately without releasing
                 * cgroup_mutex.  As @inode isn't visible to anyone else
                 * yet, trylock will always succeed without affecting
                 * lockdep checks.
                 */
                WARN_ON_ONCE(!mutex_trylock(&inode->i_mutex));
        } else if (S_ISREG(mode)) {
                inode->i_size = 0;
                inode->i_fop = &cgroup_file_operations;
                inode->i_op = &cgroup_file_inode_operations;
        }
        d_instantiate(dentry, inode);
        dget(dentry);   /* Extra count - pin the dentry in core */
        return 0;
}

/**
 * cgroup_file_mode - deduce file mode of a control file
 * @cft: the control file in question
 *
 * returns cft->mode if ->mode is not 0
 * returns S_IRUGO|S_IWUSR if it has both a read and a write handler
 * returns S_IRUGO if it has only a read handler
 * returns S_IWUSR if it has only a write hander
 */
static umode_t cgroup_file_mode(const struct cftype *cft)
{
        umode_t mode = 0;

        if (cft->mode)
                return cft->mode;

        if (cft->read || cft->read_u64 || cft->read_s64 ||
            cft->read_map || cft->read_seq_string)
                mode |= S_IRUGO;

        if (cft->write || cft->write_u64 || cft->write_s64 ||
            cft->write_string || cft->trigger)
                mode |= S_IWUSR;

        return mode;
}

static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
{
        struct dentry *dir = cgrp->dentry;
        struct cgroup *parent = __d_cgrp(dir);
        struct dentry *dentry;
        struct cfent *cfe;
        int error;
        umode_t mode;
        char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };

        if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
            !(cgrp->root->flags & CGRP_ROOT_NOPREFIX)) {
                strcpy(name, cft->ss->name);
                strcat(name, ".");
        }
        strcat(name, cft->name);

        BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));

        cfe = kzalloc(sizeof(*cfe), GFP_KERNEL);
        if (!cfe)
                return -ENOMEM;

        dentry = lookup_one_len(name, dir, strlen(name));
        if (IS_ERR(dentry)) {
                error = PTR_ERR(dentry);
                goto out;
        }

        cfe->type = (void *)cft;
        cfe->dentry = dentry;
        dentry->d_fsdata = cfe;
        simple_xattrs_init(&cfe->xattrs);

        mode = cgroup_file_mode(cft);
        error = cgroup_create_file(dentry, mode | S_IFREG, cgrp->root->sb);
        if (!error) {
                list_add_tail(&cfe->node, &parent->files);
                cfe = NULL;
        }
        dput(dentry);
out:
        kfree(cfe);
        return error;
}

/**
 * cgroup_addrm_files - add or remove files to a cgroup directory
 * @cgrp: the target cgroup
 * @cfts: array of cftypes to be added
 * @is_add: whether to add or remove
 *
 * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
 * For removals, this function never fails.  If addition fails, this
 * function doesn't remove files already added.  The caller is responsible
 * for cleaning up.
 */
static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
                              bool is_add)
{
        struct cftype *cft;
        int ret;

        lockdep_assert_held(&cgrp->dentry->d_inode->i_mutex);
        lockdep_assert_held(&cgroup_mutex);

        for (cft = cfts; cft->name[0] != '\0'; cft++) {
                /* does cft->flags tell us to skip this file on @cgrp? */
                if ((cft->flags & CFTYPE_INSANE) && cgroup_sane_behavior(cgrp))
                        continue;
                if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgrp->parent)
                        continue;
                if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgrp->parent)
                        continue;

                if (is_add) {
                        ret = cgroup_add_file(cgrp, cft);
                        if (ret) {
                                pr_warn("cgroup_addrm_files: failed to add %s, err=%d\n",
                                        cft->name, ret);
                                return ret;
                        }
                } else {
                        cgroup_rm_file(cgrp, cft);
                }
        }
        return 0;
}

static void cgroup_cfts_prepare(void)
        __acquires(&cgroup_mutex)
{
        /*
         * Thanks to the entanglement with vfs inode locking, we can't walk
         * the existing cgroups under cgroup_mutex and create files.
         * Instead, we use css_for_each_descendant_pre() and drop RCU read
         * lock before calling cgroup_addrm_files().
         */
        mutex_lock(&cgroup_mutex);
}

static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
        __releases(&cgroup_mutex)
{
        LIST_HEAD(pending);
        struct cgroup_subsys *ss = cfts[0].ss;
        struct cgroup *root = &ss->root->top_cgroup;
        struct super_block *sb = ss->root->sb;
        struct dentry *prev = NULL;
        struct inode *inode;
        struct cgroup_subsys_state *css;
        u64 update_before;
        int ret = 0;

        /* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
        if (!cfts || ss->root == &cgroup_dummy_root ||
            !atomic_inc_not_zero(&sb->s_active)) {
                mutex_unlock(&cgroup_mutex);
                return 0;
        }

        /*
         * All cgroups which are created after we drop cgroup_mutex will
         * have the updated set of files, so we only need to update the
         * cgroups created before the current @cgroup_serial_nr_next.
         */
        update_before = cgroup_serial_nr_next;

        /* add/rm files for all cgroups created before */
        css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
                struct cgroup *cgrp = css->cgroup;

                if (cgroup_is_dead(cgrp))
                        continue;

                inode = cgrp->dentry->d_inode;
                dget(cgrp->dentry);
                dput(prev);
                prev = cgrp->dentry;

                mutex_unlock(&cgroup_mutex);
                mutex_lock(&inode->i_mutex);
                mutex_lock(&cgroup_mutex);
                if (cgrp->serial_nr < update_before && !cgroup_is_dead(cgrp))
                        ret = cgroup_addrm_files(cgrp, cfts, is_add);
                mutex_unlock(&inode->i_mutex);
                if (ret)
                        break;
        }
        mutex_unlock(&cgroup_mutex);
        dput(prev);
        deactivate_super(sb);
        return ret;
}

/**
 * cgroup_add_cftypes - add an array of cftypes to a subsystem
 * @ss: target cgroup subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Register @cfts to @ss.  Files described by @cfts are created for all
 * existing cgroups to which @ss is attached and all future cgroups will
 * have them too.  This function can be called anytime whether @ss is
 * attached or not.
 *
 * Returns 0 on successful registration, -errno on failure.  Note that this
 * function currently returns 0 as long as @cfts registration is successful
 * even if some file creation attempts on existing cgroups fail.
 */
int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
        struct cftype_set *set;
        struct cftype *cft;
        int ret;

        set = kzalloc(sizeof(*set), GFP_KERNEL);
        if (!set)
                return -ENOMEM;

        for (cft = cfts; cft->name[0] != '\0'; cft++)
                cft->ss = ss;

        cgroup_cfts_prepare();
        set->cfts = cfts;
        list_add_tail(&set->node, &ss->cftsets);
        ret = cgroup_cfts_commit(cfts, true);
        if (ret)
                cgroup_rm_cftypes(cfts);
        return ret;
}
EXPORT_SYMBOL_GPL(cgroup_add_cftypes);

/**
 * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Unregister @cfts.  Files described by @cfts are removed from all
 * existing cgroups and all future cgroups won't have them either.  This
 * function can be called anytime whether @cfts' subsys is attached or not.
 *
 * Returns 0 on successful unregistration, -ENOENT if @cfts is not
 * registered.
 */
int cgroup_rm_cftypes(struct cftype *cfts)
{
        struct cftype_set *set;

        if (!cfts || !cfts[0].ss)
                return -ENOENT;

        cgroup_cfts_prepare();

        list_for_each_entry(set, &cfts[0].ss->cftsets, node) {
                if (set->cfts == cfts) {
                        list_del(&set->node);
                        kfree(set);
                        cgroup_cfts_commit(cfts, false);
                        return 0;
                }
        }

        cgroup_cfts_commit(NULL, false);
        return -ENOENT;
}

/**
 * cgroup_task_count - count the number of tasks in a cgroup.
 * @cgrp: the cgroup in question
 *
 * Return the number of tasks in the cgroup.
 */
int cgroup_task_count(const struct cgroup *cgrp)
{
        int count = 0;
        struct cgrp_cset_link *link;

        read_lock(&css_set_lock);
        list_for_each_entry(link, &cgrp->cset_links, cset_link)
                count += atomic_read(&link->cset->refcount);
        read_unlock(&css_set_lock);
        return count;
}

/*
 * To reduce the fork() overhead for systems that are not actually using
 * their cgroups capability, we don't maintain the lists running through
 * each css_set to its tasks until we see the list actually used - in other
 * words after the first call to css_task_iter_start().
 */
static void cgroup_enable_task_cg_lists(void)
{
        struct task_struct *p, *g;
        write_lock(&css_set_lock);
        use_task_css_set_links = 1;
        /*
         * We need tasklist_lock because RCU is not safe against
         * while_each_thread(). Besides, a forking task that has passed
         * cgroup_post_fork() without seeing use_task_css_set_links = 1
         * is not guaranteed to have its child immediately visible in the
         * tasklist if we walk through it with RCU.
         */
        read_lock(&tasklist_lock);
        do_each_thread(g, p) {
                task_lock(p);
                /*
                 * We should check if the process is exiting, otherwise
                 * it will race with cgroup_exit() in that the list
                 * entry won't be deleted though the process has exited.
                 * Do it while holding siglock so that we don't end up
                 * racing against cgroup_exit().
                 */
                spin_lock_irq(&p->sighand->siglock);
                if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))
                        list_add(&p->cg_list, &task_css_set(p)->tasks);
                spin_unlock_irq(&p->sighand->siglock);

                task_unlock(p);
        } while_each_thread(g, p);
        read_unlock(&tasklist_lock);
        write_unlock(&css_set_lock);
}

/**
 * css_next_child - find the next child of a given css
 * @pos_css: the current position (%NULL to initiate traversal)
 * @parent_css: css whose children to walk
 *
 * This function returns the next child of @parent_css and should be called
 * under RCU read lock.  The only requirement is that @parent_css and
 * @pos_css are accessible.  The next sibling is guaranteed to be returned
 * regardless of their states.
 */
struct cgroup_subsys_state *
css_next_child(struct cgroup_subsys_state *pos_css,
               struct cgroup_subsys_state *parent_css)
{
        struct cgroup *pos = pos_css ? pos_css->cgroup : NULL;
        struct cgroup *cgrp = parent_css->cgroup;
        struct cgroup *next;

        WARN_ON_ONCE(!rcu_read_lock_held());

        /*
         * @pos could already have been removed.  Once a cgroup is removed,
         * its ->sibling.next is no longer updated when its next sibling
         * changes.  As CGRP_DEAD assertion is serialized and happens
         * before the cgroup is taken off the ->sibling list, if we see it
         * unasserted, it's guaranteed that the next sibling hasn't
         * finished its grace period even if it's already removed, and thus
         * safe to dereference from this RCU critical section.  If
         * ->sibling.next is inaccessible, cgroup_is_dead() is guaranteed
         * to be visible as %true here.
         *
         * If @pos is dead, its next pointer can't be dereferenced;
         * however, as each cgroup is given a monotonically increasing
         * unique serial number and always appended to the sibling list,
         * the next one can be found by walking the parent's children until
         * we see a cgroup with higher serial number than @pos's.  While
         * this path can be slower, it's taken only when either the current
         * cgroup is removed or iteration and removal race.
         */
        if (!pos) {
                next = list_entry_rcu(cgrp->children.next, struct cgroup, sibling);
        } else if (likely(!cgroup_is_dead(pos))) {
                next = list_entry_rcu(pos->sibling.next, struct cgroup, sibling);
        } else {
                list_for_each_entry_rcu(next, &cgrp->children, sibling)
                        if (next->serial_nr > pos->serial_nr)
                                break;
        }

        if (&next->sibling == &cgrp->children)
                return NULL;

        return cgroup_css(next, parent_css->ss);
}
EXPORT_SYMBOL_GPL(css_next_child);

/**
 * css_next_descendant_pre - find the next descendant for pre-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: css whose descendants to walk
 *
 * To be used by css_for_each_descendant_pre().  Find the next descendant
 * to visit for pre-order traversal of @root's descendants.  @root is
 * included in the iteration and the first node to be visited.
 *
 * While this function requires RCU read locking, it doesn't require the
 * whole traversal to be contained in a single RCU critical section.  This
 * function will return the correct next descendant as long as both @pos
 * and @root are accessible and @pos is a descendant of @root.
 */
struct cgroup_subsys_state *
css_next_descendant_pre(struct cgroup_subsys_state *pos,
                        struct cgroup_subsys_state *root)
{
        struct cgroup_subsys_state *next;

        WARN_ON_ONCE(!rcu_read_lock_held());

        /* if first iteration, visit @root */
        if (!pos)
                return root;

        /* visit the first child if exists */
        next = css_next_child(NULL, pos);
        if (next)
                return next;

        /* no child, visit my or the closest ancestor's next sibling */
        while (pos != root) {
                next = css_next_child(pos, css_parent(pos));
                if (next)
                        return next;
                pos = css_parent(pos);
        }

        return NULL;
}
EXPORT_SYMBOL_GPL(css_next_descendant_pre);

/**
 * css_rightmost_descendant - return the rightmost descendant of a css
 * @pos: css of interest
 *
 * Return the rightmost descendant of @pos.  If there's no descendant, @pos
 * is returned.  This can be used during pre-order traversal to skip
 * subtree of @pos.
 *
 * While this function requires RCU read locking, it doesn't require the
 * whole traversal to be contained in a single RCU critical section.  This
 * function will return the correct rightmost descendant as long as @pos is
 * accessible.
 */
struct cgroup_subsys_state *
css_rightmost_descendant(struct cgroup_subsys_state *pos)
{
        struct cgroup_subsys_state *last, *tmp;

        WARN_ON_ONCE(!rcu_read_lock_held());

        do {
                last = pos;
                /* ->prev isn't RCU safe, walk ->next till the end */
                pos = NULL;
                css_for_each_child(tmp, last)
                        pos = tmp;
        } while (pos);

        return last;
}
EXPORT_SYMBOL_GPL(css_rightmost_descendant);

static struct cgroup_subsys_state *
css_leftmost_descendant(struct cgroup_subsys_state *pos)
{
        struct cgroup_subsys_state *last;

        do {
                last = pos;
                pos = css_next_child(NULL, pos);
        } while (pos);

        return last;
}

/**
 * css_next_descendant_post - find the next descendant for post-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: css whose descendants to walk
 *
 * To be used by css_for_each_descendant_post().  Find the next descendant
 * to visit for post-order traversal of @root's descendants.  @root is
 * included in the iteration and the last node to be visited.
 *
 * While this function requires RCU read locking, it doesn't require the
 * whole traversal to be contained in a single RCU critical section.  This
 * function will return the correct next descendant as long as both @pos
 * and @cgroup are accessible and @pos is a descendant of @cgroup.
 */
struct cgroup_subsys_state *
css_next_descendant_post(struct cgroup_subsys_state *pos,
                         struct cgroup_subsys_state *root)
{
        struct cgroup_subsys_state *next;

        WARN_ON_ONCE(!rcu_read_lock_held());

        /* if first iteration, visit leftmost descendant which may be @root */
        if (!pos)
                return css_leftmost_descendant(root);

        /* if we visited @root, we're done */
        if (pos == root)
                return NULL;

        /* if there's an unvisited sibling, visit its leftmost descendant */
        next = css_next_child(pos, css_parent(pos));
        if (next)
                return css_leftmost_descendant(next);

        /* no sibling left, visit parent */
        return css_parent(pos);
}
EXPORT_SYMBOL_GPL(css_next_descendant_post);

/**
 * css_advance_task_iter - advance a task itererator to the next css_set
 * @it: the iterator to advance
 *
 * Advance @it to the next css_set to walk.
 */
static void css_advance_task_iter(struct css_task_iter *it)
{
        struct list_head *l = it->cset_link;
        struct cgrp_cset_link *link;
        struct css_set *cset;

        /* Advance to the next non-empty css_set */
        do {
                l = l->next;
                if (l == &it->origin_css->cgroup->cset_links) {
                        it->cset_link = NULL;
                        return;
                }
                link = list_entry(l, struct cgrp_cset_link, cset_link);
                cset = link->cset;
        } while (list_empty(&cset->tasks));
        it->cset_link = l;
        it->task = cset->tasks.next;
}

/**
 * css_task_iter_start - initiate task iteration
 * @css: the css to walk tasks of
 * @it: the task iterator to use
 *
 * Initiate iteration through the tasks of @css.  The caller can call
 * css_task_iter_next() to walk through the tasks until the function
 * returns NULL.  On completion of iteration, css_task_iter_end() must be
 * called.
 *
 * Note that this function acquires a lock which is released when the
 * iteration finishes.  The caller can't sleep while iteration is in
 * progress.
 */
void css_task_iter_start(struct cgroup_subsys_state *css,
                         struct css_task_iter *it)
        __acquires(css_set_lock)
{
        /*
         * The first time anyone tries to iterate across a css, we need to
         * enable the list linking each css_set to its tasks, and fix up
         * all existing tasks.
         */
        if (!use_task_css_set_links)
                cgroup_enable_task_cg_lists();

        read_lock(&css_set_lock);

        it->origin_css = css;
        it->cset_link = &css->cgroup->cset_links;

        css_advance_task_iter(it);
}

/**
 * css_task_iter_next - return the next task for the iterator
 * @it: the task iterator being iterated
 *
 * The "next" function for task iteration.  @it should have been
 * initialized via css_task_iter_start().  Returns NULL when the iteration
 * reaches the end.
 */
struct task_struct *css_task_iter_next(struct css_task_iter *it)
{
        struct task_struct *res;
        struct list_head *l = it->task;
        struct cgrp_cset_link *link;

        /* If the iterator cg is NULL, we have no tasks */
        if (!it->cset_link)
                return NULL;
        res = list_entry(l, struct task_struct, cg_list);
        /* Advance iterator to find next entry */
        l = l->next;
        link = list_entry(it->cset_link, struct cgrp_cset_link, cset_link);
        if (l == &link->cset->tasks) {
                /*
                 * We reached the end of this task list - move on to the
                 * next cgrp_cset_link.
                 */
                css_advance_task_iter(it);
        } else {
                it->task = l;
        }
        return res;
}

/**
 * css_task_iter_end - finish task iteration
 * @it: the task iterator to finish
 *
 * Finish task iteration started by css_task_iter_start().
 */
void css_task_iter_end(struct css_task_iter *it)
        __releases(css_set_lock)
{
        read_unlock(&css_set_lock);
}

static inline int started_after_time(struct task_struct *t1,
                                     struct timespec *time,
                                     struct task_struct *t2)
{
        int start_diff = timespec_compare(&t1->start_time, time);
        if (start_diff > 0) {
                return 1;
        } else if (start_diff < 0) {
                return 0;
        } else {
                /*
                 * Arbitrarily, if two processes started at the same
                 * time, we'll say that the lower pointer value
                 * started first. Note that t2 may have exited by now
                 * so this may not be a valid pointer any longer, but
                 * that's fine - it still serves to distinguish
                 * between two tasks started (effectively) simultaneously.
                 */
                return t1 > t2;
        }
}

/*
 * This function is a callback from heap_insert() and is used to order
 * the heap.
 * In this case we order the heap in descending task start time.
 */
static inline int started_after(void *p1, void *p2)
{
        struct task_struct *t1 = p1;
        struct task_struct *t2 = p2;
        return started_after_time(t1, &t2->start_time, t2);
}

/**
 * css_scan_tasks - iterate though all the tasks in a css
 * @css: the css to iterate tasks of
 * @test: optional test callback
 * @process: process callback
 * @data: data passed to @test and @process
 * @heap: optional pre-allocated heap used for task iteration
 *
 * Iterate through all the tasks in @css, calling @test for each, and if it
 * returns %true, call @process for it also.
 *
 * @test may be NULL, meaning always true (select all tasks), which
 * effectively duplicates css_task_iter_{start,next,end}() but does not
 * lock css_set_lock for the call to @process.
 *
 * It is guaranteed that @process will act on every task that is a member
 * of @css for the duration of this call.  This function may or may not
 * call @process for tasks that exit or move to a different css during the
 * call, or are forked or move into the css during the call.
 *
 * Note that @test may be called with locks held, and may in some
 * situations be called multiple times for the same task, so it should be
 * cheap.
 *
 * If @heap is non-NULL, a heap has been pre-allocated and will be used for
 * heap operations (and its "gt" member will be overwritten), else a
 * temporary heap will be used (allocation of which may cause this function
 * to fail).
 */
int css_scan_tasks(struct cgroup_subsys_state *css,
                   bool (*test)(struct task_struct *, void *),
                   void (*process)(struct task_struct *, void *),
                   void *data, struct ptr_heap *heap)
{
        int retval, i;
        struct css_task_iter it;
        struct task_struct *p, *dropped;
        /* Never dereference latest_task, since it's not refcounted */
        struct task_struct *latest_task = NULL;
        struct ptr_heap tmp_heap;
        struct timespec latest_time = { 0, 0 };

        if (heap) {
                /* The caller supplied our heap and pre-allocated its memory */
                heap->gt = &started_after;
        } else {
                /* We need to allocate our own heap memory */
                heap = &tmp_heap;
                retval = heap_init(heap, PAGE_SIZE, GFP_KERNEL, &started_after);
                if (retval)
                        /* cannot allocate the heap */
                        return retval;
        }

 again:
        /*
         * Scan tasks in the css, using the @test callback to determine
         * which are of interest, and invoking @process callback on the
         * ones which need an update.  Since we don't want to hold any
         * locks during the task updates, gather tasks to be processed in a
         * heap structure.  The heap is sorted by descending task start
         * time.  If the statically-sized heap fills up, we overflow tasks
         * that started later, and in future iterations only consider tasks
         * that started after the latest task in the previous pass. This
         * guarantees forward progress and that we don't miss any tasks.
         */
        heap->size = 0;
        css_task_iter_start(css, &it);
        while ((p = css_task_iter_next(&it))) {
                /*
                 * Only affect tasks that qualify per the caller's callback,
                 * if he provided one
                 */
                if (test && !test(p, data))
                        continue;
                /*
                 * Only process tasks that started after the last task
                 * we processed
                 */
                if (!started_after_time(p, &latest_time, latest_task))
                        continue;
                dropped = heap_insert(heap, p);
                if (dropped == NULL) {
                        /*
                         * The new task was inserted; the heap wasn't
                         * previously full
                         */
                        get_task_struct(p);
                } else if (dropped != p) {
                        /*
                         * The new task was inserted, and pushed out a
                         * different task
                         */
                        get_task_struct(p);
                        put_task_struct(dropped);
                }
                /*
                 * Else the new task was newer than anything already in
                 * the heap and wasn't inserted
                 */
        }
        css_task_iter_end(&it);

        if (heap->size) {
                for (i = 0; i < heap->size; i++) {
                        struct task_struct *q = heap->ptrs[i];
                        if (i == 0) {
                                latest_time = q->start_time;
                                latest_task = q;
                        }
                        /* Process the task per the caller's callback */
                        process(q, data);
                        put_task_struct(q);
                }
                /*
                 * If we had to process any tasks at all, scan again
                 * in case some of them were in the middle of forking
                 * children that didn't get processed.
                 * Not the most efficient way to do it, but it avoids
                 * having to take callback_mutex in the fork path
                 */
                goto again;
        }
        if (heap == &tmp_heap)
                heap_free(&tmp_heap);
        return 0;
}

static void cgroup_transfer_one_task(struct task_struct *task, void *data)
{
        struct cgroup *new_cgroup = data;

        mutex_lock(&cgroup_mutex);
        cgroup_attach_task(new_cgroup, task, false);
        mutex_unlock(&cgroup_mutex);
}

/**
 * cgroup_trasnsfer_tasks - move tasks from one cgroup to another
 * @to: cgroup to which the tasks will be moved
 * @from: cgroup in which the tasks currently reside
 */
int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
{
        return css_scan_tasks(&from->dummy_css, NULL, cgroup_transfer_one_task,
                              to, NULL);
}

/*
 * Stuff for reading the 'tasks'/'procs' files.
 *
 * Reading this file can return large amounts of data if a cgroup has
 * *lots* of attached tasks. So it may need several calls to read(),
 * but we cannot guarantee that the information we produce is correct
 * unless we produce it entirely atomically.
 *
 */

/* which pidlist file are we talking about? */
enum cgroup_filetype {
        CGROUP_FILE_PROCS,
        CGROUP_FILE_TASKS,
};

/*
 * A pidlist is a list of pids that virtually represents the contents of one
 * of the cgroup files ("procs" or "tasks"). We keep a list of such pidlists,
 * a pair (one each for procs, tasks) for each pid namespace that's relevant
 * to the cgroup.
 */
struct cgroup_pidlist {
        /*
         * used to find which pidlist is wanted. doesn't change as long as
         * this particular list stays in the list.
        */
        struct { enum cgroup_filetype type; struct pid_namespace *ns; } key;
        /* array of xids */
        pid_t *list;
        /* how many elements the above list has */
        int length;
        /* how many files are using the current array */
        int use_count;
        /* each of these stored in a list by its cgroup */
        struct list_head links;
        /* pointer to the cgroup we belong to, for list removal purposes */
        struct cgroup *owner;
        /* protects the other fields */
        struct rw_semaphore rwsem;
};

/*
 * The following two functions "fix" the issue where there are more pids
 * than kmalloc will give memory for; in such cases, we use vmalloc/vfree.
 * TODO: replace with a kernel-wide solution to this problem
 */
#define PIDLIST_TOO_LARGE(c) ((c) * sizeof(pid_t) > (PAGE_SIZE * 2))
static void *pidlist_allocate(int count)
{
        if (PIDLIST_TOO_LARGE(count))
                return vmalloc(count * sizeof(pid_t));
        else
                return kmalloc(count * sizeof(pid_t), GFP_KERNEL);
}
static void pidlist_free(void *p)
{
        if (is_vmalloc_addr(p))
                vfree(p);
        else
                kfree(p);
}

/*
 * pidlist_uniq - given a kmalloc()ed list, strip out all duplicate entries
 * Returns the number of unique elements.
 */
static int pidlist_uniq(pid_t *list, int length)
{
        int src, dest = 1;

        /*
         * we presume the 0th element is unique, so i starts at 1. trivial
         * edge cases first; no work needs to be done for either
         */
        if (length == 0 || length == 1)
                return length;
        /* src and dest walk down the list; dest counts unique elements */
        for (src = 1; src < length; src++) {
                /* find next unique element */
                while (list[src] == list[src-1]) {
                        src++;
                        if (src == length)
                                goto after;
                }
                /* dest always points to where the next unique element goes */
                list[dest] = list[src];
                dest++;
        }
after:
        return dest;
}

static int cmppid(const void *a, const void *b)
{
        return *(pid_t *)a - *(pid_t *)b;
}

/*
 * find the appropriate pidlist for our purpose (given procs vs tasks)
 * returns with the lock on that pidlist already held, and takes care
 * of the use count, or returns NULL with no locks held if we're out of
 * memory.
 */
static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
                                                  enum cgroup_filetype type)
{
        struct cgroup_pidlist *l;
        /* don't need task_nsproxy() if we're looking at ourself */
        struct pid_namespace *ns = task_active_pid_ns(current);

        /*
         * We can't drop the pidlist_mutex before taking the l->rwsem in case
         * the last ref-holder is trying to remove l from the list at the same
         * time. Holding the pidlist_mutex precludes somebody taking whichever
         * list we find out from under us - compare release_pid_array().
         */
        mutex_lock(&cgrp->pidlist_mutex);
        list_for_each_entry(l, &cgrp->pidlists, links) {
                if (l->key.type == type && l->key.ns == ns) {
                        /* make sure l doesn't vanish out from under us */
                        down_write(&l->rwsem);
                        mutex_unlock(&cgrp->pidlist_mutex);
                        return l;
                }
        }
        /* entry not found; create a new one */
        l = kzalloc(sizeof(struct cgroup_pidlist), GFP_KERNEL);
        if (!l) {
                mutex_unlock(&cgrp->pidlist_mutex);
                return l;
        }
        init_rwsem(&l->rwsem);
        down_write(&l->rwsem);
        l->key.type = type;
        l->key.ns = get_pid_ns(ns);
        l->owner = cgrp;
        list_add(&l->links, &cgrp->pidlists);
        mutex_unlock(&cgrp->pidlist_mutex);
        return l;
}

/*
 * Load a cgroup's pidarray with either procs' tgids or tasks' pids
 */
static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
                              struct cgroup_pidlist **lp)
{
        pid_t *array;
        int length;
        int pid, n = 0; /* used for populating the array */
        struct css_task_iter it;
        struct task_struct *tsk;
        struct cgroup_pidlist *l;

        /*
         * If cgroup gets more users after we read count, we won't have
         * enough space - tough.  This race is indistinguishable to the
         * caller from the case that the additional cgroup users didn't
         * show up until sometime later on.
         */
        length = cgroup_task_count(cgrp);
        array = pidlist_allocate(length);
        if (!array)
                return -ENOMEM;
        /* now, populate the array */
        css_task_iter_start(&cgrp->dummy_css, &it);
        while ((tsk = css_task_iter_next(&it))) {
                if (unlikely(n == length))
                        break;
                /* get tgid or pid for procs or tasks file respectively */
                if (type == CGROUP_FILE_PROCS)
                        pid = task_tgid_vnr(tsk);
                else
                        pid = task_pid_vnr(tsk);
                if (pid > 0) /* make sure to only use valid results */
                        array[n++] = pid;
        }
        css_task_iter_end(&it);
        length = n;
        /* now sort & (if procs) strip out duplicates */
        sort(array, length, sizeof(pid_t), cmppid, NULL);
        if (type == CGROUP_FILE_PROCS)
                length = pidlist_uniq(array, length);
        l = cgroup_pidlist_find(cgrp, type);
        if (!l) {
                pidlist_free(array);
                return -ENOMEM;
        }
        /* store array, freeing old if necessary - lock already held */
        pidlist_free(l->list);
        l->list = array;
        l->length = length;
        l->use_count++;
        up_write(&l->rwsem);
        *lp = l;
        return 0;
}

/**
 * cgroupstats_build - build and fill cgroupstats
 * @stats: cgroupstats to fill information into
 * @dentry: A dentry entry belonging to the cgroup for which stats have
 * been requested.
 *
 * Build and fill cgroupstats so that taskstats can export it to user
 * space.
 */
int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
{
        int ret = -EINVAL;
        struct cgroup *cgrp;
        struct css_task_iter it;
        struct task_struct *tsk;

        /*
         * Validate dentry by checking the superblock operations,
         * and make sure it's a directory.
         */
        if (dentry->d_sb->s_op != &cgroup_ops ||
            !S_ISDIR(dentry->d_inode->i_mode))
                 goto err;

        ret = 0;
        cgrp = dentry->d_fsdata;

        css_task_iter_start(&cgrp->dummy_css, &it);
        while ((tsk = css_task_iter_next(&it))) {
                switch (tsk->state) {
                case TASK_RUNNING:
                        stats->nr_running++;
                        break;
                case TASK_INTERRUPTIBLE:
                        stats->nr_sleeping++;
                        break;
                case TASK_UNINTERRUPTIBLE:
                        stats->nr_uninterruptible++;
                        break;
                case TASK_STOPPED:
                        stats->nr_stopped++;
                        break;
                default:
                        if (delayacct_is_task_waiting_on_io(tsk))
                                stats->nr_io_wait++;
                        break;
                }
        }
        css_task_iter_end(&it);

err:
        return ret;
}


/*
 * seq_file methods for the tasks/procs files. The seq_file position is the
 * next pid to display; the seq_file iterator is a pointer to the pid
 * in the cgroup->l->list array.
 */

static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
{
        /*
         * Initially we receive a position value that corresponds to
         * one more than the last pid shown (or 0 on the first call or
         * after a seek to the start). Use a binary-search to find the
         * next pid to display, if any
         */
        struct cgroup_pidlist *l = s->private;
        int index = 0, pid = *pos;
        int *iter;

        down_read(&l->rwsem);
        if (pid) {
                int end = l->length;

                while (index < end) {
                        int mid = (index + end) / 2;
                        if (l->list[mid] == pid) {
                                index = mid;
                                break;
                        } else if (l->list[mid] <= pid)
                                index = mid + 1;
                        else
                                end = mid;
                }
        }
        /* If we're off the end of the array, we're done */
        if (index >= l->length)
                return NULL;
        /* Update the abstract position to be the actual pid that we found */
        iter = l->list + index;
        *pos = *iter;
        return iter;
}

static void cgroup_pidlist_stop(struct seq_file *s, void *v)
{
        struct cgroup_pidlist *l = s->private;
        up_read(&l->rwsem);
}

static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
{
        struct cgroup_pidlist *l = s->private;
        pid_t *p = v;
        pid_t *end = l->list + l->length;
        /*
         * Advance to the next pid in the array. If this goes off the
         * end, we're done
         */
        p++;
        if (p >= end) {
                return NULL;
        } else {
                *pos = *p;
                return p;
        }
}

static int cgroup_pidlist_show(struct seq_file *s, void *v)
{
        return seq_printf(s, "%d\n", *(int *)v);
}

/*
 * seq_operations functions for iterating on pidlists through seq_file -
 * independent of whether it's tasks or procs
 */
static const struct seq_operations cgroup_pidlist_seq_operations = {
        .start = cgroup_pidlist_start,
        .stop = cgroup_pidlist_stop,
        .next = cgroup_pidlist_next,
        .show = cgroup_pidlist_show,
};

static void cgroup_release_pid_array(struct cgroup_pidlist *l)
{
        /*
         * the case where we're the last user of this particular pidlist will
         * have us remove it from the cgroup's list, which entails taking the
         * mutex. since in pidlist_find the pidlist->lock depends on cgroup->
         * pidlist_mutex, we have to take pidlist_mutex first.
         */
        mutex_lock(&l->owner->pidlist_mutex);
        down_write(&l->rwsem);
        BUG_ON(!l->use_count);
        if (!--l->use_count) {
                /* we're the last user if refcount is 0; remove and free */
                list_del(&l->links);
                mutex_unlock(&l->owner->pidlist_mutex);
                pidlist_free(l->list);
                put_pid_ns(l->key.ns);
                up_write(&l->rwsem);
                kfree(l);
                return;
        }
        mutex_unlock(&l->owner->pidlist_mutex);
        up_write(&l->rwsem);
}

static int cgroup_pidlist_release(struct inode *inode, struct file *file)
{
        struct cgroup_pidlist *l;
        if (!(file->f_mode & FMODE_READ))
                return 0;
        /*
         * the seq_file will only be initialized if the file was opened for
         * reading; hence we check if it's not null only in that case.
         */
        l = ((struct seq_file *)file->private_data)->private;
        cgroup_release_pid_array(l);
        return seq_release(inode, file);
}

static const struct file_operations cgroup_pidlist_operations = {
        .read = seq_read,
        .llseek = seq_lseek,
        .write = cgroup_file_write,
        .release = cgroup_pidlist_release,
};

/*
 * The following functions handle opens on a file that displays a pidlist
 * (tasks or procs). Prepare an array of the process/thread IDs of whoever's
 * in the cgroup.
 */
/* helper function for the two below it */
static int cgroup_pidlist_open(struct file *file, enum cgroup_filetype type)
{
        struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
        struct cgroup_pidlist *l;
        int retval;

        /* Nothing to do for write-only files */
        if (!(file->f_mode & FMODE_READ))
                return 0;

        /* have the array populated */
        retval = pidlist_array_load(cgrp, type, &l);
        if (retval)
                return retval;
        /* configure file information */
        file->f_op = &cgroup_pidlist_operations;

        retval = seq_open(file, &cgroup_pidlist_seq_operations);
        if (retval) {
                cgroup_release_pid_array(l);
                return retval;
        }
        ((struct seq_file *)file->private_data)->private = l;
        return 0;
}
static int cgroup_tasks_open(struct inode *unused, struct file *file)
{
        return cgroup_pidlist_open(file, CGROUP_FILE_TASKS);
}
static int cgroup_procs_open(struct inode *unused, struct file *file)
{
        return cgroup_pidlist_open(file, CGROUP_FILE_PROCS);
}

static u64 cgroup_read_notify_on_release(struct cgroup_subsys_state *css,
                                         struct cftype *cft)
{
        return notify_on_release(css->cgroup);
}

static int cgroup_write_notify_on_release(struct cgroup_subsys_state *css,
                                          struct cftype *cft, u64 val)
{
        clear_bit(CGRP_RELEASABLE, &css->cgroup->flags);
        if (val)
                set_bit(CGRP_NOTIFY_ON_RELEASE, &css->cgroup->flags);
        else
                clear_bit(CGRP_NOTIFY_ON_RELEASE, &css->cgroup->flags);
        return 0;
}

/*
 * When dput() is called asynchronously, if umount has been done and
 * then deactivate_super() in cgroup_free_fn() kills the superblock,
 * there's a small window that vfs will see the root dentry with non-zero
 * refcnt and trigger BUG().
 *
 * That's why we hold a reference before dput() and drop it right after.
 */
static void cgroup_dput(struct cgroup *cgrp)
{
        struct super_block *sb = cgrp->root->sb;

        atomic_inc(&sb->s_active);
        dput(cgrp->dentry);
        deactivate_super(sb);
}

/*
 * Unregister event and free resources.
 *
 * Gets called from workqueue.
 */
static void cgroup_event_remove(struct work_struct *work)
{
        struct cgroup_event *event = container_of(work, struct cgroup_event,
                        remove);
        struct cgroup_subsys_state *css = event->css;

        remove_wait_queue(event->wqh, &event->wait);

        event->cft->unregister_event(css, event->cft, event->eventfd);

        /* Notify userspace the event is going away. */
        eventfd_signal(event->eventfd, 1);

        eventfd_ctx_put(event->eventfd);
        kfree(event);
        css_put(css);
}

/*
 * Gets called on POLLHUP on eventfd when user closes it.
 *
 * Called with wqh->lock held and interrupts disabled.
 */
static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
                int sync, void *key)
{
        struct cgroup_event *event = container_of(wait,
                        struct cgroup_event, wait);
        struct cgroup *cgrp = event->css->cgroup;
        unsigned long flags = (unsigned long)key;

        if (flags & POLLHUP) {
                /*
                 * If the event has been detached at cgroup removal, we
                 * can simply return knowing the other side will cleanup
                 * for us.
                 *
                 * We can't race against event freeing since the other
                 * side will require wqh->lock via remove_wait_queue(),
                 * which we hold.
                 */
                spin_lock(&cgrp->event_list_lock);
                if (!list_empty(&event->list)) {
                        list_del_init(&event->list);
                        /*
                         * We are in atomic context, but cgroup_event_remove()
                         * may sleep, so we have to call it in workqueue.
                         */
                        schedule_work(&event->remove);
                }
                spin_unlock(&cgrp->event_list_lock);
        }

        return 0;
}

static void cgroup_event_ptable_queue_proc(struct file *file,
                wait_queue_head_t *wqh, poll_table *pt)
{
        struct cgroup_event *event = container_of(pt,
                        struct cgroup_event, pt);

        event->wqh = wqh;
        add_wait_queue(wqh, &event->wait);
}

/*
 * Parse input and register new cgroup event handler.
 *
 * Input must be in format '<event_fd> <control_fd> <args>'.
 * Interpretation of args is defined by control file implementation.
 */
static int cgroup_write_event_control(struct cgroup_subsys_state *dummy_css,
                                      struct cftype *cft, const char *buffer)
{
        struct cgroup *cgrp = dummy_css->cgroup;
        struct cgroup_event *event;
        struct cgroup_subsys_state *cfile_css;
        unsigned int efd, cfd;
        struct fd efile;
        struct fd cfile;
        char *endp;
        int ret;

        efd = simple_strtoul(buffer, &endp, 10);
        if (*endp != ' ')
                return -EINVAL;
        buffer = endp + 1;

        cfd = simple_strtoul(buffer, &endp, 10);
        if ((*endp != ' ') && (*endp != '\0'))
                return -EINVAL;
        buffer = endp + 1;

        event = kzalloc(sizeof(*event), GFP_KERNEL);
        if (!event)
                return -ENOMEM;

        INIT_LIST_HEAD(&event->list);
        init_poll_funcptr(&event->pt, cgroup_event_ptable_queue_proc);
        init_waitqueue_func_entry(&event->wait, cgroup_event_wake);
        INIT_WORK(&event->remove, cgroup_event_remove);

        efile = fdget(efd);
        if (!efile.file) {
                ret = -EBADF;
                goto out_kfree;
        }

        event->eventfd = eventfd_ctx_fileget(efile.file);
        if (IS_ERR(event->eventfd)) {
                ret = PTR_ERR(event->eventfd);
                goto out_put_efile;
        }

        cfile = fdget(cfd);
        if (!cfile.file) {
                ret = -EBADF;
                goto out_put_eventfd;
        }

        /* the process need read permission on control file */
        /* AV: shouldn't we check that it's been opened for read instead? */
        ret = inode_permission(file_inode(cfile.file), MAY_READ);
        if (ret < 0)
                goto out_put_cfile;

        event->cft = __file_cft(cfile.file);
        if (IS_ERR(event->cft)) {
                ret = PTR_ERR(event->cft);
                goto out_put_cfile;
        }

        if (!event->cft->ss) {
                ret = -EBADF;
                goto out_put_cfile;
        }

        /*
         * Determine the css of @cfile, verify it belongs to the same
         * cgroup as cgroup.event_control, and associate @event with it.
         * Remaining events are automatically removed on cgroup destruction
         * but the removal is asynchronous, so take an extra ref.
         */
        rcu_read_lock();

        ret = -EINVAL;
        event->css = cgroup_css(cgrp, event->cft->ss);
        cfile_css = css_from_dir(cfile.file->f_dentry->d_parent, event->cft->ss);
        if (event->css && event->css == cfile_css && css_tryget(event->css))
                ret = 0;

        rcu_read_unlock();
        if (ret)
                goto out_put_cfile;

        if (!event->cft->register_event || !event->cft->unregister_event) {
                ret = -EINVAL;
                goto out_put_css;
        }

        ret = event->cft->register_event(event->css, event->cft,
                        event->eventfd, buffer);
        if (ret)
                goto out_put_css;

        efile.file->f_op->poll(efile.file, &event->pt);

        spin_lock(&cgrp->event_list_lock);
        list_add(&event->list, &cgrp->event_list);
        spin_unlock(&cgrp->event_list_lock);

        fdput(cfile);
        fdput(efile);

        return 0;

out_put_css:
        css_put(event->css);
out_put_cfile:
        fdput(cfile);
out_put_eventfd:
        eventfd_ctx_put(event->eventfd);
out_put_efile:
        fdput(efile);
out_kfree:
        kfree(event);

        return ret;
}

static u64 cgroup_clone_children_read(struct cgroup_subsys_state *css,
                                      struct cftype *cft)
{
        return test_bit(CGRP_CPUSET_CLONE_CHILDREN, &css->cgroup->flags);
}

static int cgroup_clone_children_write(struct cgroup_subsys_state *css,
                                       struct cftype *cft, u64 val)
{
        if (val)
                set_bit(CGRP_CPUSET_CLONE_CHILDREN, &css->cgroup->flags);
        else
                clear_bit(CGRP_CPUSET_CLONE_CHILDREN, &css->cgroup->flags);
        return 0;
}

static struct cftype cgroup_base_files[] = {
        {
                .name = "cgroup.procs",
                .open = cgroup_procs_open,
                .write_u64 = cgroup_procs_write,
                .release = cgroup_pidlist_release,
                .mode = S_IRUGO | S_IWUSR,
        },
        {
                .name = "cgroup.event_control",
                .write_string = cgroup_write_event_control,
                .mode = S_IWUGO,
        },
        {
                .name = "cgroup.clone_children",
                .flags = CFTYPE_INSANE,
                .read_u64 = cgroup_clone_children_read,
                .write_u64 = cgroup_clone_children_write,
        },
        {
                .name = "cgroup.sane_behavior",
                .flags = CFTYPE_ONLY_ON_ROOT,
                .read_seq_string = cgroup_sane_behavior_show,
        },

        /*
         * Historical crazy stuff.  These don't have "cgroup."  prefix and
         * don't exist if sane_behavior.  If you're depending on these, be
         * prepared to be burned.
         */
        {
                .name = "tasks",
                .flags = CFTYPE_INSANE,         /* use "procs" instead */
                .open = cgroup_tasks_open,
                .write_u64 = cgroup_tasks_write,
                .release = cgroup_pidlist_release,
                .mode = S_IRUGO | S_IWUSR,
        },
        {
                .name = "notify_on_release",
                .flags = CFTYPE_INSANE,
                .read_u64 = cgroup_read_notify_on_release,
                .write_u64 = cgroup_write_notify_on_release,
        },
        {
                .name = "release_agent",
                .flags = CFTYPE_INSANE | CFTYPE_ONLY_ON_ROOT,
                .read_seq_string = cgroup_release_agent_show,
                .write_string = cgroup_release_agent_write,
                .max_write_len = PATH_MAX,
        },
        { }     /* terminate */
};

/**
 * cgroup_populate_dir - create subsys files in a cgroup directory
 * @cgrp: target cgroup
 * @subsys_mask: mask of the subsystem ids whose files should be added
 *
 * On failure, no file is added.
 */
static int cgroup_populate_dir(struct cgroup *cgrp, unsigned long subsys_mask)
{
        struct cgroup_subsys *ss;
        int i, ret = 0;

        /* process cftsets of each subsystem */
        for_each_subsys(ss, i) {
                struct cftype_set *set;

                if (!test_bit(i, &subsys_mask))
                        continue;

                list_for_each_entry(set, &ss->cftsets, node) {
                        ret = cgroup_addrm_files(cgrp, set->cfts, true);
                        if (ret < 0)
                                goto err;
                }
        }

        /* This cgroup is ready now */
        for_each_root_subsys(cgrp->root, ss) {
                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
                struct css_id *id = rcu_dereference_protected(css->id, true);

                /*
                 * Update id->css pointer and make this css visible from
                 * CSS ID functions. This pointer will be dereferened
                 * from RCU-read-side without locks.
                 */
                if (id)
                        rcu_assign_pointer(id->css, css);
        }

        return 0;
err:
        cgroup_clear_dir(cgrp, subsys_mask);
        return ret;
}

/*
 * css destruction is four-stage process.
 *
 * 1. Destruction starts.  Killing of the percpu_ref is initiated.
 *    Implemented in kill_css().
 *
 * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
 *    and thus css_tryget() is guaranteed to fail, the css can be offlined
 *    by invoking offline_css().  After offlining, the base ref is put.
 *    Implemented in css_killed_work_fn().
 *
 * 3. When the percpu_ref reaches zero, the only possible remaining
 *    accessors are inside RCU read sections.  css_release() schedules the
 *    RCU callback.
 *
 * 4. After the grace period, the css can be freed.  Implemented in
 *    css_free_work_fn().
 *
 * It is actually hairier because both step 2 and 4 require process context
 * and thus involve punting to css->destroy_work adding two additional
 * steps to the already complex sequence.
 */
static void css_free_work_fn(struct work_struct *work)
{
        struct cgroup_subsys_state *css =
                container_of(work, struct cgroup_subsys_state, destroy_work);
        struct cgroup *cgrp = css->cgroup;

        if (css->parent)
                css_put(css->parent);

        css->ss->css_free(css);
        cgroup_dput(cgrp);
}

static void css_free_rcu_fn(struct rcu_head *rcu_head)
{
        struct cgroup_subsys_state *css =
                container_of(rcu_head, struct cgroup_subsys_state, rcu_head);

        /*
         * css holds an extra ref to @cgrp->dentry which is put on the last
         * css_put().  dput() requires process context which we don't have.
         */
        INIT_WORK(&css->destroy_work, css_free_work_fn);
        queue_work(cgroup_destroy_wq, &css->destroy_work);
}

static void css_release(struct percpu_ref *ref)
{
        struct cgroup_subsys_state *css =
                container_of(ref, struct cgroup_subsys_state, refcnt);

        call_rcu(&css->rcu_head, css_free_rcu_fn);
}

static void init_css(struct cgroup_subsys_state *css, struct cgroup_subsys *ss,
                     struct cgroup *cgrp)
{
        css->cgroup = cgrp;
        css->ss = ss;
        css->flags = 0;
        css->id = NULL;

        if (cgrp->parent)
                css->parent = cgroup_css(cgrp->parent, ss);
        else
                css->flags |= CSS_ROOT;

        BUG_ON(cgroup_css(cgrp, ss));
}

/* invoke ->css_online() on a new CSS and mark it online if successful */
static int online_css(struct cgroup_subsys_state *css)
{
        struct cgroup_subsys *ss = css->ss;
        int ret = 0;

        lockdep_assert_held(&cgroup_mutex);

        if (ss->css_online)
                ret = ss->css_online(css);
        if (!ret) {
                css->flags |= CSS_ONLINE;
                css->cgroup->nr_css++;
                rcu_assign_pointer(css->cgroup->subsys[ss->subsys_id], css);
        }
        return ret;
}

/* if the CSS is online, invoke ->css_offline() on it and mark it offline */
static void offline_css(struct cgroup_subsys_state *css)
{
        struct cgroup_subsys *ss = css->ss;

        lockdep_assert_held(&cgroup_mutex);

        if (!(css->flags & CSS_ONLINE))
                return;

        if (ss->css_offline)
                ss->css_offline(css);

        css->flags &= ~CSS_ONLINE;
        css->cgroup->nr_css--;
        RCU_INIT_POINTER(css->cgroup->subsys[ss->subsys_id], css);
}

/*
 * cgroup_create - create a cgroup
 * @parent: cgroup that will be parent of the new cgroup
 * @dentry: dentry of the new cgroup
 * @mode: mode to set on new inode
 *
 * Must be called with the mutex on the parent inode held
 */
static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                             umode_t mode)
{
        struct cgroup_subsys_state *css_ar[CGROUP_SUBSYS_COUNT] = { };
        struct cgroup *cgrp;
        struct cgroup_name *name;
        struct cgroupfs_root *root = parent->root;
        int err;
        struct cgroup_subsys *ss;
        struct super_block *sb = root->sb;

        /* allocate the cgroup and its ID, 0 is reserved for the root */
        cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
        if (!cgrp)
                return -ENOMEM;

        name = cgroup_alloc_name(dentry);
        if (!name) {
                err = -ENOMEM;
                goto err_free_cgrp;
        }
        rcu_assign_pointer(cgrp->name, name);

        /*
         * Only live parents can have children.  Note that the liveliness
         * check isn't strictly necessary because cgroup_mkdir() and
         * cgroup_rmdir() are fully synchronized by i_mutex; however, do it
         * anyway so that locking is contained inside cgroup proper and we
         * don't get nasty surprises if we ever grow another caller.
         */
        if (!cgroup_lock_live_group(parent)) {
                err = -ENODEV;
                goto err_free_name;
        }

        /* Grab a reference on the superblock so the hierarchy doesn't
         * get deleted on unmount if there are child cgroups.  This
         * can be done outside cgroup_mutex, since the sb can't
         * disappear while someone has an open control file on the
         * fs */
        atomic_inc(&sb->s_active);

        /*
         * Temporarily set the pointer to NULL, so idr_find() won't return
         * a half-baked cgroup.
         */
        cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
        if (cgrp->id < 0) {
                err = -ENOMEM;
                goto err_unlock;
        }

        init_cgroup_housekeeping(cgrp);

        dentry->d_fsdata = cgrp;
        cgrp->dentry = dentry;

        cgrp->parent = parent;
        cgrp->dummy_css.parent = &parent->dummy_css;
        cgrp->root = parent->root;

        if (notify_on_release(parent))
                set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);

        if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
                set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);

        for_each_root_subsys(root, ss) {
                struct cgroup_subsys_state *css;

                css = ss->css_alloc(cgroup_css(parent, ss));
                if (IS_ERR(css)) {
                        err = PTR_ERR(css);
                        goto err_free_all;
                }
                css_ar[ss->subsys_id] = css;

                err = percpu_ref_init(&css->refcnt, css_release);
                if (err)
                        goto err_free_all;

                init_css(css, ss, cgrp);

                if (ss->use_id) {
                        err = alloc_css_id(css);
                        if (err)
                                goto err_free_all;
                }
        }

        /*
         * Create directory.  cgroup_create_file() returns with the new
         * directory locked on success so that it can be populated without
         * dropping cgroup_mutex.
         */
        err = cgroup_create_file(dentry, S_IFDIR | mode, sb);
        if (err < 0)
                goto err_free_all;
        lockdep_assert_held(&dentry->d_inode->i_mutex);

        cgrp->serial_nr = cgroup_serial_nr_next++;

        /* allocation complete, commit to creation */
        list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
        root->number_of_cgroups++;

        /* hold a ref to the parent's dentry */
        dget(parent->dentry);

        /* creation succeeded, notify subsystems */
        for_each_root_subsys(root, ss) {
                struct cgroup_subsys_state *css = css_ar[ss->subsys_id];

                err = online_css(css);
                if (err)
                        goto err_destroy;

                /* each css holds a ref to the cgroup's dentry and parent css */
                dget(dentry);
                css_get(css->parent);

                /* mark it consumed for error path */
                css_ar[ss->subsys_id] = NULL;

                if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
                    parent->parent) {
                        pr_warning("cgroup: %s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
                                   current->comm, current->pid, ss->name);
                        if (!strcmp(ss->name, "memory"))
                                pr_warning("cgroup: \"memory\" requires setting use_hierarchy to 1 on the root.\n");
                        ss->warned_broken_hierarchy = true;
                }
        }

        idr_replace(&root->cgroup_idr, cgrp, cgrp->id);

        err = cgroup_addrm_files(cgrp, cgroup_base_files, true);
        if (err)
                goto err_destroy;

        err = cgroup_populate_dir(cgrp, root->subsys_mask);
        if (err)
                goto err_destroy;

        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&cgrp->dentry->d_inode->i_mutex);

        return 0;

err_free_all:
        for_each_root_subsys(root, ss) {
                struct cgroup_subsys_state *css = css_ar[ss->subsys_id];

                if (css) {
                        percpu_ref_cancel_init(&css->refcnt);
                        ss->css_free(css);
                }
        }
        idr_remove(&root->cgroup_idr, cgrp->id);
err_unlock:
        mutex_unlock(&cgroup_mutex);
        /* Release the reference count that we took on the superblock */
        deactivate_super(sb);
err_free_name:
        kfree(rcu_dereference_raw(cgrp->name));
err_free_cgrp:
        kfree(cgrp);
        return err;

err_destroy:
        for_each_root_subsys(root, ss) {
                struct cgroup_subsys_state *css = css_ar[ss->subsys_id];

                if (css) {
                        percpu_ref_cancel_init(&css->refcnt);
                        ss->css_free(css);
                }
        }
        cgroup_destroy_locked(cgrp);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&dentry->d_inode->i_mutex);
        return err;
}

static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
{
        struct cgroup *c_parent = dentry->d_parent->d_fsdata;

        /* the vfs holds inode->i_mutex already */
        return cgroup_create(c_parent, dentry, mode | S_IFDIR);
}

/*
 * This is called when the refcnt of a css is confirmed to be killed.
 * css_tryget() is now guaranteed to fail.
 */
static void css_killed_work_fn(struct work_struct *work)
{
        struct cgroup_subsys_state *css =
                container_of(work, struct cgroup_subsys_state, destroy_work);
        struct cgroup *cgrp = css->cgroup;

        mutex_lock(&cgroup_mutex);

        /*
         * css_tryget() is guaranteed to fail now.  Tell subsystems to
         * initate destruction.
         */
        offline_css(css);

        /*
         * If @cgrp is marked dead, it's waiting for refs of all css's to
         * be disabled before proceeding to the second phase of cgroup
         * destruction.  If we are the last one, kick it off.
         */
        if (!cgrp->nr_css && cgroup_is_dead(cgrp))
                cgroup_destroy_css_killed(cgrp);

        mutex_unlock(&cgroup_mutex);

        /*
         * Put the css refs from kill_css().  Each css holds an extra
         * reference to the cgroup's dentry and cgroup removal proceeds
         * regardless of css refs.  On the last put of each css, whenever
         * that may be, the extra dentry ref is put so that dentry
         * destruction happens only after all css's are released.
         */
        css_put(css);
}

/* css kill confirmation processing requires process context, bounce */
static void css_killed_ref_fn(struct percpu_ref *ref)
{
        struct cgroup_subsys_state *css =
                container_of(ref, struct cgroup_subsys_state, refcnt);

        INIT_WORK(&css->destroy_work, css_killed_work_fn);
        queue_work(cgroup_destroy_wq, &css->destroy_work);
}

/**
 * kill_css - destroy a css
 * @css: css to destroy
 *
 * This function initiates destruction of @css by removing cgroup interface
 * files and putting its base reference.  ->css_offline() will be invoked
 * asynchronously once css_tryget() is guaranteed to fail and when the
 * reference count reaches zero, @css will be released.
 */
static void kill_css(struct cgroup_subsys_state *css)
{
        cgroup_clear_dir(css->cgroup, 1 << css->ss->subsys_id);

        /*
         * Killing would put the base ref, but we need to keep it alive
         * until after ->css_offline().
         */
        css_get(css);

        /*
         * cgroup core guarantees that, by the time ->css_offline() is
         * invoked, no new css reference will be given out via
         * css_tryget().  We can't simply call percpu_ref_kill() and
         * proceed to offlining css's because percpu_ref_kill() doesn't
         * guarantee that the ref is seen as killed on all CPUs on return.
         *
         * Use percpu_ref_kill_and_confirm() to get notifications as each
         * css is confirmed to be seen as killed on all CPUs.
         */
        percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
}

/**
 * cgroup_destroy_locked - the first stage of cgroup destruction
 * @cgrp: cgroup to be destroyed
 *
 * css's make use of percpu refcnts whose killing latency shouldn't be
 * exposed to userland and are RCU protected.  Also, cgroup core needs to
 * guarantee that css_tryget() won't succeed by the time ->css_offline() is
 * invoked.  To satisfy all the requirements, destruction is implemented in
 * the following two steps.
 *
 * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
 *     userland visible parts and start killing the percpu refcnts of
 *     css's.  Set up so that the next stage will be kicked off once all
 *     the percpu refcnts are confirmed to be killed.
 *
 * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
 *     rest of destruction.  Once all cgroup references are gone, the
 *     cgroup is RCU-freed.
 *
 * This function implements s1.  After this step, @cgrp is gone as far as
 * the userland is concerned and a new cgroup with the same name may be
 * created.  As cgroup doesn't care about the names internally, this
 * doesn't cause any problem.
 */
static int cgroup_destroy_locked(struct cgroup *cgrp)
        __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
{
        struct dentry *d = cgrp->dentry;
        struct cgroup_event *event, *tmp;
        struct cgroup_subsys *ss;
        struct cgroup *child;
        bool empty;

        lockdep_assert_held(&d->d_inode->i_mutex);
        lockdep_assert_held(&cgroup_mutex);

        /*
         * css_set_lock synchronizes access to ->cset_links and prevents
         * @cgrp from being removed while __put_css_set() is in progress.
         */
        read_lock(&css_set_lock);
        empty = list_empty(&cgrp->cset_links);
        read_unlock(&css_set_lock);
        if (!empty)
                return -EBUSY;

        /*
         * Make sure there's no live children.  We can't test ->children
         * emptiness as dead children linger on it while being destroyed;
         * otherwise, "rmdir parent/child parent" may fail with -EBUSY.
         */
        empty = true;
        rcu_read_lock();
        list_for_each_entry_rcu(child, &cgrp->children, sibling) {
                empty = cgroup_is_dead(child);
                if (!empty)
                        break;
        }
        rcu_read_unlock();
        if (!empty)
                return -EBUSY;

        /*
         * Initiate massacre of all css's.  cgroup_destroy_css_killed()
         * will be invoked to perform the rest of destruction once the
         * percpu refs of all css's are confirmed to be killed.
         */
        for_each_root_subsys(cgrp->root, ss) {
                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);

                if (css)
                        kill_css(css);
        }

        /*
         * Mark @cgrp dead.  This prevents further task migration and child
         * creation by disabling cgroup_lock_live_group().  Note that
         * CGRP_DEAD assertion is depended upon by css_next_child() to
         * resume iteration after dropping RCU read lock.  See
         * css_next_child() for details.
         */
        set_bit(CGRP_DEAD, &cgrp->flags);

        /* CGRP_DEAD is set, remove from ->release_list for the last time */
        raw_spin_lock(&release_list_lock);
        if (!list_empty(&cgrp->release_list))
                list_del_init(&cgrp->release_list);
        raw_spin_unlock(&release_list_lock);

        /*
         * If @cgrp has css's attached, the second stage of cgroup
         * destruction is kicked off from css_killed_work_fn() after the
         * refs of all attached css's are killed.  If @cgrp doesn't have
         * any css, we kick it off here.
         */
        if (!cgrp->nr_css)
                cgroup_destroy_css_killed(cgrp);

        /*
         * Clear the base files and remove @cgrp directory.  The removal
         * puts the base ref but we aren't quite done with @cgrp yet, so
         * hold onto it.
         */
        cgroup_addrm_files(cgrp, cgroup_base_files, false);
        dget(d);
        cgroup_d_remove_dir(d);

        /*
         * Unregister events and notify userspace.
         * Notify userspace about cgroup removing only after rmdir of cgroup
         * directory to avoid race between userspace and kernelspace.
         */
        spin_lock(&cgrp->event_list_lock);
        list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
                list_del_init(&event->list);
                schedule_work(&event->remove);
        }
        spin_unlock(&cgrp->event_list_lock);

        return 0;
};

/**
 * cgroup_destroy_css_killed - the second step of cgroup destruction
 * @work: cgroup->destroy_free_work
 *
 * This function is invoked from a work item for a cgroup which is being
 * destroyed after all css's are offlined and performs the rest of
 * destruction.  This is the second step of destruction described in the
 * comment above cgroup_destroy_locked().
 */
static void cgroup_destroy_css_killed(struct cgroup *cgrp)
{
        struct cgroup *parent = cgrp->parent;
        struct dentry *d = cgrp->dentry;

        lockdep_assert_held(&cgroup_mutex);

        /* delete this cgroup from parent->children */
        list_del_rcu(&cgrp->sibling);

        /*
         * We should remove the cgroup object from idr before its grace
         * period starts, so we won't be looking up a cgroup while the
         * cgroup is being freed.
         */
        idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
        cgrp->id = -1;

        dput(d);

        set_bit(CGRP_RELEASABLE, &parent->flags);
        check_for_release(parent);
}

static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
        int ret;

        mutex_lock(&cgroup_mutex);
        ret = cgroup_destroy_locked(dentry->d_fsdata);
        mutex_unlock(&cgroup_mutex);

        return ret;
}

static void __init_or_module cgroup_init_cftsets(struct cgroup_subsys *ss)
{
        INIT_LIST_HEAD(&ss->cftsets);

        /*
         * base_cftset is embedded in subsys itself, no need to worry about
         * deregistration.
         */
        if (ss->base_cftypes) {
                struct cftype *cft;

                for (cft = ss->base_cftypes; cft->name[0] != '\0'; cft++)
                        cft->ss = ss;

                ss->base_cftset.cfts = ss->base_cftypes;
                list_add_tail(&ss->base_cftset.node, &ss->cftsets);
        }
}

static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
{
        struct cgroup_subsys_state *css;

        printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);

        mutex_lock(&cgroup_mutex);

        /* init base cftset */
        cgroup_init_cftsets(ss);

        /* Create the top cgroup state for this subsystem */
        list_add(&ss->sibling, &cgroup_dummy_root.subsys_list);
        ss->root = &cgroup_dummy_root;
        css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss));
        /* We don't handle early failures gracefully */
        BUG_ON(IS_ERR(css));
        init_css(css, ss, cgroup_dummy_top);

        /* Update the init_css_set to contain a subsys
         * pointer to this state - since the subsystem is
         * newly registered, all tasks and hence the
         * init_css_set is in the subsystem's top cgroup. */
        init_css_set.subsys[ss->subsys_id] = css;

        need_forkexit_callback |= ss->fork || ss->exit;

        /* At system boot, before all subsystems have been
         * registered, no tasks have been forked, so we don't
         * need to invoke fork callbacks here. */
        BUG_ON(!list_empty(&init_task.tasks));

        BUG_ON(online_css(css));

        mutex_unlock(&cgroup_mutex);

        /* this function shouldn't be used with modular subsystems, since they
         * need to register a subsys_id, among other things */
        BUG_ON(ss->module);
}

/**
 * cgroup_load_subsys: load and register a modular subsystem at runtime
 * @ss: the subsystem to load
 *
 * This function should be called in a modular subsystem's initcall. If the
 * subsystem is built as a module, it will be assigned a new subsys_id and set
 * up for use. If the subsystem is built-in anyway, work is delegated to the
 * simpler cgroup_init_subsys.
 */
int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
{
        struct cgroup_subsys_state *css;
        int i, ret;
        struct hlist_node *tmp;
        struct css_set *cset;
        unsigned long key;

        /* check name and function validity */
        if (ss->name == NULL || strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN ||
            ss->css_alloc == NULL || ss->css_free == NULL)
                return -EINVAL;

        /*
         * we don't support callbacks in modular subsystems. this check is
         * before the ss->module check for consistency; a subsystem that could
         * be a module should still have no callbacks even if the user isn't
         * compiling it as one.
         */
        if (ss->fork || ss->exit)
                return -EINVAL;

        /*
         * an optionally modular subsystem is built-in: we want to do nothing,
         * since cgroup_init_subsys will have already taken care of it.
         */
        if (ss->module == NULL) {
                /* a sanity check */
                BUG_ON(cgroup_subsys[ss->subsys_id] != ss);
                return 0;
        }

        /* init base cftset */
        cgroup_init_cftsets(ss);

        mutex_lock(&cgroup_mutex);
        cgroup_subsys[ss->subsys_id] = ss;

        /*
         * no ss->css_alloc seems to need anything important in the ss
         * struct, so this can happen first (i.e. before the dummy root
         * attachment).
         */
        css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss));
        if (IS_ERR(css)) {
                /* failure case - need to deassign the cgroup_subsys[] slot. */
                cgroup_subsys[ss->subsys_id] = NULL;
                mutex_unlock(&cgroup_mutex);
                return PTR_ERR(css);
        }

        list_add(&ss->sibling, &cgroup_dummy_root.subsys_list);
        ss->root = &cgroup_dummy_root;

        /* our new subsystem will be attached to the dummy hierarchy. */
        init_css(css, ss, cgroup_dummy_top);
        /* init_idr must be after init_css() because it sets css->id. */
        if (ss->use_id) {
                ret = cgroup_init_idr(ss, css);
                if (ret)
                        goto err_unload;
        }

        /*
         * Now we need to entangle the css into the existing css_sets. unlike
         * in cgroup_init_subsys, there are now multiple css_sets, so each one
         * will need a new pointer to it; done by iterating the css_set_table.
         * furthermore, modifying the existing css_sets will corrupt the hash
         * table state, so each changed css_set will need its hash recomputed.
         * this is all done under the css_set_lock.
         */
        write_lock(&css_set_lock);
        hash_for_each_safe(css_set_table, i, tmp, cset, hlist) {
                /* skip entries that we already rehashed */
                if (cset->subsys[ss->subsys_id])
                        continue;
                /* remove existing entry */
                hash_del(&cset->hlist);
                /* set new value */
                cset->subsys[ss->subsys_id] = css;
                /* recompute hash and restore entry */
                key = css_set_hash(cset->subsys);
                hash_add(css_set_table, &cset->hlist, key);
        }
        write_unlock(&css_set_lock);

        ret = online_css(css);
        if (ret)
                goto err_unload;

        /* success! */
        mutex_unlock(&cgroup_mutex);
        return 0;

err_unload:
        mutex_unlock(&cgroup_mutex);
        /* @ss can't be mounted here as try_module_get() would fail */
        cgroup_unload_subsys(ss);
        return ret;
}
EXPORT_SYMBOL_GPL(cgroup_load_subsys);

/**
 * cgroup_unload_subsys: unload a modular subsystem
 * @ss: the subsystem to unload
 *
 * This function should be called in a modular subsystem's exitcall. When this
 * function is invoked, the refcount on the subsystem's module will be 0, so
 * the subsystem will not be attached to any hierarchy.
 */
void cgroup_unload_subsys(struct cgroup_subsys *ss)
{
        struct cgrp_cset_link *link;

        BUG_ON(ss->module == NULL);

        /*
         * we shouldn't be called if the subsystem is in use, and the use of
         * try_module_get() in rebind_subsystems() should ensure that it
         * doesn't start being used while we're killing it off.
         */
        BUG_ON(ss->root != &cgroup_dummy_root);

        mutex_lock(&cgroup_mutex);

        offline_css(cgroup_css(cgroup_dummy_top, ss));

        if (ss->use_id)
                idr_destroy(&ss->idr);

        /* deassign the subsys_id */
        cgroup_subsys[ss->subsys_id] = NULL;

        /* remove subsystem from the dummy root's list of subsystems */
        list_del_init(&ss->sibling);

        /*
         * disentangle the css from all css_sets attached to the dummy
         * top. as in loading, we need to pay our respects to the hashtable
         * gods.
         */
        write_lock(&css_set_lock);
        list_for_each_entry(link, &cgroup_dummy_top->cset_links, cset_link) {
                struct css_set *cset = link->cset;
                unsigned long key;

                hash_del(&cset->hlist);
                cset->subsys[ss->subsys_id] = NULL;
                key = css_set_hash(cset->subsys);
                hash_add(css_set_table, &cset->hlist, key);
        }
        write_unlock(&css_set_lock);

        /*
         * remove subsystem's css from the cgroup_dummy_top and free it -
         * need to free before marking as null because ss->css_free needs
         * the cgrp->subsys pointer to find their state. note that this
         * also takes care of freeing the css_id.
         */
        ss->css_free(cgroup_css(cgroup_dummy_top, ss));
        RCU_INIT_POINTER(cgroup_dummy_top->subsys[ss->subsys_id], NULL);

        mutex_unlock(&cgroup_mutex);
}
EXPORT_SYMBOL_GPL(cgroup_unload_subsys);

/**
 * cgroup_init_early - cgroup initialization at system boot
 *
 * Initialize cgroups at system boot, and initialize any
 * subsystems that request early init.
 */
int __init cgroup_init_early(void)
{
        struct cgroup_subsys *ss;
        int i;

        atomic_set(&init_css_set.refcount, 1);
        INIT_LIST_HEAD(&init_css_set.cgrp_links);
        INIT_LIST_HEAD(&init_css_set.tasks);
        INIT_HLIST_NODE(&init_css_set.hlist);
        css_set_count = 1;
        init_cgroup_root(&cgroup_dummy_root);
        cgroup_root_count = 1;
        RCU_INIT_POINTER(init_task.cgroups, &init_css_set);

        init_cgrp_cset_link.cset = &init_css_set;
        init_cgrp_cset_link.cgrp = cgroup_dummy_top;
        list_add(&init_cgrp_cset_link.cset_link, &cgroup_dummy_top->cset_links);
        list_add(&init_cgrp_cset_link.cgrp_link, &init_css_set.cgrp_links);

        /* at bootup time, we don't worry about modular subsystems */
        for_each_builtin_subsys(ss, i) {
                BUG_ON(!ss->name);
                BUG_ON(strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN);
                BUG_ON(!ss->css_alloc);
                BUG_ON(!ss->css_free);
                if (ss->subsys_id != i) {
                        printk(KERN_ERR "cgroup: Subsys %s id == %d\n",
                               ss->name, ss->subsys_id);
                        BUG();
                }

                if (ss->early_init)
                        cgroup_init_subsys(ss);
        }
        return 0;
}

/**
 * cgroup_init - cgroup initialization
 *
 * Register cgroup filesystem and /proc file, and initialize
 * any subsystems that didn't request early init.
 */
int __init cgroup_init(void)
{
        struct cgroup_subsys *ss;
        unsigned long key;
        int i, err;

        err = bdi_init(&cgroup_backing_dev_info);
        if (err)
                return err;

        for_each_builtin_subsys(ss, i) {
                if (!ss->early_init)
                        cgroup_init_subsys(ss);
                if (ss->use_id)
                        cgroup_init_idr(ss, init_css_set.subsys[ss->subsys_id]);
        }

        /* allocate id for the dummy hierarchy */
        mutex_lock(&cgroup_mutex);
        mutex_lock(&cgroup_root_mutex);

        /* Add init_css_set to the hash table */
        key = css_set_hash(init_css_set.subsys);
        hash_add(css_set_table, &init_css_set.hlist, key);

        BUG_ON(cgroup_init_root_id(&cgroup_dummy_root, 0, 1));

        err = idr_alloc(&cgroup_dummy_root.cgroup_idr, cgroup_dummy_top,
                        0, 1, GFP_KERNEL);
        BUG_ON(err < 0);

        mutex_unlock(&cgroup_root_mutex);
        mutex_unlock(&cgroup_mutex);

        cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);
        if (!cgroup_kobj) {
                err = -ENOMEM;
                goto out;
        }

        err = register_filesystem(&cgroup_fs_type);
        if (err < 0) {
                kobject_put(cgroup_kobj);
                goto out;
        }

        proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations);

out:
        if (err)
                bdi_destroy(&cgroup_backing_dev_info);

        return err;
}

static int __init cgroup_wq_init(void)
{
        /*
         * There isn't much point in executing destruction path in
         * parallel.  Good chunk is serialized with cgroup_mutex anyway.
         * Use 1 for @max_active.
         *
         * We would prefer to do this in cgroup_init() above, but that
         * is called before init_workqueues(): so leave this until after.
         */
        cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
        BUG_ON(!cgroup_destroy_wq);
        return 0;
}
core_initcall(cgroup_wq_init);

/*
 * proc_cgroup_show()
 *  - Print task's cgroup paths into seq_file, one line for each hierarchy
 *  - Used for /proc/<pid>/cgroup.
 *  - No need to task_lock(tsk) on this tsk->cgroup reference, as it
 *    doesn't really matter if tsk->cgroup changes after we read it,
 *    and we take cgroup_mutex, keeping cgroup_attach_task() from changing it
 *    anyway.  No need to check that tsk->cgroup != NULL, thanks to
 *    the_top_cgroup_hack in cgroup_exit(), which sets an exiting tasks
 *    cgroup to top_cgroup.
 */

/* TODO: Use a proper seq_file iterator */
int proc_cgroup_show(struct seq_file *m, void *v)
{
        struct pid *pid;
        struct task_struct *tsk;
        char *buf;
        int retval;
        struct cgroupfs_root *root;

        retval = -ENOMEM;
        buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
        if (!buf)
                goto out;

        retval = -ESRCH;
        pid = m->private;
        tsk = get_pid_task(pid, PIDTYPE_PID);
        if (!tsk)
                goto out_free;

        retval = 0;

        mutex_lock(&cgroup_mutex);

        for_each_active_root(root) {
                struct cgroup_subsys *ss;
                struct cgroup *cgrp;
                int count = 0;

                seq_printf(m, "%d:", root->hierarchy_id);
                for_each_root_subsys(root, ss)
                        seq_printf(m, "%s%s", count++ ? "," : "", ss->name);
                if (strlen(root->name))
                        seq_printf(m, "%sname=%s", count ? "," : "",
                                   root->name);
                seq_putc(m, ':');
                cgrp = task_cgroup_from_root(tsk, root);
                retval = cgroup_path(cgrp, buf, PAGE_SIZE);
                if (retval < 0)
                        goto out_unlock;
                seq_puts(m, buf);
                seq_putc(m, '\n');
        }

out_unlock:
        mutex_unlock(&cgroup_mutex);
        put_task_struct(tsk);
out_free:
        kfree(buf);
out:
        return retval;
}

/* Display information about each subsystem and each hierarchy */
static int proc_cgroupstats_show(struct seq_file *m, void *v)
{
        struct cgroup_subsys *ss;
        int i;

        seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
        /*
         * ideally we don't want subsystems moving around while we do this.
         * cgroup_mutex is also necessary to guarantee an atomic snapshot of
         * subsys/hierarchy state.
         */
        mutex_lock(&cgroup_mutex);

        for_each_subsys(ss, i)
                seq_printf(m, "%s\t%d\t%d\t%d\n",
                           ss->name, ss->root->hierarchy_id,
                           ss->root->number_of_cgroups, !ss->disabled);

        mutex_unlock(&cgroup_mutex);
        return 0;
}

static int cgroupstats_open(struct inode *inode, struct file *file)
{
        return single_open(file, proc_cgroupstats_show, NULL);
}

static const struct file_operations proc_cgroupstats_operations = {
        .open = cgroupstats_open,
        .read = seq_read,
        .llseek = seq_lseek,
        .release = single_release,
};

/**
 * cgroup_fork - attach newly forked task to its parents cgroup.
 * @child: pointer to task_struct of forking parent process.
 *
 * Description: A task inherits its parent's cgroup at fork().
 *
 * A pointer to the shared css_set was automatically copied in
 * fork.c by dup_task_struct().  However, we ignore that copy, since
 * it was not made under the protection of RCU or cgroup_mutex, so
 * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
 * have already changed current->cgroups, allowing the previously
 * referenced cgroup group to be removed and freed.
 *
 * At the point that cgroup_fork() is called, 'current' is the parent
 * task, and the passed argument 'child' points to the child task.
 */
void cgroup_fork(struct task_struct *child)
{
        task_lock(current);
        get_css_set(task_css_set(current));
        child->cgroups = current->cgroups;
        task_unlock(current);
        INIT_LIST_HEAD(&child->cg_list);
}

/**
 * cgroup_post_fork - called on a new task after adding it to the task list
 * @child: the task in question
 *
 * Adds the task to the list running through its css_set if necessary and
 * call the subsystem fork() callbacks.  Has to be after the task is
 * visible on the task list in case we race with the first call to
 * cgroup_task_iter_start() - to guarantee that the new task ends up on its
 * list.
 */
void cgroup_post_fork(struct task_struct *child)
{
        struct cgroup_subsys *ss;
        int i;

        /*
         * use_task_css_set_links is set to 1 before we walk the tasklist
         * under the tasklist_lock and we read it here after we added the child
         * to the tasklist under the tasklist_lock as well. If the child wasn't
         * yet in the tasklist when we walked through it from
         * cgroup_enable_task_cg_lists(), then use_task_css_set_links value
         * should be visible now due to the paired locking and barriers implied
         * by LOCK/UNLOCK: it is written before the tasklist_lock unlock
         * in cgroup_enable_task_cg_lists() and read here after the tasklist_lock
         * lock on fork.
         */
        if (use_task_css_set_links) {
                write_lock(&css_set_lock);
                task_lock(child);
                if (list_empty(&child->cg_list))
                        list_add(&child->cg_list, &task_css_set(child)->tasks);
                task_unlock(child);
                write_unlock(&css_set_lock);
        }

        /*
         * Call ss->fork().  This must happen after @child is linked on
         * css_set; otherwise, @child might change state between ->fork()
         * and addition to css_set.
         */
        if (need_forkexit_callback) {
                /*
                 * fork/exit callbacks are supported only for builtin
                 * subsystems, and the builtin section of the subsys
                 * array is immutable, so we don't need to lock the
                 * subsys array here. On the other hand, modular section
                 * of the array can be freed at module unload, so we
                 * can't touch that.
                 */
                for_each_builtin_subsys(ss, i)
                        if (ss->fork)
                                ss->fork(child);
        }
}

/**
 * cgroup_exit - detach cgroup from exiting task
 * @tsk: pointer to task_struct of exiting process
 * @run_callback: run exit callbacks?
 *
 * Description: Detach cgroup from @tsk and release it.
 *
 * Note that cgroups marked notify_on_release force every task in
 * them to take the global cgroup_mutex mutex when exiting.
 * This could impact scaling on very large systems.  Be reluctant to
 * use notify_on_release cgroups where very high task exit scaling
 * is required on large systems.
 *
 * the_top_cgroup_hack:
 *
 *    Set the exiting tasks cgroup to the root cgroup (top_cgroup).
 *
 *    We call cgroup_exit() while the task is still competent to
 *    handle notify_on_release(), then leave the task attached to the
 *    root cgroup in each hierarchy for the remainder of its exit.
 *
 *    To do this properly, we would increment the reference count on
 *    top_cgroup, and near the very end of the kernel/exit.c do_exit()
 *    code we would add a second cgroup function call, to drop that
 *    reference.  This would just create an unnecessary hot spot on
 *    the top_cgroup reference count, to no avail.
 *
 *    Normally, holding a reference to a cgroup without bumping its
 *    count is unsafe.   The cgroup could go away, or someone could
 *    attach us to a different cgroup, decrementing the count on
 *    the first cgroup that we never incremented.  But in this case,
 *    top_cgroup isn't going away, and either task has PF_EXITING set,
 *    which wards off any cgroup_attach_task() attempts, or task is a failed
 *    fork, never visible to cgroup_attach_task.
 */
void cgroup_exit(struct task_struct *tsk, int run_callbacks)
{
        struct cgroup_subsys *ss;
        struct css_set *cset;
        int i;

        /*
         * Unlink from the css_set task list if necessary.
         * Optimistically check cg_list before taking
         * css_set_lock
         */
        if (!list_empty(&tsk->cg_list)) {
                write_lock(&css_set_lock);
                if (!list_empty(&tsk->cg_list))
                        list_del_init(&tsk->cg_list);
                write_unlock(&css_set_lock);
        }

        /* Reassign the task to the init_css_set. */
        task_lock(tsk);
        cset = task_css_set(tsk);
        RCU_INIT_POINTER(tsk->cgroups, &init_css_set);

        if (run_callbacks && need_forkexit_callback) {
                /*
                 * fork/exit callbacks are supported only for builtin
                 * subsystems, see cgroup_post_fork() for details.
                 */
                for_each_builtin_subsys(ss, i) {
                        if (ss->exit) {
                                struct cgroup_subsys_state *old_css = cset->subsys[i];
                                struct cgroup_subsys_state *css = task_css(tsk, i);

                                ss->exit(css, old_css, tsk);
                        }
                }
        }
        task_unlock(tsk);

        put_css_set_taskexit(cset);
}

static void check_for_release(struct cgroup *cgrp)
{
        if (cgroup_is_releasable(cgrp) &&
            list_empty(&cgrp->cset_links) && list_empty(&cgrp->children)) {
                /*
                 * Control Group is currently removeable. If it's not
                 * already queued for a userspace notification, queue
                 * it now
                 */
                int need_schedule_work = 0;

                raw_spin_lock(&release_list_lock);
                if (!cgroup_is_dead(cgrp) &&
                    list_empty(&cgrp->release_list)) {
                        list_add(&cgrp->release_list, &release_list);
                        need_schedule_work = 1;
                }
                raw_spin_unlock(&release_list_lock);
                if (need_schedule_work)
                        schedule_work(&release_agent_work);
        }
}

/*
 * Notify userspace when a cgroup is released, by running the
 * configured release agent with the name of the cgroup (path
 * relative to the root of cgroup file system) as the argument.
 *
 * Most likely, this user command will try to rmdir this cgroup.
 *
 * This races with the possibility that some other task will be
 * attached to this cgroup before it is removed, or that some other
 * user task will 'mkdir' a child cgroup of this cgroup.  That's ok.
 * The presumed 'rmdir' will fail quietly if this cgroup is no longer
 * unused, and this cgroup will be reprieved from its death sentence,
 * to continue to serve a useful existence.  Next time it's released,
 * we will get notified again, if it still has 'notify_on_release' set.
 *
 * The final arg to call_usermodehelper() is UMH_WAIT_EXEC, which
 * means only wait until the task is successfully execve()'d.  The
 * separate release agent task is forked by call_usermodehelper(),
 * then control in this thread returns here, without waiting for the
 * release agent task.  We don't bother to wait because the caller of
 * this routine has no use for the exit status of the release agent
 * task, so no sense holding our caller up for that.
 */
static void cgroup_release_agent(struct work_struct *work)
{
        BUG_ON(work != &release_agent_work);
        mutex_lock(&cgroup_mutex);
        raw_spin_lock(&release_list_lock);
        while (!list_empty(&release_list)) {
                char *argv[3], *envp[3];
                int i;
                char *pathbuf = NULL, *agentbuf = NULL;
                struct cgroup *cgrp = list_entry(release_list.next,
                                                    struct cgroup,
                                                    release_list);
                list_del_init(&cgrp->release_list);
                raw_spin_unlock(&release_list_lock);
                pathbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
                if (!pathbuf)
                        goto continue_free;
                if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0)
                        goto continue_free;
                agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
                if (!agentbuf)
                        goto continue_free;

                i = 0;
                argv[i++] = agentbuf;
                argv[i++] = pathbuf;
                argv[i] = NULL;

                i = 0;
                /* minimal command environment */
                envp[i++] = "HOME=/";
                envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
                envp[i] = NULL;

                /* Drop the lock while we invoke the usermode helper,
                 * since the exec could involve hitting disk and hence
                 * be a slow process */
                mutex_unlock(&cgroup_mutex);
                call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
                mutex_lock(&cgroup_mutex);
 continue_free:
                kfree(pathbuf);
                kfree(agentbuf);
                raw_spin_lock(&release_list_lock);
        }
        raw_spin_unlock(&release_list_lock);
        mutex_unlock(&cgroup_mutex);
}

static int __init cgroup_disable(char *str)
{
        struct cgroup_subsys *ss;
        char *token;
        int i;

        while ((token = strsep(&str, ",")) != NULL) {
                if (!*token)
                        continue;

                /*
                 * cgroup_disable, being at boot time, can't know about
                 * module subsystems, so we don't worry about them.
                 */
                for_each_builtin_subsys(ss, i) {
                        if (!strcmp(token, ss->name)) {
                                ss->disabled = 1;
                                printk(KERN_INFO "Disabling %s control group"
                                        " subsystem\n", ss->name);
                                break;
                        }
                }
        }
        return 1;
}
__setup("cgroup_disable=", cgroup_disable);

/*
 * Functons for CSS ID.
 */

/* to get ID other than 0, this should be called when !cgroup_is_dead() */
unsigned short css_id(struct cgroup_subsys_state *css)
{
        struct css_id *cssid;

        /*
         * This css_id() can return correct value when somone has refcnt
         * on this or this is under rcu_read_lock(). Once css->id is allocated,
         * it's unchanged until freed.
         */
        cssid = rcu_dereference_raw(css->id);

        if (cssid)
                return cssid->id;
        return 0;
}
EXPORT_SYMBOL_GPL(css_id);

/**
 *  css_is_ancestor - test "root" css is an ancestor of "child"
 * @child: the css to be tested.
 * @root: the css supporsed to be an ancestor of the child.
 *
 * Returns true if "root" is an ancestor of "child" in its hierarchy. Because
 * this function reads css->id, the caller must hold rcu_read_lock().
 * But, considering usual usage, the csses should be valid objects after test.
 * Assuming that the caller will do some action to the child if this returns
 * returns true, the caller must take "child";s reference count.
 * If "child" is valid object and this returns true, "root" is valid, too.
 */

bool css_is_ancestor(struct cgroup_subsys_state *child,
                    const struct cgroup_subsys_state *root)
{
        struct css_id *child_id;
        struct css_id *root_id;

        child_id  = rcu_dereference(child->id);
        if (!child_id)
                return false;
        root_id = rcu_dereference(root->id);
        if (!root_id)
                return false;
        if (child_id->depth < root_id->depth)
                return false;
        if (child_id->stack[root_id->depth] != root_id->id)
                return false;
        return true;
}

void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
{
        struct css_id *id = rcu_dereference_protected(css->id, true);

        /* When this is called before css_id initialization, id can be NULL */
        if (!id)
                return;

        BUG_ON(!ss->use_id);

        rcu_assign_pointer(id->css, NULL);
        rcu_assign_pointer(css->id, NULL);
        spin_lock(&ss->id_lock);
        idr_remove(&ss->idr, id->id);
        spin_unlock(&ss->id_lock);
        kfree_rcu(id, rcu_head);
}
EXPORT_SYMBOL_GPL(free_css_id);

/*
 * This is called by init or create(). Then, calls to this function are
 * always serialized (By cgroup_mutex() at create()).
 */

static struct css_id *get_new_cssid(struct cgroup_subsys *ss, int depth)
{
        struct css_id *newid;
        int ret, size;

        BUG_ON(!ss->use_id);

        size = sizeof(*newid) + sizeof(unsigned short) * (depth + 1);
        newid = kzalloc(size, GFP_KERNEL);
        if (!newid)
                return ERR_PTR(-ENOMEM);

        idr_preload(GFP_KERNEL);
        spin_lock(&ss->id_lock);
        /* Don't use 0. allocates an ID of 1-65535 */
        ret = idr_alloc(&ss->idr, newid, 1, CSS_ID_MAX + 1, GFP_NOWAIT);
        spin_unlock(&ss->id_lock);
        idr_preload_end();

        /* Returns error when there are no free spaces for new ID.*/
        if (ret < 0)
                goto err_out;

        newid->id = ret;
        newid->depth = depth;
        return newid;
err_out:
        kfree(newid);
        return ERR_PTR(ret);

}

static int __init_or_module cgroup_init_idr(struct cgroup_subsys *ss,
                                            struct cgroup_subsys_state *rootcss)
{
        struct css_id *newid;

        spin_lock_init(&ss->id_lock);
        idr_init(&ss->idr);

        newid = get_new_cssid(ss, 0);
        if (IS_ERR(newid))
                return PTR_ERR(newid);

        newid->stack[0] = newid->id;
        RCU_INIT_POINTER(newid->css, rootcss);
        RCU_INIT_POINTER(rootcss->id, newid);
        return 0;
}

static int alloc_css_id(struct cgroup_subsys_state *child_css)
{
        struct cgroup_subsys_state *parent_css = css_parent(child_css);
        struct css_id *child_id, *parent_id;
        int i, depth;

        parent_id = rcu_dereference_protected(parent_css->id, true);
        depth = parent_id->depth + 1;

        child_id = get_new_cssid(child_css->ss, depth);
        if (IS_ERR(child_id))
                return PTR_ERR(child_id);

        for (i = 0; i < depth; i++)
                child_id->stack[i] = parent_id->stack[i];
        child_id->stack[depth] = child_id->id;
        /*
         * child_id->css pointer will be set after this cgroup is available
         * see cgroup_populate_dir()
         */
        rcu_assign_pointer(child_css->id, child_id);

        return 0;
}

/**
 * css_lookup - lookup css by id
 * @ss: cgroup subsys to be looked into.
 * @id: the id
 *
 * Returns pointer to cgroup_subsys_state if there is valid one with id.
 * NULL if not. Should be called under rcu_read_lock()
 */
struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
{
        struct css_id *cssid = NULL;

        BUG_ON(!ss->use_id);
        cssid = idr_find(&ss->idr, id);

        if (unlikely(!cssid))
                return NULL;

        return rcu_dereference(cssid->css);
}
EXPORT_SYMBOL_GPL(css_lookup);

/**
 * css_from_dir - get corresponding css from the dentry of a cgroup dir
 * @dentry: directory dentry of interest
 * @ss: subsystem of interest
 *
 * Must be called under RCU read lock.  The caller is responsible for
 * pinning the returned css if it needs to be accessed outside the RCU
 * critical section.
 */
struct cgroup_subsys_state *css_from_dir(struct dentry *dentry,
                                         struct cgroup_subsys *ss)
{
        struct cgroup *cgrp;

        WARN_ON_ONCE(!rcu_read_lock_held());

        /* is @dentry a cgroup dir? */
        if (!dentry->d_inode ||
            dentry->d_inode->i_op != &cgroup_dir_inode_operations)
                return ERR_PTR(-EBADF);

        cgrp = __d_cgrp(dentry);
        return cgroup_css(cgrp, ss) ?: ERR_PTR(-ENOENT);
}

/**
 * css_from_id - lookup css by id
 * @id: the cgroup id
 * @ss: cgroup subsys to be looked into
 *
 * Returns the css if there's valid one with @id, otherwise returns NULL.
 * Should be called under rcu_read_lock().
 */
struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
{
        struct cgroup *cgrp;

        rcu_lockdep_assert(rcu_read_lock_held() ||
                           lockdep_is_held(&cgroup_mutex),
                           "css_from_id() needs proper protection");

        cgrp = idr_find(&ss->root->cgroup_idr, id);
        if (cgrp)
                return cgroup_css(cgrp, ss);
        return NULL;
}

#ifdef CONFIG_CGROUP_DEBUG
static struct cgroup_subsys_state *
debug_css_alloc(struct cgroup_subsys_state *parent_css)
{
        struct cgroup_subsys_state *css = kzalloc(sizeof(*css), GFP_KERNEL);

        if (!css)
                return ERR_PTR(-ENOMEM);

        return css;
}

static void debug_css_free(struct cgroup_subsys_state *css)
{
        kfree(css);
}

static u64 debug_taskcount_read(struct cgroup_subsys_state *css,
                                struct cftype *cft)
{
        return cgroup_task_count(css->cgroup);
}

static u64 current_css_set_read(struct cgroup_subsys_state *css,
                                struct cftype *cft)
{
        return (u64)(unsigned long)current->cgroups;
}

static u64 current_css_set_refcount_read(struct cgroup_subsys_state *css,
                                         struct cftype *cft)
{
        u64 count;

        rcu_read_lock();
        count = atomic_read(&task_css_set(current)->refcount);
        rcu_read_unlock();
        return count;
}

static int current_css_set_cg_links_read(struct cgroup_subsys_state *css,
                                         struct cftype *cft,
                                         struct seq_file *seq)
{
        struct cgrp_cset_link *link;
        struct css_set *cset;

        read_lock(&css_set_lock);
        rcu_read_lock();
        cset = rcu_dereference(current->cgroups);
        list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
                struct cgroup *c = link->cgrp;
                const char *name;

                if (c->dentry)
                        name = c->dentry->d_name.name;
                else
                        name = "?";
                seq_printf(seq, "Root %d group %s\n",
                           c->root->hierarchy_id, name);
        }
        rcu_read_unlock();
        read_unlock(&css_set_lock);
        return 0;
}

#define MAX_TASKS_SHOWN_PER_CSS 25
static int cgroup_css_links_read(struct cgroup_subsys_state *css,
                                 struct cftype *cft, struct seq_file *seq)
{
        struct cgrp_cset_link *link;

        read_lock(&css_set_lock);
        list_for_each_entry(link, &css->cgroup->cset_links, cset_link) {
                struct css_set *cset = link->cset;
                struct task_struct *task;
                int count = 0;
                seq_printf(seq, "css_set %p\n", cset);
                list_for_each_entry(task, &cset->tasks, cg_list) {
                        if (count++ > MAX_TASKS_SHOWN_PER_CSS) {
                                seq_puts(seq, "  ...\n");
                                break;
                        } else {
                                seq_printf(seq, "  task %d\n",
                                           task_pid_vnr(task));
                        }
                }
        }
        read_unlock(&css_set_lock);
        return 0;
}

static u64 releasable_read(struct cgroup_subsys_state *css, struct cftype *cft)
{
        return test_bit(CGRP_RELEASABLE, &css->cgroup->flags);
}

static struct cftype debug_files[] =  {
        {
                .name = "taskcount",
                .read_u64 = debug_taskcount_read,
        },

        {
                .name = "current_css_set",
                .read_u64 = current_css_set_read,
        },

        {
                .name = "current_css_set_refcount",
                .read_u64 = current_css_set_refcount_read,
        },

        {
                .name = "current_css_set_cg_links",
                .read_seq_string = current_css_set_cg_links_read,
        },

        {
                .name = "cgroup_css_links",
                .read_seq_string = cgroup_css_links_read,
        },

        {
                .name = "releasable",
                .read_u64 = releasable_read,
        },

        { }     /* terminate */
};

struct cgroup_subsys debug_subsys = {
        .name = "debug",
        .css_alloc = debug_css_alloc,
        .css_free = debug_css_free,
        .subsys_id = debug_subsys_id,
        .base_cftypes = debug_files,
};
#endif /* CONFIG_CGROUP_DEBUG */