powerpc/perf: Avoid FAB_*_MATCH checks for power9

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

/*
 *  Fast Userspace Mutexes (which I call "Futexes!").
 *  (C) Rusty Russell, IBM 2002
 *
 *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
 *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved
 *
 *  Removed page pinning, fix privately mapped COW pages and other cleanups
 *  (C) Copyright 2003, 2004 Jamie Lokier
 *
 *  Robust futex support started by Ingo Molnar
 *  (C) Copyright 2006 Red Hat Inc, All Rights Reserved
 *  Thanks to Thomas Gleixner for suggestions, analysis and fixes.
 *
 *  PI-futex support started by Ingo Molnar and Thomas Gleixner
 *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *  Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
 *
 *  PRIVATE futexes by Eric Dumazet
 *  Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
 *
 *  Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
 *  Copyright (C) IBM Corporation, 2009
 *  Thanks to Thomas Gleixner for conceptual design and careful reviews.
 *
 *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
 *  enough at me, Linus for the original (flawed) idea, Matthew
 *  Kirkwood for proof-of-concept implementation.
 *
 *  "The futexes are also cursed."
 *  "But they come in a choice of three flavours!"
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/jhash.h>
#include <linux/init.h>
#include <linux/futex.h>
#include <linux/mount.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
#include <linux/signal.h>
#include <linux/export.h>
#include <linux/magic.h>
#include <linux/pid.h>
#include <linux/nsproxy.h>
#include <linux/ptrace.h>
#include <linux/sched/rt.h>
#include <linux/hugetlb.h>
#include <linux/freezer.h>
#include <linux/bootmem.h>
#include <linux/fault-inject.h>

#include <asm/futex.h>

#include "locking/rtmutex_common.h"

/*
 * READ this before attempting to hack on futexes!
 *
 * Basic futex operation and ordering guarantees
 * =============================================
 *
 * The waiter reads the futex value in user space and calls
 * futex_wait(). This function computes the hash bucket and acquires
 * the hash bucket lock. After that it reads the futex user space value
 * again and verifies that the data has not changed. If it has not changed
 * it enqueues itself into the hash bucket, releases the hash bucket lock
 * and schedules.
 *
 * The waker side modifies the user space value of the futex and calls
 * futex_wake(). This function computes the hash bucket and acquires the
 * hash bucket lock. Then it looks for waiters on that futex in the hash
 * bucket and wakes them.
 *
 * In futex wake up scenarios where no tasks are blocked on a futex, taking
 * the hb spinlock can be avoided and simply return. In order for this
 * optimization to work, ordering guarantees must exist so that the waiter
 * being added to the list is acknowledged when the list is concurrently being
 * checked by the waker, avoiding scenarios like the following:
 *
 * CPU 0                               CPU 1
 * val = *futex;
 * sys_futex(WAIT, futex, val);
 *   futex_wait(futex, val);
 *   uval = *futex;
 *                                     *futex = newval;
 *                                     sys_futex(WAKE, futex);
 *                                       futex_wake(futex);
 *                                       if (queue_empty())
 *                                         return;
 *   if (uval == val)
 *      lock(hash_bucket(futex));
 *      queue();
 *     unlock(hash_bucket(futex));
 *     schedule();
 *
 * This would cause the waiter on CPU 0 to wait forever because it
 * missed the transition of the user space value from val to newval
 * and the waker did not find the waiter in the hash bucket queue.
 *
 * The correct serialization ensures that a waiter either observes
 * the changed user space value before blocking or is woken by a
 * concurrent waker:
 *
 * CPU 0                                 CPU 1
 * val = *futex;
 * sys_futex(WAIT, futex, val);
 *   futex_wait(futex, val);
 *
 *   waiters++; (a)
 *   smp_mb(); (A) <-- paired with -.
 *                                  |
 *   lock(hash_bucket(futex));      |
 *                                  |
 *   uval = *futex;                 |
 *                                  |        *futex = newval;
 *                                  |        sys_futex(WAKE, futex);
 *                                  |          futex_wake(futex);
 *                                  |
 *                                  `--------> smp_mb(); (B)
 *   if (uval == val)
 *     queue();
 *     unlock(hash_bucket(futex));
 *     schedule();                         if (waiters)
 *                                           lock(hash_bucket(futex));
 *   else                                    wake_waiters(futex);
 *     waiters--; (b)                        unlock(hash_bucket(futex));
 *
 * Where (A) orders the waiters increment and the futex value read through
 * atomic operations (see hb_waiters_inc) and where (B) orders the write
 * to futex and the waiters read -- this is done by the barriers for both
 * shared and private futexes in get_futex_key_refs().
 *
 * This yields the following case (where X:=waiters, Y:=futex):
 *
 *      X = Y = 0
 *
 *      w[X]=1          w[Y]=1
 *      MB              MB
 *      r[Y]=y          r[X]=x
 *
 * Which guarantees that x==0 && y==0 is impossible; which translates back into
 * the guarantee that we cannot both miss the futex variable change and the
 * enqueue.
 *
 * Note that a new waiter is accounted for in (a) even when it is possible that
 * the wait call can return error, in which case we backtrack from it in (b).
 * Refer to the comment in queue_lock().
 *
 * Similarly, in order to account for waiters being requeued on another
 * address we always increment the waiters for the destination bucket before
 * acquiring the lock. It then decrements them again  after releasing it -
 * the code that actually moves the futex(es) between hash buckets (requeue_futex)
 * will do the additional required waiter count housekeeping. This is done for
 * double_lock_hb() and double_unlock_hb(), respectively.
 */

#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
int __read_mostly futex_cmpxchg_enabled;
#endif

/*
 * Futex flags used to encode options to functions and preserve them across
 * restarts.
 */
#ifdef CONFIG_MMU
# define FLAGS_SHARED           0x01
#else
/*
 * NOMMU does not have per process address space. Let the compiler optimize
 * code away.
 */
# define FLAGS_SHARED           0x00
#endif
#define FLAGS_CLOCKRT           0x02
#define FLAGS_HAS_TIMEOUT       0x04

/*
 * Priority Inheritance state:
 */
struct futex_pi_state {
        /*
         * list of 'owned' pi_state instances - these have to be
         * cleaned up in do_exit() if the task exits prematurely:
         */
        struct list_head list;

        /*
         * The PI object:
         */
        struct rt_mutex pi_mutex;

        struct task_struct *owner;
        atomic_t refcount;

        union futex_key key;
};

/**
 * struct futex_q - The hashed futex queue entry, one per waiting task
 * @list:               priority-sorted list of tasks waiting on this futex
 * @task:               the task waiting on the futex
 * @lock_ptr:           the hash bucket lock
 * @key:                the key the futex is hashed on
 * @pi_state:           optional priority inheritance state
 * @rt_waiter:          rt_waiter storage for use with requeue_pi
 * @requeue_pi_key:     the requeue_pi target futex key
 * @bitset:             bitset for the optional bitmasked wakeup
 *
 * We use this hashed waitqueue, instead of a normal wait_queue_t, so
 * we can wake only the relevant ones (hashed queues may be shared).
 *
 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
 * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
 * The order of wakeup is always to make the first condition true, then
 * the second.
 *
 * PI futexes are typically woken before they are removed from the hash list via
 * the rt_mutex code. See unqueue_me_pi().
 */
struct futex_q {
        struct plist_node list;

        struct task_struct *task;
        spinlock_t *lock_ptr;
        union futex_key key;
        struct futex_pi_state *pi_state;
        struct rt_mutex_waiter *rt_waiter;
        union futex_key *requeue_pi_key;
        u32 bitset;
};

static const struct futex_q futex_q_init = {
        /* list gets initialized in queue_me()*/
        .key = FUTEX_KEY_INIT,
        .bitset = FUTEX_BITSET_MATCH_ANY
};

/*
 * Hash buckets are shared by all the futex_keys that hash to the same
 * location.  Each key may have multiple futex_q structures, one for each task
 * waiting on a futex.
 */
struct futex_hash_bucket {
        atomic_t waiters;
        spinlock_t lock;
        struct plist_head chain;
} ____cacheline_aligned_in_smp;

/*
 * The base of the bucket array and its size are always used together
 * (after initialization only in hash_futex()), so ensure that they
 * reside in the same cacheline.
 */
static struct {
        struct futex_hash_bucket *queues;
        unsigned long            hashsize;
} __futex_data __read_mostly __aligned(2*sizeof(long));
#define futex_queues   (__futex_data.queues)
#define futex_hashsize (__futex_data.hashsize)


/*
 * Fault injections for futexes.
 */
#ifdef CONFIG_FAIL_FUTEX

static struct {
        struct fault_attr attr;

        bool ignore_private;
} fail_futex = {
        .attr = FAULT_ATTR_INITIALIZER,
        .ignore_private = false,
};

static int __init setup_fail_futex(char *str)
{
        return setup_fault_attr(&fail_futex.attr, str);
}
__setup("fail_futex=", setup_fail_futex);

static bool should_fail_futex(bool fshared)
{
        if (fail_futex.ignore_private && !fshared)
                return false;

        return should_fail(&fail_futex.attr, 1);
}

#ifdef CONFIG_FAULT_INJECTION_DEBUG_FS

static int __init fail_futex_debugfs(void)
{
        umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
        struct dentry *dir;

        dir = fault_create_debugfs_attr("fail_futex", NULL,
                                        &fail_futex.attr);
        if (IS_ERR(dir))
                return PTR_ERR(dir);

        if (!debugfs_create_bool("ignore-private", mode, dir,
                                 &fail_futex.ignore_private)) {
                debugfs_remove_recursive(dir);
                return -ENOMEM;
        }

        return 0;
}

late_initcall(fail_futex_debugfs);

#endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */

#else
static inline bool should_fail_futex(bool fshared)
{
        return false;
}
#endif /* CONFIG_FAIL_FUTEX */

static inline void futex_get_mm(union futex_key *key)
{
        atomic_inc(&key->private.mm->mm_count);
        /*
         * Ensure futex_get_mm() implies a full barrier such that
         * get_futex_key() implies a full barrier. This is relied upon
         * as smp_mb(); (B), see the ordering comment above.
         */
        smp_mb__after_atomic();
}

/*
 * Reflects a new waiter being added to the waitqueue.
 */
static inline void hb_waiters_inc(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
        atomic_inc(&hb->waiters);
        /*
         * Full barrier (A), see the ordering comment above.
         */
        smp_mb__after_atomic();
#endif
}

/*
 * Reflects a waiter being removed from the waitqueue by wakeup
 * paths.
 */
static inline void hb_waiters_dec(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
        atomic_dec(&hb->waiters);
#endif
}

static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
        return atomic_read(&hb->waiters);
#else
        return 1;
#endif
}

/**
 * hash_futex - Return the hash bucket in the global hash
 * @key:        Pointer to the futex key for which the hash is calculated
 *
 * We hash on the keys returned from get_futex_key (see below) and return the
 * corresponding hash bucket in the global hash.
 */
static struct futex_hash_bucket *hash_futex(union futex_key *key)
{
        u32 hash = jhash2((u32*)&key->both.word,
                          (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
                          key->both.offset);
        return &futex_queues[hash & (futex_hashsize - 1)];
}


/**
 * match_futex - Check whether two futex keys are equal
 * @key1:       Pointer to key1
 * @key2:       Pointer to key2
 *
 * Return 1 if two futex_keys are equal, 0 otherwise.
 */
static inline int match_futex(union futex_key *key1, union futex_key *key2)
{
        return (key1 && key2
                && key1->both.word == key2->both.word
                && key1->both.ptr == key2->both.ptr
                && key1->both.offset == key2->both.offset);
}

/*
 * Take a reference to the resource addressed by a key.
 * Can be called while holding spinlocks.
 *
 */
static void get_futex_key_refs(union futex_key *key)
{
        if (!key->both.ptr)
                return;

        /*
         * On MMU less systems futexes are always "private" as there is no per
         * process address space. We need the smp wmb nevertheless - yes,
         * arch/blackfin has MMU less SMP ...
         */
        if (!IS_ENABLED(CONFIG_MMU)) {
                smp_mb(); /* explicit smp_mb(); (B) */
                return;
        }

        switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
        case FUT_OFF_INODE:
                ihold(key->shared.inode); /* implies smp_mb(); (B) */
                break;
        case FUT_OFF_MMSHARED:
                futex_get_mm(key); /* implies smp_mb(); (B) */
                break;
        default:
                /*
                 * Private futexes do not hold reference on an inode or
                 * mm, therefore the only purpose of calling get_futex_key_refs
                 * is because we need the barrier for the lockless waiter check.
                 */
                smp_mb(); /* explicit smp_mb(); (B) */
        }
}

/*
 * Drop a reference to the resource addressed by a key.
 * The hash bucket spinlock must not be held. This is
 * a no-op for private futexes, see comment in the get
 * counterpart.
 */
static void drop_futex_key_refs(union futex_key *key)
{
        if (!key->both.ptr) {
                /* If we're here then we tried to put a key we failed to get */
                WARN_ON_ONCE(1);
                return;
        }

        if (!IS_ENABLED(CONFIG_MMU))
                return;

        switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
        case FUT_OFF_INODE:
                iput(key->shared.inode);
                break;
        case FUT_OFF_MMSHARED:
                mmdrop(key->private.mm);
                break;
        }
}

/**
 * get_futex_key() - Get parameters which are the keys for a futex
 * @uaddr:      virtual address of the futex
 * @fshared:    0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
 * @key:        address where result is stored.
 * @rw:         mapping needs to be read/write (values: VERIFY_READ,
 *              VERIFY_WRITE)
 *
 * Return: a negative error code or 0
 *
 * The key words are stored in *key on success.
 *
 * For shared mappings, it's (page->index, file_inode(vma->vm_file),
 * offset_within_page).  For private mappings, it's (uaddr, current->mm).
 * We can usually work out the index without swapping in the page.
 *
 * lock_page() might sleep, the caller should not hold a spinlock.
 */
static int
get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
{
        unsigned long address = (unsigned long)uaddr;
        struct mm_struct *mm = current->mm;
        struct page *page, *tail;
        struct address_space *mapping;
        int err, ro = 0;

        /*
         * The futex address must be "naturally" aligned.
         */
        key->both.offset = address % PAGE_SIZE;
        if (unlikely((address % sizeof(u32)) != 0))
                return -EINVAL;
        address -= key->both.offset;

        if (unlikely(!access_ok(rw, uaddr, sizeof(u32))))
                return -EFAULT;

        if (unlikely(should_fail_futex(fshared)))
                return -EFAULT;

        /*
         * PROCESS_PRIVATE futexes are fast.
         * As the mm cannot disappear under us and the 'key' only needs
         * virtual address, we dont even have to find the underlying vma.
         * Note : We do have to check 'uaddr' is a valid user address,
         *        but access_ok() should be faster than find_vma()
         */
        if (!fshared) {
                key->private.mm = mm;
                key->private.address = address;
                get_futex_key_refs(key);  /* implies smp_mb(); (B) */
                return 0;
        }

again:
        /* Ignore any VERIFY_READ mapping (futex common case) */
        if (unlikely(should_fail_futex(fshared)))
                return -EFAULT;

        err = get_user_pages_fast(address, 1, 1, &page);
        /*
         * If write access is not required (eg. FUTEX_WAIT), try
         * and get read-only access.
         */
        if (err == -EFAULT && rw == VERIFY_READ) {
                err = get_user_pages_fast(address, 1, 0, &page);
                ro = 1;
        }
        if (err < 0)
                return err;
        else
                err = 0;

        /*
         * The treatment of mapping from this point on is critical. The page
         * lock protects many things but in this context the page lock
         * stabilizes mapping, prevents inode freeing in the shared
         * file-backed region case and guards against movement to swap cache.
         *
         * Strictly speaking the page lock is not needed in all cases being
         * considered here and page lock forces unnecessarily serialization
         * From this point on, mapping will be re-verified if necessary and
         * page lock will be acquired only if it is unavoidable
         *
         * Mapping checks require the head page for any compound page so the
         * head page and mapping is looked up now. For anonymous pages, it
         * does not matter if the page splits in the future as the key is
         * based on the address. For filesystem-backed pages, the tail is
         * required as the index of the page determines the key. For
         * base pages, there is no tail page and tail == page.
         */
        tail = page;
        page = compound_head(page);
        mapping = READ_ONCE(page->mapping);

        /*
         * If page->mapping is NULL, then it cannot be a PageAnon
         * page; but it might be the ZERO_PAGE or in the gate area or
         * in a special mapping (all cases which we are happy to fail);
         * or it may have been a good file page when get_user_pages_fast
         * found it, but truncated or holepunched or subjected to
         * invalidate_complete_page2 before we got the page lock (also
         * cases which we are happy to fail).  And we hold a reference,
         * so refcount care in invalidate_complete_page's remove_mapping
         * prevents drop_caches from setting mapping to NULL beneath us.
         *
         * The case we do have to guard against is when memory pressure made
         * shmem_writepage move it from filecache to swapcache beneath us:
         * an unlikely race, but we do need to retry for page->mapping.
         */
        if (unlikely(!mapping)) {
                int shmem_swizzled;

                /*
                 * Page lock is required to identify which special case above
                 * applies. If this is really a shmem page then the page lock
                 * will prevent unexpected transitions.
                 */
                lock_page(page);
                shmem_swizzled = PageSwapCache(page) || page->mapping;
                unlock_page(page);
                put_page(page);

                if (shmem_swizzled)
                        goto again;

                return -EFAULT;
        }

        /*
         * Private mappings are handled in a simple way.
         *
         * If the futex key is stored on an anonymous page, then the associated
         * object is the mm which is implicitly pinned by the calling process.
         *
         * NOTE: When userspace waits on a MAP_SHARED mapping, even if
         * it's a read-only handle, it's expected that futexes attach to
         * the object not the particular process.
         */
        if (PageAnon(page)) {
                /*
                 * A RO anonymous page will never change and thus doesn't make
                 * sense for futex operations.
                 */
                if (unlikely(should_fail_futex(fshared)) || ro) {
                        err = -EFAULT;
                        goto out;
                }

                key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
                key->private.mm = mm;
                key->private.address = address;

                get_futex_key_refs(key); /* implies smp_mb(); (B) */

        } else {
                struct inode *inode;

                /*
                 * The associated futex object in this case is the inode and
                 * the page->mapping must be traversed. Ordinarily this should
                 * be stabilised under page lock but it's not strictly
                 * necessary in this case as we just want to pin the inode, not
                 * update the radix tree or anything like that.
                 *
                 * The RCU read lock is taken as the inode is finally freed
                 * under RCU. If the mapping still matches expectations then the
                 * mapping->host can be safely accessed as being a valid inode.
                 */
                rcu_read_lock();

                if (READ_ONCE(page->mapping) != mapping) {
                        rcu_read_unlock();
                        put_page(page);

                        goto again;
                }

                inode = READ_ONCE(mapping->host);
                if (!inode) {
                        rcu_read_unlock();
                        put_page(page);

                        goto again;
                }

                /*
                 * Take a reference unless it is about to be freed. Previously
                 * this reference was taken by ihold under the page lock
                 * pinning the inode in place so i_lock was unnecessary. The
                 * only way for this check to fail is if the inode was
                 * truncated in parallel so warn for now if this happens.
                 *
                 * We are not calling into get_futex_key_refs() in file-backed
                 * cases, therefore a successful atomic_inc return below will
                 * guarantee that get_futex_key() will still imply smp_mb(); (B).
                 */
                if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
                        rcu_read_unlock();
                        put_page(page);

                        goto again;
                }

                /* Should be impossible but lets be paranoid for now */
                if (WARN_ON_ONCE(inode->i_mapping != mapping)) {
                        err = -EFAULT;
                        rcu_read_unlock();
                        iput(inode);

                        goto out;
                }

                key->both.offset |= FUT_OFF_INODE; /* inode-based key */
                key->shared.inode = inode;
                key->shared.pgoff = basepage_index(tail);
                rcu_read_unlock();
        }

out:
        put_page(page);
        return err;
}

static inline void put_futex_key(union futex_key *key)
{
        drop_futex_key_refs(key);
}

/**
 * fault_in_user_writeable() - Fault in user address and verify RW access
 * @uaddr:      pointer to faulting user space address
 *
 * Slow path to fixup the fault we just took in the atomic write
 * access to @uaddr.
 *
 * We have no generic implementation of a non-destructive write to the
 * user address. We know that we faulted in the atomic pagefault
 * disabled section so we can as well avoid the #PF overhead by
 * calling get_user_pages() right away.
 */
static int fault_in_user_writeable(u32 __user *uaddr)
{
        struct mm_struct *mm = current->mm;
        int ret;

        down_read(&mm->mmap_sem);
        ret = fixup_user_fault(current, mm, (unsigned long)uaddr,
                               FAULT_FLAG_WRITE, NULL);
        up_read(&mm->mmap_sem);

        return ret < 0 ? ret : 0;
}

/**
 * futex_top_waiter() - Return the highest priority waiter on a futex
 * @hb:         the hash bucket the futex_q's reside in
 * @key:        the futex key (to distinguish it from other futex futex_q's)
 *
 * Must be called with the hb lock held.
 */
static struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb,
                                        union futex_key *key)
{
        struct futex_q *this;

        plist_for_each_entry(this, &hb->chain, list) {
                if (match_futex(&this->key, key))
                        return this;
        }
        return NULL;
}

static int cmpxchg_futex_value_locked(u32 *curval, u32 __user *uaddr,
                                      u32 uval, u32 newval)
{
        int ret;

        pagefault_disable();
        ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
        pagefault_enable();

        return ret;
}

static int get_futex_value_locked(u32 *dest, u32 __user *from)
{
        int ret;

        pagefault_disable();
        ret = __get_user(*dest, from);
        pagefault_enable();

        return ret ? -EFAULT : 0;
}


/*
 * PI code:
 */
static int refill_pi_state_cache(void)
{
        struct futex_pi_state *pi_state;

        if (likely(current->pi_state_cache))
                return 0;

        pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);

        if (!pi_state)
                return -ENOMEM;

        INIT_LIST_HEAD(&pi_state->list);
        /* pi_mutex gets initialized later */
        pi_state->owner = NULL;
        atomic_set(&pi_state->refcount, 1);
        pi_state->key = FUTEX_KEY_INIT;

        current->pi_state_cache = pi_state;

        return 0;
}

static struct futex_pi_state * alloc_pi_state(void)
{
        struct futex_pi_state *pi_state = current->pi_state_cache;

        WARN_ON(!pi_state);
        current->pi_state_cache = NULL;

        return pi_state;
}

/*
 * Drops a reference to the pi_state object and frees or caches it
 * when the last reference is gone.
 *
 * Must be called with the hb lock held.
 */
static void put_pi_state(struct futex_pi_state *pi_state)
{
        if (!pi_state)
                return;

        if (!atomic_dec_and_test(&pi_state->refcount))
                return;

        /*
         * If pi_state->owner is NULL, the owner is most probably dying
         * and has cleaned up the pi_state already
         */
        if (pi_state->owner) {
                raw_spin_lock_irq(&pi_state->owner->pi_lock);
                list_del_init(&pi_state->list);
                raw_spin_unlock_irq(&pi_state->owner->pi_lock);

                rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner);
        }

        if (current->pi_state_cache)
                kfree(pi_state);
        else {
                /*
                 * pi_state->list is already empty.
                 * clear pi_state->owner.
                 * refcount is at 0 - put it back to 1.
                 */
                pi_state->owner = NULL;
                atomic_set(&pi_state->refcount, 1);
                current->pi_state_cache = pi_state;
        }
}

/*
 * Look up the task based on what TID userspace gave us.
 * We dont trust it.
 */
static struct task_struct * futex_find_get_task(pid_t pid)
{
        struct task_struct *p;

        rcu_read_lock();
        p = find_task_by_vpid(pid);
        if (p)
                get_task_struct(p);

        rcu_read_unlock();

        return p;
}

/*
 * This task is holding PI mutexes at exit time => bad.
 * Kernel cleans up PI-state, but userspace is likely hosed.
 * (Robust-futex cleanup is separate and might save the day for userspace.)
 */
void exit_pi_state_list(struct task_struct *curr)
{
        struct list_head *next, *head = &curr->pi_state_list;
        struct futex_pi_state *pi_state;
        struct futex_hash_bucket *hb;
        union futex_key key = FUTEX_KEY_INIT;

        if (!futex_cmpxchg_enabled)
                return;
        /*
         * We are a ZOMBIE and nobody can enqueue itself on
         * pi_state_list anymore, but we have to be careful
         * versus waiters unqueueing themselves:
         */
        raw_spin_lock_irq(&curr->pi_lock);
        while (!list_empty(head)) {

                next = head->next;
                pi_state = list_entry(next, struct futex_pi_state, list);
                key = pi_state->key;
                hb = hash_futex(&key);
                raw_spin_unlock_irq(&curr->pi_lock);

                spin_lock(&hb->lock);

                raw_spin_lock_irq(&curr->pi_lock);
                /*
                 * We dropped the pi-lock, so re-check whether this
                 * task still owns the PI-state:
                 */
                if (head->next != next) {
                        spin_unlock(&hb->lock);
                        continue;
                }

                WARN_ON(pi_state->owner != curr);
                WARN_ON(list_empty(&pi_state->list));
                list_del_init(&pi_state->list);
                pi_state->owner = NULL;
                raw_spin_unlock_irq(&curr->pi_lock);

                rt_mutex_unlock(&pi_state->pi_mutex);

                spin_unlock(&hb->lock);

                raw_spin_lock_irq(&curr->pi_lock);
        }
        raw_spin_unlock_irq(&curr->pi_lock);
}

/*
 * We need to check the following states:
 *
 *      Waiter | pi_state | pi->owner | uTID      | uODIED | ?
 *
 * [1]  NULL   | ---      | ---       | 0         | 0/1    | Valid
 * [2]  NULL   | ---      | ---       | >0        | 0/1    | Valid
 *
 * [3]  Found  | NULL     | --        | Any       | 0/1    | Invalid
 *
 * [4]  Found  | Found    | NULL      | 0         | 1      | Valid
 * [5]  Found  | Found    | NULL      | >0        | 1      | Invalid
 *
 * [6]  Found  | Found    | task      | 0         | 1      | Valid
 *
 * [7]  Found  | Found    | NULL      | Any       | 0      | Invalid
 *
 * [8]  Found  | Found    | task      | ==taskTID | 0/1    | Valid
 * [9]  Found  | Found    | task      | 0         | 0      | Invalid
 * [10] Found  | Found    | task      | !=taskTID | 0/1    | Invalid
 *
 * [1]  Indicates that the kernel can acquire the futex atomically. We
 *      came came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
 *
 * [2]  Valid, if TID does not belong to a kernel thread. If no matching
 *      thread is found then it indicates that the owner TID has died.
 *
 * [3]  Invalid. The waiter is queued on a non PI futex
 *
 * [4]  Valid state after exit_robust_list(), which sets the user space
 *      value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
 *
 * [5]  The user space value got manipulated between exit_robust_list()
 *      and exit_pi_state_list()
 *
 * [6]  Valid state after exit_pi_state_list() which sets the new owner in
 *      the pi_state but cannot access the user space value.
 *
 * [7]  pi_state->owner can only be NULL when the OWNER_DIED bit is set.
 *
 * [8]  Owner and user space value match
 *
 * [9]  There is no transient state which sets the user space TID to 0
 *      except exit_robust_list(), but this is indicated by the
 *      FUTEX_OWNER_DIED bit. See [4]
 *
 * [10] There is no transient state which leaves owner and user space
 *      TID out of sync.
 */

/*
 * Validate that the existing waiter has a pi_state and sanity check
 * the pi_state against the user space value. If correct, attach to
 * it.
 */
static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
                              struct futex_pi_state **ps)
{
        pid_t pid = uval & FUTEX_TID_MASK;

        /*
         * Userspace might have messed up non-PI and PI futexes [3]
         */
        if (unlikely(!pi_state))
                return -EINVAL;

        WARN_ON(!atomic_read(&pi_state->refcount));

        /*
         * Handle the owner died case:
         */
        if (uval & FUTEX_OWNER_DIED) {
                /*
                 * exit_pi_state_list sets owner to NULL and wakes the
                 * topmost waiter. The task which acquires the
                 * pi_state->rt_mutex will fixup owner.
                 */
                if (!pi_state->owner) {
                        /*
                         * No pi state owner, but the user space TID
                         * is not 0. Inconsistent state. [5]
                         */
                        if (pid)
                                return -EINVAL;
                        /*
                         * Take a ref on the state and return success. [4]
                         */
                        goto out_state;
                }

                /*
                 * If TID is 0, then either the dying owner has not
                 * yet executed exit_pi_state_list() or some waiter
                 * acquired the rtmutex in the pi state, but did not
                 * yet fixup the TID in user space.
                 *
                 * Take a ref on the state and return success. [6]
                 */
                if (!pid)
                        goto out_state;
        } else {
                /*
                 * If the owner died bit is not set, then the pi_state
                 * must have an owner. [7]
                 */
                if (!pi_state->owner)
                        return -EINVAL;
        }

        /*
         * Bail out if user space manipulated the futex value. If pi
         * state exists then the owner TID must be the same as the
         * user space TID. [9/10]
         */
        if (pid != task_pid_vnr(pi_state->owner))
                return -EINVAL;
out_state:
        atomic_inc(&pi_state->refcount);
        *ps = pi_state;
        return 0;
}

/*
 * Lookup the task for the TID provided from user space and attach to
 * it after doing proper sanity checks.
 */
static int attach_to_pi_owner(u32 uval, union futex_key *key,
                              struct futex_pi_state **ps)
{
        pid_t pid = uval & FUTEX_TID_MASK;
        struct futex_pi_state *pi_state;
        struct task_struct *p;

        /*
         * We are the first waiter - try to look up the real owner and attach
         * the new pi_state to it, but bail out when TID = 0 [1]
         */
        if (!pid)
                return -ESRCH;
        p = futex_find_get_task(pid);
        if (!p)
                return -ESRCH;

        if (unlikely(p->flags & PF_KTHREAD)) {
                put_task_struct(p);
                return -EPERM;
        }

        /*
         * We need to look at the task state flags to figure out,
         * whether the task is exiting. To protect against the do_exit
         * change of the task flags, we do this protected by
         * p->pi_lock:
         */
        raw_spin_lock_irq(&p->pi_lock);
        if (unlikely(p->flags & PF_EXITING)) {
                /*
                 * The task is on the way out. When PF_EXITPIDONE is
                 * set, we know that the task has finished the
                 * cleanup:
                 */
                int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;

                raw_spin_unlock_irq(&p->pi_lock);
                put_task_struct(p);
                return ret;
        }

        /*
         * No existing pi state. First waiter. [2]
         */
        pi_state = alloc_pi_state();

        /*
         * Initialize the pi_mutex in locked state and make @p
         * the owner of it:
         */
        rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);

        /* Store the key for possible exit cleanups: */
        pi_state->key = *key;

        WARN_ON(!list_empty(&pi_state->list));
        list_add(&pi_state->list, &p->pi_state_list);
        pi_state->owner = p;
        raw_spin_unlock_irq(&p->pi_lock);

        put_task_struct(p);

        *ps = pi_state;

        return 0;
}

static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
                           union futex_key *key, struct futex_pi_state **ps)
{
        struct futex_q *match = futex_top_waiter(hb, key);

        /*
         * If there is a waiter on that futex, validate it and
         * attach to the pi_state when the validation succeeds.
         */
        if (match)
                return attach_to_pi_state(uval, match->pi_state, ps);

        /*
         * We are the first waiter - try to look up the owner based on
         * @uval and attach to it.
         */
        return attach_to_pi_owner(uval, key, ps);
}

static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
{
        u32 uninitialized_var(curval);

        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)))
                return -EFAULT;

        /*If user space value changed, let the caller retry */
        return curval != uval ? -EAGAIN : 0;
}

/**
 * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
 * @uaddr:              the pi futex user address
 * @hb:                 the pi futex hash bucket
 * @key:                the futex key associated with uaddr and hb
 * @ps:                 the pi_state pointer where we store the result of the
 *                      lookup
 * @task:               the task to perform the atomic lock work for.  This will
 *                      be "current" except in the case of requeue pi.
 * @set_waiters:        force setting the FUTEX_WAITERS bit (1) or not (0)
 *
 * Return:
 *  0 - ready to wait;
 *  1 - acquired the lock;
 * <0 - error
 *
 * The hb->lock and futex_key refs shall be held by the caller.
 */
static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
                                union futex_key *key,
                                struct futex_pi_state **ps,
                                struct task_struct *task, int set_waiters)
{
        u32 uval, newval, vpid = task_pid_vnr(task);
        struct futex_q *match;
        int ret;

        /*
         * Read the user space value first so we can validate a few
         * things before proceeding further.
         */
        if (get_futex_value_locked(&uval, uaddr))
                return -EFAULT;

        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        /*
         * Detect deadlocks.
         */
        if ((unlikely((uval & FUTEX_TID_MASK) == vpid)))
                return -EDEADLK;

        if ((unlikely(should_fail_futex(true))))
                return -EDEADLK;

        /*
         * Lookup existing state first. If it exists, try to attach to
         * its pi_state.
         */
        match = futex_top_waiter(hb, key);
        if (match)
                return attach_to_pi_state(uval, match->pi_state, ps);

        /*
         * No waiter and user TID is 0. We are here because the
         * waiters or the owner died bit is set or called from
         * requeue_cmp_pi or for whatever reason something took the
         * syscall.
         */
        if (!(uval & FUTEX_TID_MASK)) {
                /*
                 * We take over the futex. No other waiters and the user space
                 * TID is 0. We preserve the owner died bit.
                 */
                newval = uval & FUTEX_OWNER_DIED;
                newval |= vpid;

                /* The futex requeue_pi code can enforce the waiters bit */
                if (set_waiters)
                        newval |= FUTEX_WAITERS;

                ret = lock_pi_update_atomic(uaddr, uval, newval);
                /* If the take over worked, return 1 */
                return ret < 0 ? ret : 1;
        }

        /*
         * First waiter. Set the waiters bit before attaching ourself to
         * the owner. If owner tries to unlock, it will be forced into
         * the kernel and blocked on hb->lock.
         */
        newval = uval | FUTEX_WAITERS;
        ret = lock_pi_update_atomic(uaddr, uval, newval);
        if (ret)
                return ret;
        /*
         * If the update of the user space value succeeded, we try to
         * attach to the owner. If that fails, no harm done, we only
         * set the FUTEX_WAITERS bit in the user space variable.
         */
        return attach_to_pi_owner(uval, key, ps);
}

/**
 * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
 * @q:  The futex_q to unqueue
 *
 * The q->lock_ptr must not be NULL and must be held by the caller.
 */
static void __unqueue_futex(struct futex_q *q)
{
        struct futex_hash_bucket *hb;

        if (WARN_ON_SMP(!q->lock_ptr || !spin_is_locked(q->lock_ptr))
            || WARN_ON(plist_node_empty(&q->list)))
                return;

        hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
        plist_del(&q->list, &hb->chain);
        hb_waiters_dec(hb);
}

/*
 * The hash bucket lock must be held when this is called.
 * Afterwards, the futex_q must not be accessed. Callers
 * must ensure to later call wake_up_q() for the actual
 * wakeups to occur.
 */
static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
{
        struct task_struct *p = q->task;

        if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
                return;

        /*
         * Queue the task for later wakeup for after we've released
         * the hb->lock. wake_q_add() grabs reference to p.
         */
        wake_q_add(wake_q, p);
        __unqueue_futex(q);
        /*
         * The waiting task can free the futex_q as soon as
         * q->lock_ptr = NULL is written, without taking any locks. A
         * memory barrier is required here to prevent the following
         * store to lock_ptr from getting ahead of the plist_del.
         */
        smp_wmb();
        q->lock_ptr = NULL;
}

static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this,
                         struct futex_hash_bucket *hb)
{
        struct task_struct *new_owner;
        struct futex_pi_state *pi_state = this->pi_state;
        u32 uninitialized_var(curval), newval;
        DEFINE_WAKE_Q(wake_q);
        bool deboost;
        int ret = 0;

        if (!pi_state)
                return -EINVAL;

        /*
         * If current does not own the pi_state then the futex is
         * inconsistent and user space fiddled with the futex value.
         */
        if (pi_state->owner != current)
                return -EINVAL;

        raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
        new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);

        /*
         * It is possible that the next waiter (the one that brought
         * this owner to the kernel) timed out and is no longer
         * waiting on the lock.
         */
        if (!new_owner)
                new_owner = this->task;

        /*
         * We pass it to the next owner. The WAITERS bit is always
         * kept enabled while there is PI state around. We cleanup the
         * owner died bit, because we are the owner.
         */
        newval = FUTEX_WAITERS | task_pid_vnr(new_owner);

        if (unlikely(should_fail_futex(true)))
                ret = -EFAULT;

        if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) {
                ret = -EFAULT;
        } else if (curval != uval) {
                /*
                 * If a unconditional UNLOCK_PI operation (user space did not
                 * try the TID->0 transition) raced with a waiter setting the
                 * FUTEX_WAITERS flag between get_user() and locking the hash
                 * bucket lock, retry the operation.
                 */
                if ((FUTEX_TID_MASK & curval) == uval)
                        ret = -EAGAIN;
                else
                        ret = -EINVAL;
        }
        if (ret) {
                raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
                return ret;
        }

        raw_spin_lock(&pi_state->owner->pi_lock);
        WARN_ON(list_empty(&pi_state->list));
        list_del_init(&pi_state->list);
        raw_spin_unlock(&pi_state->owner->pi_lock);

        raw_spin_lock(&new_owner->pi_lock);
        WARN_ON(!list_empty(&pi_state->list));
        list_add(&pi_state->list, &new_owner->pi_state_list);
        pi_state->owner = new_owner;
        raw_spin_unlock(&new_owner->pi_lock);

        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);

        deboost = rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);

        /*
         * First unlock HB so the waiter does not spin on it once he got woken
         * up. Second wake up the waiter before the priority is adjusted. If we
         * deboost first (and lose our higher priority), then the task might get
         * scheduled away before the wake up can take place.
         */
        spin_unlock(&hb->lock);
        wake_up_q(&wake_q);
        if (deboost)
                rt_mutex_adjust_prio(current);

        return 0;
}

/*
 * Express the locking dependencies for lockdep:
 */
static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
        if (hb1 <= hb2) {
                spin_lock(&hb1->lock);
                if (hb1 < hb2)
                        spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
        } else { /* hb1 > hb2 */
                spin_lock(&hb2->lock);
                spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
        }
}

static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
        spin_unlock(&hb1->lock);
        if (hb1 != hb2)
                spin_unlock(&hb2->lock);
}

/*
 * Wake up waiters matching bitset queued on this futex (uaddr).
 */
static int
futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
{
        struct futex_hash_bucket *hb;
        struct futex_q *this, *next;
        union futex_key key = FUTEX_KEY_INIT;
        int ret;
        DEFINE_WAKE_Q(wake_q);

        if (!bitset)
                return -EINVAL;

        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_READ);
        if (unlikely(ret != 0))
                goto out;

        hb = hash_futex(&key);

        /* Make sure we really have tasks to wakeup */
        if (!hb_waiters_pending(hb))
                goto out_put_key;

        spin_lock(&hb->lock);

        plist_for_each_entry_safe(this, next, &hb->chain, list) {
                if (match_futex (&this->key, &key)) {
                        if (this->pi_state || this->rt_waiter) {
                                ret = -EINVAL;
                                break;
                        }

                        /* Check if one of the bits is set in both bitsets */
                        if (!(this->bitset & bitset))
                                continue;

                        mark_wake_futex(&wake_q, this);
                        if (++ret >= nr_wake)
                                break;
                }
        }

        spin_unlock(&hb->lock);
        wake_up_q(&wake_q);
out_put_key:
        put_futex_key(&key);
out:
        return ret;
}

/*
 * Wake up all waiters hashed on the physical page that is mapped
 * to this virtual address:
 */
static int
futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
              int nr_wake, int nr_wake2, int op)
{
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
        struct futex_hash_bucket *hb1, *hb2;
        struct futex_q *this, *next;
        int ret, op_ret;
        DEFINE_WAKE_Q(wake_q);

retry:
        ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
        if (unlikely(ret != 0))
                goto out;
        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
        if (unlikely(ret != 0))
                goto out_put_key1;

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

retry_private:
        double_lock_hb(hb1, hb2);
        op_ret = futex_atomic_op_inuser(op, uaddr2);
        if (unlikely(op_ret < 0)) {

                double_unlock_hb(hb1, hb2);

#ifndef CONFIG_MMU
                /*
                 * we don't get EFAULT from MMU faults if we don't have an MMU,
                 * but we might get them from range checking
                 */
                ret = op_ret;
                goto out_put_keys;
#endif

                if (unlikely(op_ret != -EFAULT)) {
                        ret = op_ret;
                        goto out_put_keys;
                }

                ret = fault_in_user_writeable(uaddr2);
                if (ret)
                        goto out_put_keys;

                if (!(flags & FLAGS_SHARED))
                        goto retry_private;

                put_futex_key(&key2);
                put_futex_key(&key1);
                goto retry;
        }

        plist_for_each_entry_safe(this, next, &hb1->chain, list) {
                if (match_futex (&this->key, &key1)) {
                        if (this->pi_state || this->rt_waiter) {
                                ret = -EINVAL;
                                goto out_unlock;
                        }
                        mark_wake_futex(&wake_q, this);
                        if (++ret >= nr_wake)
                                break;
                }
        }

        if (op_ret > 0) {
                op_ret = 0;
                plist_for_each_entry_safe(this, next, &hb2->chain, list) {
                        if (match_futex (&this->key, &key2)) {
                                if (this->pi_state || this->rt_waiter) {
                                        ret = -EINVAL;
                                        goto out_unlock;
                                }
                                mark_wake_futex(&wake_q, this);
                                if (++op_ret >= nr_wake2)
                                        break;
                        }
                }
                ret += op_ret;
        }

out_unlock:
        double_unlock_hb(hb1, hb2);
        wake_up_q(&wake_q);
out_put_keys:
        put_futex_key(&key2);
out_put_key1:
        put_futex_key(&key1);
out:
        return ret;
}

/**
 * requeue_futex() - Requeue a futex_q from one hb to another
 * @q:          the futex_q to requeue
 * @hb1:        the source hash_bucket
 * @hb2:        the target hash_bucket
 * @key2:       the new key for the requeued futex_q
 */
static inline
void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
                   struct futex_hash_bucket *hb2, union futex_key *key2)
{

        /*
         * If key1 and key2 hash to the same bucket, no need to
         * requeue.
         */
        if (likely(&hb1->chain != &hb2->chain)) {
                plist_del(&q->list, &hb1->chain);
                hb_waiters_dec(hb1);
                hb_waiters_inc(hb2);
                plist_add(&q->list, &hb2->chain);
                q->lock_ptr = &hb2->lock;
        }
        get_futex_key_refs(key2);
        q->key = *key2;
}

/**
 * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
 * @q:          the futex_q
 * @key:        the key of the requeue target futex
 * @hb:         the hash_bucket of the requeue target futex
 *
 * During futex_requeue, with requeue_pi=1, it is possible to acquire the
 * target futex if it is uncontended or via a lock steal.  Set the futex_q key
 * to the requeue target futex so the waiter can detect the wakeup on the right
 * futex, but remove it from the hb and NULL the rt_waiter so it can detect
 * atomic lock acquisition.  Set the q->lock_ptr to the requeue target hb->lock
 * to protect access to the pi_state to fixup the owner later.  Must be called
 * with both q->lock_ptr and hb->lock held.
 */
static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
                           struct futex_hash_bucket *hb)
{
        get_futex_key_refs(key);
        q->key = *key;

        __unqueue_futex(q);

        WARN_ON(!q->rt_waiter);
        q->rt_waiter = NULL;

        q->lock_ptr = &hb->lock;

        wake_up_state(q->task, TASK_NORMAL);
}

/**
 * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
 * @pifutex:            the user address of the to futex
 * @hb1:                the from futex hash bucket, must be locked by the caller
 * @hb2:                the to futex hash bucket, must be locked by the caller
 * @key1:               the from futex key
 * @key2:               the to futex key
 * @ps:                 address to store the pi_state pointer
 * @set_waiters:        force setting the FUTEX_WAITERS bit (1) or not (0)
 *
 * Try and get the lock on behalf of the top waiter if we can do it atomically.
 * Wake the top waiter if we succeed.  If the caller specified set_waiters,
 * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
 * hb1 and hb2 must be held by the caller.
 *
 * Return:
 *  0 - failed to acquire the lock atomically;
 * >0 - acquired the lock, return value is vpid of the top_waiter
 * <0 - error
 */
static int futex_proxy_trylock_atomic(u32 __user *pifutex,
                                 struct futex_hash_bucket *hb1,
                                 struct futex_hash_bucket *hb2,
                                 union futex_key *key1, union futex_key *key2,
                                 struct futex_pi_state **ps, int set_waiters)
{
        struct futex_q *top_waiter = NULL;
        u32 curval;
        int ret, vpid;

        if (get_futex_value_locked(&curval, pifutex))
                return -EFAULT;

        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        /*
         * Find the top_waiter and determine if there are additional waiters.
         * If the caller intends to requeue more than 1 waiter to pifutex,
         * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
         * as we have means to handle the possible fault.  If not, don't set
         * the bit unecessarily as it will force the subsequent unlock to enter
         * the kernel.
         */
        top_waiter = futex_top_waiter(hb1, key1);

        /* There are no waiters, nothing for us to do. */
        if (!top_waiter)
                return 0;

        /* Ensure we requeue to the expected futex. */
        if (!match_futex(top_waiter->requeue_pi_key, key2))
                return -EINVAL;

        /*
         * Try to take the lock for top_waiter.  Set the FUTEX_WAITERS bit in
         * the contended case or if set_waiters is 1.  The pi_state is returned
         * in ps in contended cases.
         */
        vpid = task_pid_vnr(top_waiter->task);
        ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
                                   set_waiters);
        if (ret == 1) {
                requeue_pi_wake_futex(top_waiter, key2, hb2);
                return vpid;
        }
        return ret;
}

/**
 * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
 * @uaddr1:     source futex user address
 * @flags:      futex flags (FLAGS_SHARED, etc.)
 * @uaddr2:     target futex user address
 * @nr_wake:    number of waiters to wake (must be 1 for requeue_pi)
 * @nr_requeue: number of waiters to requeue (0-INT_MAX)
 * @cmpval:     @uaddr1 expected value (or %NULL)
 * @requeue_pi: if we are attempting to requeue from a non-pi futex to a
 *              pi futex (pi to pi requeue is not supported)
 *
 * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
 * uaddr2 atomically on behalf of the top waiter.
 *
 * Return:
 * >=0 - on success, the number of tasks requeued or woken;
 *  <0 - on error
 */
static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
                         u32 __user *uaddr2, int nr_wake, int nr_requeue,
                         u32 *cmpval, int requeue_pi)
{
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
        int drop_count = 0, task_count = 0, ret;
        struct futex_pi_state *pi_state = NULL;
        struct futex_hash_bucket *hb1, *hb2;
        struct futex_q *this, *next;
        DEFINE_WAKE_Q(wake_q);

        if (requeue_pi) {
                /*
                 * Requeue PI only works on two distinct uaddrs. This
                 * check is only valid for private futexes. See below.
                 */
                if (uaddr1 == uaddr2)
                        return -EINVAL;

                /*
                 * requeue_pi requires a pi_state, try to allocate it now
                 * without any locks in case it fails.
                 */
                if (refill_pi_state_cache())
                        return -ENOMEM;
                /*
                 * requeue_pi must wake as many tasks as it can, up to nr_wake
                 * + nr_requeue, since it acquires the rt_mutex prior to
                 * returning to userspace, so as to not leave the rt_mutex with
                 * waiters and no owner.  However, second and third wake-ups
                 * cannot be predicted as they involve race conditions with the
                 * first wake and a fault while looking up the pi_state.  Both
                 * pthread_cond_signal() and pthread_cond_broadcast() should
                 * use nr_wake=1.
                 */
                if (nr_wake != 1)
                        return -EINVAL;
        }

retry:
        ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
        if (unlikely(ret != 0))
                goto out;
        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
                            requeue_pi ? VERIFY_WRITE : VERIFY_READ);
        if (unlikely(ret != 0))
                goto out_put_key1;

        /*
         * The check above which compares uaddrs is not sufficient for
         * shared futexes. We need to compare the keys:
         */
        if (requeue_pi && match_futex(&key1, &key2)) {
                ret = -EINVAL;
                goto out_put_keys;
        }

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

retry_private:
        hb_waiters_inc(hb2);
        double_lock_hb(hb1, hb2);

        if (likely(cmpval != NULL)) {
                u32 curval;

                ret = get_futex_value_locked(&curval, uaddr1);

                if (unlikely(ret)) {
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);

                        ret = get_user(curval, uaddr1);
                        if (ret)
                                goto out_put_keys;

                        if (!(flags & FLAGS_SHARED))
                                goto retry_private;

                        put_futex_key(&key2);
                        put_futex_key(&key1);
                        goto retry;
                }
                if (curval != *cmpval) {
                        ret = -EAGAIN;
                        goto out_unlock;
                }
        }

        if (requeue_pi && (task_count - nr_wake < nr_requeue)) {
                /*
                 * Attempt to acquire uaddr2 and wake the top waiter. If we
                 * intend to requeue waiters, force setting the FUTEX_WAITERS
                 * bit.  We force this here where we are able to easily handle
                 * faults rather in the requeue loop below.
                 */
                ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
                                                 &key2, &pi_state, nr_requeue);

                /*
                 * At this point the top_waiter has either taken uaddr2 or is
                 * waiting on it.  If the former, then the pi_state will not
                 * exist yet, look it up one more time to ensure we have a
                 * reference to it. If the lock was taken, ret contains the
                 * vpid of the top waiter task.
                 * If the lock was not taken, we have pi_state and an initial
                 * refcount on it. In case of an error we have nothing.
                 */
                if (ret > 0) {
                        WARN_ON(pi_state);
                        drop_count++;
                        task_count++;
                        /*
                         * If we acquired the lock, then the user space value
                         * of uaddr2 should be vpid. It cannot be changed by
                         * the top waiter as it is blocked on hb2 lock if it
                         * tries to do so. If something fiddled with it behind
                         * our back the pi state lookup might unearth it. So
                         * we rather use the known value than rereading and
                         * handing potential crap to lookup_pi_state.
                         *
                         * If that call succeeds then we have pi_state and an
                         * initial refcount on it.
                         */
                        ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
                }

                switch (ret) {
                case 0:
                        /* We hold a reference on the pi state. */
                        break;

                        /* If the above failed, then pi_state is NULL */
                case -EFAULT:
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);
                        put_futex_key(&key2);
                        put_futex_key(&key1);
                        ret = fault_in_user_writeable(uaddr2);
                        if (!ret)
                                goto retry;
                        goto out;
                case -EAGAIN:
                        /*
                         * Two reasons for this:
                         * - Owner is exiting and we just wait for the
                         *   exit to complete.
                         * - The user space value changed.
                         */
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);
                        put_futex_key(&key2);
                        put_futex_key(&key1);
                        cond_resched();
                        goto retry;
                default:
                        goto out_unlock;
                }
        }

        plist_for_each_entry_safe(this, next, &hb1->chain, list) {
                if (task_count - nr_wake >= nr_requeue)
                        break;

                if (!match_futex(&this->key, &key1))
                        continue;

                /*
                 * FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
                 * be paired with each other and no other futex ops.
                 *
                 * We should never be requeueing a futex_q with a pi_state,
                 * which is awaiting a futex_unlock_pi().
                 */
                if ((requeue_pi && !this->rt_waiter) ||
                    (!requeue_pi && this->rt_waiter) ||
                    this->pi_state) {
                        ret = -EINVAL;
                        break;
                }

                /*
                 * Wake nr_wake waiters.  For requeue_pi, if we acquired the
                 * lock, we already woke the top_waiter.  If not, it will be
                 * woken by futex_unlock_pi().
                 */
                if (++task_count <= nr_wake && !requeue_pi) {
                        mark_wake_futex(&wake_q, this);
                        continue;
                }

                /* Ensure we requeue to the expected futex for requeue_pi. */
                if (requeue_pi && !match_futex(this->requeue_pi_key, &key2)) {
                        ret = -EINVAL;
                        break;
                }

                /*
                 * Requeue nr_requeue waiters and possibly one more in the case
                 * of requeue_pi if we couldn't acquire the lock atomically.
                 */
                if (requeue_pi) {
                        /*
                         * Prepare the waiter to take the rt_mutex. Take a
                         * refcount on the pi_state and store the pointer in
                         * the futex_q object of the waiter.
                         */
                        atomic_inc(&pi_state->refcount);
                        this->pi_state = pi_state;
                        ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
                                                        this->rt_waiter,
                                                        this->task);
                        if (ret == 1) {
                                /*
                                 * We got the lock. We do neither drop the
                                 * refcount on pi_state nor clear
                                 * this->pi_state because the waiter needs the
                                 * pi_state for cleaning up the user space
                                 * value. It will drop the refcount after
                                 * doing so.
                                 */
                                requeue_pi_wake_futex(this, &key2, hb2);
                                drop_count++;
                                continue;
                        } else if (ret) {
                                /*
                                 * rt_mutex_start_proxy_lock() detected a
                                 * potential deadlock when we tried to queue
                                 * that waiter. Drop the pi_state reference
                                 * which we took above and remove the pointer
                                 * to the state from the waiters futex_q
                                 * object.
                                 */
                                this->pi_state = NULL;
                                put_pi_state(pi_state);
                                /*
                                 * We stop queueing more waiters and let user
                                 * space deal with the mess.
                                 */
                                break;
                        }
                }
                requeue_futex(this, hb1, hb2, &key2);
                drop_count++;
        }

        /*
         * We took an extra initial reference to the pi_state either
         * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
         * need to drop it here again.
         */
        put_pi_state(pi_state);

out_unlock:
        double_unlock_hb(hb1, hb2);
        wake_up_q(&wake_q);
        hb_waiters_dec(hb2);

        /*
         * drop_futex_key_refs() must be called outside the spinlocks. During
         * the requeue we moved futex_q's from the hash bucket at key1 to the
         * one at key2 and updated their key pointer.  We no longer need to
         * hold the references to key1.
         */
        while (--drop_count >= 0)
                drop_futex_key_refs(&key1);

out_put_keys:
        put_futex_key(&key2);
out_put_key1:
        put_futex_key(&key1);
out:
        return ret ? ret : task_count;
}

/* The key must be already stored in q->key. */
static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
        __acquires(&hb->lock)
{
        struct futex_hash_bucket *hb;

        hb = hash_futex(&q->key);

        /*
         * Increment the counter before taking the lock so that
         * a potential waker won't miss a to-be-slept task that is
         * waiting for the spinlock. This is safe as all queue_lock()
         * users end up calling queue_me(). Similarly, for housekeeping,
         * decrement the counter at queue_unlock() when some error has
         * occurred and we don't end up adding the task to the list.
         */
        hb_waiters_inc(hb);

        q->lock_ptr = &hb->lock;

        spin_lock(&hb->lock); /* implies smp_mb(); (A) */
        return hb;
}

static inline void
queue_unlock(struct futex_hash_bucket *hb)
        __releases(&hb->lock)
{
        spin_unlock(&hb->lock);
        hb_waiters_dec(hb);
}

/**
 * queue_me() - Enqueue the futex_q on the futex_hash_bucket
 * @q:  The futex_q to enqueue
 * @hb: The destination hash bucket
 *
 * The hb->lock must be held by the caller, and is released here. A call to
 * queue_me() is typically paired with exactly one call to unqueue_me().  The
 * exceptions involve the PI related operations, which may use unqueue_me_pi()
 * or nothing if the unqueue is done as part of the wake process and the unqueue
 * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
 * an example).
 */
static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
        __releases(&hb->lock)
{
        int prio;

        /*
         * The priority used to register this element is
         * - either the real thread-priority for the real-time threads
         * (i.e. threads with a priority lower than MAX_RT_PRIO)
         * - or MAX_RT_PRIO for non-RT threads.
         * Thus, all RT-threads are woken first in priority order, and
         * the others are woken last, in FIFO order.
         */
        prio = min(current->normal_prio, MAX_RT_PRIO);

        plist_node_init(&q->list, prio);
        plist_add(&q->list, &hb->chain);
        q->task = current;
        spin_unlock(&hb->lock);
}

/**
 * unqueue_me() - Remove the futex_q from its futex_hash_bucket
 * @q:  The futex_q to unqueue
 *
 * The q->lock_ptr must not be held by the caller. A call to unqueue_me() must
 * be paired with exactly one earlier call to queue_me().
 *
 * Return:
 *   1 - if the futex_q was still queued (and we removed unqueued it);
 *   0 - if the futex_q was already removed by the waking thread
 */
static int unqueue_me(struct futex_q *q)
{
        spinlock_t *lock_ptr;
        int ret = 0;

        /* In the common case we don't take the spinlock, which is nice. */
retry:
        /*
         * q->lock_ptr can change between this read and the following spin_lock.
         * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
         * optimizing lock_ptr out of the logic below.
         */
        lock_ptr = READ_ONCE(q->lock_ptr);
        if (lock_ptr != NULL) {
                spin_lock(lock_ptr);
                /*
                 * q->lock_ptr can change between reading it and
                 * spin_lock(), causing us to take the wrong lock.  This
                 * corrects the race condition.
                 *
                 * Reasoning goes like this: if we have the wrong lock,
                 * q->lock_ptr must have changed (maybe several times)
                 * between reading it and the spin_lock().  It can
                 * change again after the spin_lock() but only if it was
                 * already changed before the spin_lock().  It cannot,
                 * however, change back to the original value.  Therefore
                 * we can detect whether we acquired the correct lock.
                 */
                if (unlikely(lock_ptr != q->lock_ptr)) {
                        spin_unlock(lock_ptr);
                        goto retry;
                }
                __unqueue_futex(q);

                BUG_ON(q->pi_state);

                spin_unlock(lock_ptr);
                ret = 1;
        }

        drop_futex_key_refs(&q->key);
        return ret;
}

/*
 * PI futexes can not be requeued and must remove themself from the
 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry
 * and dropped here.
 */
static void unqueue_me_pi(struct futex_q *q)
        __releases(q->lock_ptr)
{
        __unqueue_futex(q);

        BUG_ON(!q->pi_state);
        put_pi_state(q->pi_state);
        q->pi_state = NULL;

        spin_unlock(q->lock_ptr);
}

/*
 * Fixup the pi_state owner with the new owner.
 *
 * Must be called with hash bucket lock held and mm->sem held for non
 * private futexes.
 */
static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
                                struct task_struct *newowner)
{
        u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
        struct futex_pi_state *pi_state = q->pi_state;
        struct task_struct *oldowner = pi_state->owner;
        u32 uval, uninitialized_var(curval), newval;
        int ret;

        /* Owner died? */
        if (!pi_state->owner)
                newtid |= FUTEX_OWNER_DIED;

        /*
         * We are here either because we stole the rtmutex from the
         * previous highest priority waiter or we are the highest priority
         * waiter but failed to get the rtmutex the first time.
         * We have to replace the newowner TID in the user space variable.
         * This must be atomic as we have to preserve the owner died bit here.
         *
         * Note: We write the user space value _before_ changing the pi_state
         * because we can fault here. Imagine swapped out pages or a fork
         * that marked all the anonymous memory readonly for cow.
         *
         * Modifying pi_state _before_ the user space value would
         * leave the pi_state in an inconsistent state when we fault
         * here, because we need to drop the hash bucket lock to
         * handle the fault. This might be observed in the PID check
         * in lookup_pi_state.
         */
retry:
        if (get_futex_value_locked(&uval, uaddr))
                goto handle_fault;

        while (1) {
                newval = (uval & FUTEX_OWNER_DIED) | newtid;

                if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
                        goto handle_fault;
                if (curval == uval)
                        break;
                uval = curval;
        }

        /*
         * We fixed up user space. Now we need to fix the pi_state
         * itself.
         */
        if (pi_state->owner != NULL) {
                raw_spin_lock_irq(&pi_state->owner->pi_lock);
                WARN_ON(list_empty(&pi_state->list));
                list_del_init(&pi_state->list);
                raw_spin_unlock_irq(&pi_state->owner->pi_lock);
        }

        pi_state->owner = newowner;

        raw_spin_lock_irq(&newowner->pi_lock);
        WARN_ON(!list_empty(&pi_state->list));
        list_add(&pi_state->list, &newowner->pi_state_list);
        raw_spin_unlock_irq(&newowner->pi_lock);
        return 0;

        /*
         * To handle the page fault we need to drop the hash bucket
         * lock here. That gives the other task (either the highest priority
         * waiter itself or the task which stole the rtmutex) the
         * chance to try the fixup of the pi_state. So once we are
         * back from handling the fault we need to check the pi_state
         * after reacquiring the hash bucket lock and before trying to
         * do another fixup. When the fixup has been done already we
         * simply return.
         */
handle_fault:
        spin_unlock(q->lock_ptr);

        ret = fault_in_user_writeable(uaddr);

        spin_lock(q->lock_ptr);

        /*
         * Check if someone else fixed it for us:
         */
        if (pi_state->owner != oldowner)
                return 0;

        if (ret)
                return ret;

        goto retry;
}

static long futex_wait_restart(struct restart_block *restart);

/**
 * fixup_owner() - Post lock pi_state and corner case management
 * @uaddr:      user address of the futex
 * @q:          futex_q (contains pi_state and access to the rt_mutex)
 * @locked:     if the attempt to take the rt_mutex succeeded (1) or not (0)
 *
 * After attempting to lock an rt_mutex, this function is called to cleanup
 * the pi_state owner as well as handle race conditions that may allow us to
 * acquire the lock. Must be called with the hb lock held.
 *
 * Return:
 *  1 - success, lock taken;
 *  0 - success, lock not taken;
 * <0 - on error (-EFAULT)
 */
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{
        struct task_struct *owner;
        int ret = 0;

        if (locked) {
                /*
                 * Got the lock. We might not be the anticipated owner if we
                 * did a lock-steal - fix up the PI-state in that case:
                 */
                if (q->pi_state->owner != current)
                        ret = fixup_pi_state_owner(uaddr, q, current);
                goto out;
        }

        /*
         * Catch the rare case, where the lock was released when we were on the
         * way back before we locked the hash bucket.
         */
        if (q->pi_state->owner == current) {
                /*
                 * Try to get the rt_mutex now. This might fail as some other
                 * task acquired the rt_mutex after we removed ourself from the
                 * rt_mutex waiters list.
                 */
                if (rt_mutex_trylock(&q->pi_state->pi_mutex)) {
                        locked = 1;
                        goto out;
                }

                /*
                 * pi_state is incorrect, some other task did a lock steal and
                 * we returned due to timeout or signal without taking the
                 * rt_mutex. Too late.
                 */
                raw_spin_lock_irq(&q->pi_state->pi_mutex.wait_lock);
                owner = rt_mutex_owner(&q->pi_state->pi_mutex);
                if (!owner)
                        owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
                raw_spin_unlock_irq(&q->pi_state->pi_mutex.wait_lock);
                ret = fixup_pi_state_owner(uaddr, q, owner);
                goto out;
        }

        /*
         * Paranoia check. If we did not take the lock, then we should not be
         * the owner of the rt_mutex.
         */
        if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
                printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
                                "pi-state %p\n", ret,
                                q->pi_state->pi_mutex.owner,
                                q->pi_state->owner);

out:
        return ret ? ret : locked;
}

/**
 * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
 * @hb:         the futex hash bucket, must be locked by the caller
 * @q:          the futex_q to queue up on
 * @timeout:    the prepared hrtimer_sleeper, or null for no timeout
 */
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
                                struct hrtimer_sleeper *timeout)
{
        /*
         * The task state is guaranteed to be set before another task can
         * wake it. set_current_state() is implemented using smp_store_mb() and
         * queue_me() calls spin_unlock() upon completion, both serializing
         * access to the hash list and forcing another memory barrier.
         */
        set_current_state(TASK_INTERRUPTIBLE);
        queue_me(q, hb);

        /* Arm the timer */
        if (timeout)
                hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);

        /*
         * If we have been removed from the hash list, then another task
         * has tried to wake us, and we can skip the call to schedule().
         */
        if (likely(!plist_node_empty(&q->list))) {
                /*
                 * If the timer has already expired, current will already be
                 * flagged for rescheduling. Only call schedule if there
                 * is no timeout, or if it has yet to expire.
                 */
                if (!timeout || timeout->task)
                        freezable_schedule();
        }
        __set_current_state(TASK_RUNNING);
}

/**
 * futex_wait_setup() - Prepare to wait on a futex
 * @uaddr:      the futex userspace address
 * @val:        the expected value
 * @flags:      futex flags (FLAGS_SHARED, etc.)
 * @q:          the associated futex_q
 * @hb:         storage for hash_bucket pointer to be returned to caller
 *
 * Setup the futex_q and locate the hash_bucket.  Get the futex value and
 * compare it with the expected value.  Handle atomic faults internally.
 * Return with the hb lock held and a q.key reference on success, and unlocked
 * with no q.key reference on failure.
 *
 * Return:
 *  0 - uaddr contains val and hb has been locked;
 * <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
 */
static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
                           struct futex_q *q, struct futex_hash_bucket **hb)
{
        u32 uval;
        int ret;

        /*
         * Access the page AFTER the hash-bucket is locked.
         * Order is important:
         *
         *   Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
         *   Userspace waker:  if (cond(var)) { var = new; futex_wake(&var); }
         *
         * The basic logical guarantee of a futex is that it blocks ONLY
         * if cond(var) is known to be true at the time of blocking, for
         * any cond.  If we locked the hash-bucket after testing *uaddr, that
         * would open a race condition where we could block indefinitely with
         * cond(var) false, which would violate the guarantee.
         *
         * On the other hand, we insert q and release the hash-bucket only
         * after testing *uaddr.  This guarantees that futex_wait() will NOT
         * absorb a wakeup if *uaddr does not match the desired values
         * while the syscall executes.
         */
retry:
        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, VERIFY_READ);
        if (unlikely(ret != 0))
                return ret;

retry_private:
        *hb = queue_lock(q);

        ret = get_futex_value_locked(&uval, uaddr);

        if (ret) {
                queue_unlock(*hb);

                ret = get_user(uval, uaddr);
                if (ret)
                        goto out;

                if (!(flags & FLAGS_SHARED))
                        goto retry_private;

                put_futex_key(&q->key);
                goto retry;
        }

        if (uval != val) {
                queue_unlock(*hb);
                ret = -EWOULDBLOCK;
        }

out:
        if (ret)
                put_futex_key(&q->key);
        return ret;
}

static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
                      ktime_t *abs_time, u32 bitset)
{
        struct hrtimer_sleeper timeout, *to = NULL;
        struct restart_block *restart;
        struct futex_hash_bucket *hb;
        struct futex_q q = futex_q_init;
        int ret;

        if (!bitset)
                return -EINVAL;
        q.bitset = bitset;

        if (abs_time) {
                to = &timeout;

                hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
                                      CLOCK_REALTIME : CLOCK_MONOTONIC,
                                      HRTIMER_MODE_ABS);
                hrtimer_init_sleeper(to, current);
                hrtimer_set_expires_range_ns(&to->timer, *abs_time,
                                             current->timer_slack_ns);
        }

retry:
        /*
         * Prepare to wait on uaddr. On success, holds hb lock and increments
         * q.key refs.
         */
        ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
        if (ret)
                goto out;

        /* queue_me and wait for wakeup, timeout, or a signal. */
        futex_wait_queue_me(hb, &q, to);

        /* If we were woken (and unqueued), we succeeded, whatever. */
        ret = 0;
        /* unqueue_me() drops q.key ref */
        if (!unqueue_me(&q))
                goto out;
        ret = -ETIMEDOUT;
        if (to && !to->task)
                goto out;

        /*
         * We expect signal_pending(current), but we might be the
         * victim of a spurious wakeup as well.
         */
        if (!signal_pending(current))
                goto retry;

        ret = -ERESTARTSYS;
        if (!abs_time)
                goto out;

        restart = &current->restart_block;
        restart->fn = futex_wait_restart;
        restart->futex.uaddr = uaddr;
        restart->futex.val = val;
        restart->futex.time = *abs_time;
        restart->futex.bitset = bitset;
        restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;

        ret = -ERESTART_RESTARTBLOCK;

out:
        if (to) {
                hrtimer_cancel(&to->timer);
                destroy_hrtimer_on_stack(&to->timer);
        }
        return ret;
}


static long futex_wait_restart(struct restart_block *restart)
{
        u32 __user *uaddr = restart->futex.uaddr;
        ktime_t t, *tp = NULL;

        if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
                t = restart->futex.time;
                tp = &t;
        }
        restart->fn = do_no_restart_syscall;

        return (long)futex_wait(uaddr, restart->futex.flags,
                                restart->futex.val, tp, restart->futex.bitset);
}


/*
 * Userspace tried a 0 -> TID atomic transition of the futex value
 * and failed. The kernel side here does the whole locking operation:
 * if there are waiters then it will block as a consequence of relying
 * on rt-mutexes, it does PI, etc. (Due to races the kernel might see
 * a 0 value of the futex too.).
 *
 * Also serves as futex trylock_pi()'ing, and due semantics.
 */
static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
                         ktime_t *time, int trylock)
{
        struct hrtimer_sleeper timeout, *to = NULL;
        struct futex_hash_bucket *hb;
        struct futex_q q = futex_q_init;
        int res, ret;

        if (refill_pi_state_cache())
                return -ENOMEM;

        if (time) {
                to = &timeout;
                hrtimer_init_on_stack(&to->timer, CLOCK_REALTIME,
                                      HRTIMER_MODE_ABS);
                hrtimer_init_sleeper(to, current);
                hrtimer_set_expires(&to->timer, *time);
        }

retry:
        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, VERIFY_WRITE);
        if (unlikely(ret != 0))
                goto out;

retry_private:
        hb = queue_lock(&q);

        ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0);
        if (unlikely(ret)) {
                /*
                 * Atomic work succeeded and we got the lock,
                 * or failed. Either way, we do _not_ block.
                 */
                switch (ret) {
                case 1:
                        /* We got the lock. */
                        ret = 0;
                        goto out_unlock_put_key;
                case -EFAULT:
                        goto uaddr_faulted;
                case -EAGAIN:
                        /*
                         * Two reasons for this:
                         * - Task is exiting and we just wait for the
                         *   exit to complete.
                         * - The user space value changed.
                         */
                        queue_unlock(hb);
                        put_futex_key(&q.key);
                        cond_resched();
                        goto retry;
                default:
                        goto out_unlock_put_key;
                }
        }

        /*
         * Only actually queue now that the atomic ops are done:
         */
        queue_me(&q, hb);

        WARN_ON(!q.pi_state);
        /*
         * Block on the PI mutex:
         */
        if (!trylock) {
                ret = rt_mutex_timed_futex_lock(&q.pi_state->pi_mutex, to);
        } else {
                ret = rt_mutex_trylock(&q.pi_state->pi_mutex);
                /* Fixup the trylock return value: */
                ret = ret ? 0 : -EWOULDBLOCK;
        }

        spin_lock(q.lock_ptr);
        /*
         * Fixup the pi_state owner and possibly acquire the lock if we
         * haven't already.
         */
        res = fixup_owner(uaddr, &q, !ret);
        /*
         * If fixup_owner() returned an error, proprogate that.  If it acquired
         * the lock, clear our -ETIMEDOUT or -EINTR.
         */
        if (res)
                ret = (res < 0) ? res : 0;

        /*
         * If fixup_owner() faulted and was unable to handle the fault, unlock
         * it and return the fault to userspace.
         */
        if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
                rt_mutex_unlock(&q.pi_state->pi_mutex);

        /* Unqueue and drop the lock */
        unqueue_me_pi(&q);

        goto out_put_key;

out_unlock_put_key:
        queue_unlock(hb);

out_put_key:
        put_futex_key(&q.key);
out:
        if (to)
                destroy_hrtimer_on_stack(&to->timer);
        return ret != -EINTR ? ret : -ERESTARTNOINTR;

uaddr_faulted:
        queue_unlock(hb);

        ret = fault_in_user_writeable(uaddr);
        if (ret)
                goto out_put_key;

        if (!(flags & FLAGS_SHARED))
                goto retry_private;

        put_futex_key(&q.key);
        goto retry;
}

/*
 * Userspace attempted a TID -> 0 atomic transition, and failed.
 * This is the in-kernel slowpath: we look up the PI state (if any),
 * and do the rt-mutex unlock.
 */
static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
{
        u32 uninitialized_var(curval), uval, vpid = task_pid_vnr(current);
        union futex_key key = FUTEX_KEY_INIT;
        struct futex_hash_bucket *hb;
        struct futex_q *match;
        int ret;

retry:
        if (get_user(uval, uaddr))
                return -EFAULT;
        /*
         * We release only a lock we actually own:
         */
        if ((uval & FUTEX_TID_MASK) != vpid)
                return -EPERM;

        ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_WRITE);
        if (ret)
                return ret;

        hb = hash_futex(&key);
        spin_lock(&hb->lock);

        /*
         * Check waiters first. We do not trust user space values at
         * all and we at least want to know if user space fiddled
         * with the futex value instead of blindly unlocking.
         */
        match = futex_top_waiter(hb, &key);
        if (match) {
                ret = wake_futex_pi(uaddr, uval, match, hb);
                /*
                 * In case of success wake_futex_pi dropped the hash
                 * bucket lock.
                 */
                if (!ret)
                        goto out_putkey;
                /*
                 * The atomic access to the futex value generated a
                 * pagefault, so retry the user-access and the wakeup:
                 */
                if (ret == -EFAULT)
                        goto pi_faulted;
                /*
                 * A unconditional UNLOCK_PI op raced against a waiter
                 * setting the FUTEX_WAITERS bit. Try again.
                 */
                if (ret == -EAGAIN) {
                        spin_unlock(&hb->lock);
                        put_futex_key(&key);
                        goto retry;
                }
                /*
                 * wake_futex_pi has detected invalid state. Tell user
                 * space.
                 */
                goto out_unlock;
        }

        /*
         * We have no kernel internal state, i.e. no waiters in the
         * kernel. Waiters which are about to queue themselves are stuck
         * on hb->lock. So we can safely ignore them. We do neither
         * preserve the WAITERS bit not the OWNER_DIED one. We are the
         * owner.
         */
        if (cmpxchg_futex_value_locked(&curval, uaddr, uval, 0))
                goto pi_faulted;

        /*
         * If uval has changed, let user space handle it.
         */
        ret = (curval == uval) ? 0 : -EAGAIN;

out_unlock:
        spin_unlock(&hb->lock);
out_putkey:
        put_futex_key(&key);
        return ret;

pi_faulted:
        spin_unlock(&hb->lock);
        put_futex_key(&key);

        ret = fault_in_user_writeable(uaddr);
        if (!ret)
                goto retry;

        return ret;
}

/**
 * handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex
 * @hb:         the hash_bucket futex_q was original enqueued on
 * @q:          the futex_q woken while waiting to be requeued
 * @key2:       the futex_key of the requeue target futex
 * @timeout:    the timeout associated with the wait (NULL if none)
 *
 * Detect if the task was woken on the initial futex as opposed to the requeue
 * target futex.  If so, determine if it was a timeout or a signal that caused
 * the wakeup and return the appropriate error code to the caller.  Must be
 * called with the hb lock held.
 *
 * Return:
 *  0 = no early wakeup detected;
 * <0 = -ETIMEDOUT or -ERESTARTNOINTR
 */
static inline
int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
                                   struct futex_q *q, union futex_key *key2,
                                   struct hrtimer_sleeper *timeout)
{
        int ret = 0;

        /*
         * With the hb lock held, we avoid races while we process the wakeup.
         * We only need to hold hb (and not hb2) to ensure atomicity as the
         * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
         * It can't be requeued from uaddr2 to something else since we don't
         * support a PI aware source futex for requeue.
         */
        if (!match_futex(&q->key, key2)) {
                WARN_ON(q->lock_ptr && (&hb->lock != q->lock_ptr));
                /*
                 * We were woken prior to requeue by a timeout or a signal.
                 * Unqueue the futex_q and determine which it was.
                 */
                plist_del(&q->list, &hb->chain);
                hb_waiters_dec(hb);

                /* Handle spurious wakeups gracefully */
                ret = -EWOULDBLOCK;
                if (timeout && !timeout->task)
                        ret = -ETIMEDOUT;
                else if (signal_pending(current))
                        ret = -ERESTARTNOINTR;
        }
        return ret;
}

/**
 * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
 * @uaddr:      the futex we initially wait on (non-pi)
 * @flags:      futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
 *              the same type, no requeueing from private to shared, etc.
 * @val:        the expected value of uaddr
 * @abs_time:   absolute timeout
 * @bitset:     32 bit wakeup bitset set by userspace, defaults to all
 * @uaddr2:     the pi futex we will take prior to returning to user-space
 *
 * The caller will wait on uaddr and will be requeued by futex_requeue() to
 * uaddr2 which must be PI aware and unique from uaddr.  Normal wakeup will wake
 * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
 * userspace.  This ensures the rt_mutex maintains an owner when it has waiters;
 * without one, the pi logic would not know which task to boost/deboost, if
 * there was a need to.
 *
 * We call schedule in futex_wait_queue_me() when we enqueue and return there
 * via the following--
 * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
 * 2) wakeup on uaddr2 after a requeue
 * 3) signal
 * 4) timeout
 *
 * If 3, cleanup and return -ERESTARTNOINTR.
 *
 * If 2, we may then block on trying to take the rt_mutex and return via:
 * 5) successful lock
 * 6) signal
 * 7) timeout
 * 8) other lock acquisition failure
 *
 * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
 *
 * If 4 or 7, we cleanup and return with -ETIMEDOUT.
 *
 * Return:
 *  0 - On success;
 * <0 - On error
 */
static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
                                 u32 val, ktime_t *abs_time, u32 bitset,
                                 u32 __user *uaddr2)
{
        struct hrtimer_sleeper timeout, *to = NULL;
        struct rt_mutex_waiter rt_waiter;
        struct futex_hash_bucket *hb;
        union futex_key key2 = FUTEX_KEY_INIT;
        struct futex_q q = futex_q_init;
        int res, ret;

        if (uaddr == uaddr2)
                return -EINVAL;

        if (!bitset)
                return -EINVAL;

        if (abs_time) {
                to = &timeout;
                hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
                                      CLOCK_REALTIME : CLOCK_MONOTONIC,
                                      HRTIMER_MODE_ABS);
                hrtimer_init_sleeper(to, current);
                hrtimer_set_expires_range_ns(&to->timer, *abs_time,
                                             current->timer_slack_ns);
        }

        /*
         * The waiter is allocated on our stack, manipulated by the requeue
         * code while we sleep on uaddr.
         */
        debug_rt_mutex_init_waiter(&rt_waiter);
        RB_CLEAR_NODE(&rt_waiter.pi_tree_entry);
        RB_CLEAR_NODE(&rt_waiter.tree_entry);
        rt_waiter.task = NULL;

        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
        if (unlikely(ret != 0))
                goto out;

        q.bitset = bitset;
        q.rt_waiter = &rt_waiter;
        q.requeue_pi_key = &key2;

        /*
         * Prepare to wait on uaddr. On success, increments q.key (key1) ref
         * count.
         */
        ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
        if (ret)
                goto out_key2;

        /*
         * The check above which compares uaddrs is not sufficient for
         * shared futexes. We need to compare the keys:
         */
        if (match_futex(&q.key, &key2)) {
                queue_unlock(hb);
                ret = -EINVAL;
                goto out_put_keys;
        }

        /* Queue the futex_q, drop the hb lock, wait for wakeup. */
        futex_wait_queue_me(hb, &q, to);

        spin_lock(&hb->lock);
        ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
        spin_unlock(&hb->lock);
        if (ret)
                goto out_put_keys;

        /*
         * In order for us to be here, we know our q.key == key2, and since
         * we took the hb->lock above, we also know that futex_requeue() has
         * completed and we no longer have to concern ourselves with a wakeup
         * race with the atomic proxy lock acquisition by the requeue code. The
         * futex_requeue dropped our key1 reference and incremented our key2
         * reference count.
         */

        /* Check if the requeue code acquired the second futex for us. */
        if (!q.rt_waiter) {
                /*
                 * Got the lock. We might not be the anticipated owner if we
                 * did a lock-steal - fix up the PI-state in that case.
                 */
                if (q.pi_state && (q.pi_state->owner != current)) {
                        spin_lock(q.lock_ptr);
                        ret = fixup_pi_state_owner(uaddr2, &q, current);
                        if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current)
                                rt_mutex_unlock(&q.pi_state->pi_mutex);
                        /*
                         * Drop the reference to the pi state which
                         * the requeue_pi() code acquired for us.
                         */
                        put_pi_state(q.pi_state);
                        spin_unlock(q.lock_ptr);
                }
        } else {
                struct rt_mutex *pi_mutex;

                /*
                 * We have been woken up by futex_unlock_pi(), a timeout, or a
                 * signal.  futex_unlock_pi() will not destroy the lock_ptr nor
                 * the pi_state.
                 */
                WARN_ON(!q.pi_state);
                pi_mutex = &q.pi_state->pi_mutex;
                ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter);
                debug_rt_mutex_free_waiter(&rt_waiter);

                spin_lock(q.lock_ptr);
                /*
                 * Fixup the pi_state owner and possibly acquire the lock if we
                 * haven't already.
                 */
                res = fixup_owner(uaddr2, &q, !ret);
                /*
                 * If fixup_owner() returned an error, proprogate that.  If it
                 * acquired the lock, clear -ETIMEDOUT or -EINTR.
                 */
                if (res)
                        ret = (res < 0) ? res : 0;

                /*
                 * If fixup_pi_state_owner() faulted and was unable to handle
                 * the fault, unlock the rt_mutex and return the fault to
                 * userspace.
                 */
                if (ret && rt_mutex_owner(pi_mutex) == current)
                        rt_mutex_unlock(pi_mutex);

                /* Unqueue and drop the lock. */
                unqueue_me_pi(&q);
        }

        if (ret == -EINTR) {
                /*
                 * We've already been requeued, but cannot restart by calling
                 * futex_lock_pi() directly. We could restart this syscall, but
                 * it would detect that the user space "val" changed and return
                 * -EWOULDBLOCK.  Save the overhead of the restart and return
                 * -EWOULDBLOCK directly.
                 */
                ret = -EWOULDBLOCK;
        }

out_put_keys:
        put_futex_key(&q.key);
out_key2:
        put_futex_key(&key2);

out:
        if (to) {
                hrtimer_cancel(&to->timer);
                destroy_hrtimer_on_stack(&to->timer);
        }
        return ret;
}

/*
 * Support for robust futexes: the kernel cleans up held futexes at
 * thread exit time.
 *
 * Implementation: user-space maintains a per-thread list of locks it
 * is holding. Upon do_exit(), the kernel carefully walks this list,
 * and marks all locks that are owned by this thread with the
 * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
 * always manipulated with the lock held, so the list is private and
 * per-thread. Userspace also maintains a per-thread 'list_op_pending'
 * field, to allow the kernel to clean up if the thread dies after
 * acquiring the lock, but just before it could have added itself to
 * the list. There can only be one such pending lock.
 */

/**
 * sys_set_robust_list() - Set the robust-futex list head of a task
 * @head:       pointer to the list-head
 * @len:        length of the list-head, as userspace expects
 */
SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
                size_t, len)
{
        if (!futex_cmpxchg_enabled)
                return -ENOSYS;
        /*
         * The kernel knows only one size for now:
         */
        if (unlikely(len != sizeof(*head)))
                return -EINVAL;

        current->robust_list = head;

        return 0;
}

/**
 * sys_get_robust_list() - Get the robust-futex list head of a task
 * @pid:        pid of the process [zero for current task]
 * @head_ptr:   pointer to a list-head pointer, the kernel fills it in
 * @len_ptr:    pointer to a length field, the kernel fills in the header size
 */
SYSCALL_DEFINE3(get_robust_list, int, pid,
                struct robust_list_head __user * __user *, head_ptr,
                size_t __user *, len_ptr)
{
        struct robust_list_head __user *head;
        unsigned long ret;
        struct task_struct *p;

        if (!futex_cmpxchg_enabled)
                return -ENOSYS;

        rcu_read_lock();

        ret = -ESRCH;
        if (!pid)
                p = current;
        else {
                p = find_task_by_vpid(pid);
                if (!p)
                        goto err_unlock;
        }

        ret = -EPERM;
        if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
                goto err_unlock;

        head = p->robust_list;
        rcu_read_unlock();

        if (put_user(sizeof(*head), len_ptr))
                return -EFAULT;
        return put_user(head, head_ptr);

err_unlock:
        rcu_read_unlock();

        return ret;
}

/*
 * Process a futex-list entry, check whether it's owned by the
 * dying task, and do notification if so:
 */
int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
{
        u32 uval, uninitialized_var(nval), mval;

retry:
        if (get_user(uval, uaddr))
                return -1;

        if ((uval & FUTEX_TID_MASK) == task_pid_vnr(curr)) {
                /*
                 * Ok, this dying thread is truly holding a futex
                 * of interest. Set the OWNER_DIED bit atomically
                 * via cmpxchg, and if the value had FUTEX_WAITERS
                 * set, wake up a waiter (if any). (We have to do a
                 * futex_wake() even if OWNER_DIED is already set -
                 * to handle the rare but possible case of recursive
                 * thread-death.) The rest of the cleanup is done in
                 * userspace.
                 */
                mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
                /*
                 * We are not holding a lock here, but we want to have
                 * the pagefault_disable/enable() protection because
                 * we want to handle the fault gracefully. If the
                 * access fails we try to fault in the futex with R/W
                 * verification via get_user_pages. get_user() above
                 * does not guarantee R/W access. If that fails we
                 * give up and leave the futex locked.
                 */
                if (cmpxchg_futex_value_locked(&nval, uaddr, uval, mval)) {
                        if (fault_in_user_writeable(uaddr))
                                return -1;
                        goto retry;
                }
                if (nval != uval)
                        goto retry;

                /*
                 * Wake robust non-PI futexes here. The wakeup of
                 * PI futexes happens in exit_pi_state():
                 */
                if (!pi && (uval & FUTEX_WAITERS))
                        futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
        }
        return 0;
}

/*
 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
 */
static inline int fetch_robust_entry(struct robust_list __user **entry,
                                     struct robust_list __user * __user *head,
                                     unsigned int *pi)
{
        unsigned long uentry;

        if (get_user(uentry, (unsigned long __user *)head))
                return -EFAULT;

        *entry = (void __user *)(uentry & ~1UL);
        *pi = uentry & 1;

        return 0;
}

/*
 * Walk curr->robust_list (very carefully, it's a userspace list!)
 * and mark any locks found there dead, and notify any waiters.
 *
 * We silently return on any sign of list-walking problem.
 */
void exit_robust_list(struct task_struct *curr)
{
        struct robust_list_head __user *head = curr->robust_list;
        struct robust_list __user *entry, *next_entry, *pending;
        unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
        unsigned int uninitialized_var(next_pi);
        unsigned long futex_offset;
        int rc;

        if (!futex_cmpxchg_enabled)
                return;

        /*
         * Fetch the list head (which was registered earlier, via
         * sys_set_robust_list()):
         */
        if (fetch_robust_entry(&entry, &head->list.next, &pi))
                return;
        /*
         * Fetch the relative futex offset:
         */
        if (get_user(futex_offset, &head->futex_offset))
                return;
        /*
         * Fetch any possibly pending lock-add first, and handle it
         * if it exists:
         */
        if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
                return;

        next_entry = NULL;      /* avoid warning with gcc */
        while (entry != &head->list) {
                /*
                 * Fetch the next entry in the list before calling
                 * handle_futex_death:
                 */
                rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
                /*
                 * A pending lock might already be on the list, so
                 * don't process it twice:
                 */
                if (entry != pending)
                        if (handle_futex_death((void __user *)entry + futex_offset,
                                                curr, pi))
                                return;
                if (rc)
                        return;
                entry = next_entry;
                pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
                if (!--limit)
                        break;

                cond_resched();
        }

        if (pending)
                handle_futex_death((void __user *)pending + futex_offset,
                                   curr, pip);
}

long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
                u32 __user *uaddr2, u32 val2, u32 val3)
{
        int cmd = op & FUTEX_CMD_MASK;
        unsigned int flags = 0;

        if (!(op & FUTEX_PRIVATE_FLAG))
                flags |= FLAGS_SHARED;

        if (op & FUTEX_CLOCK_REALTIME) {
                flags |= FLAGS_CLOCKRT;
                if (cmd != FUTEX_WAIT && cmd != FUTEX_WAIT_BITSET && \
                    cmd != FUTEX_WAIT_REQUEUE_PI)
                        return -ENOSYS;
        }

        switch (cmd) {
        case FUTEX_LOCK_PI:
        case FUTEX_UNLOCK_PI:
        case FUTEX_TRYLOCK_PI:
        case FUTEX_WAIT_REQUEUE_PI:
        case FUTEX_CMP_REQUEUE_PI:
                if (!futex_cmpxchg_enabled)
                        return -ENOSYS;
        }

        switch (cmd) {
        case FUTEX_WAIT:
                val3 = FUTEX_BITSET_MATCH_ANY;
        case FUTEX_WAIT_BITSET:
                return futex_wait(uaddr, flags, val, timeout, val3);
        case FUTEX_WAKE:
                val3 = FUTEX_BITSET_MATCH_ANY;
        case FUTEX_WAKE_BITSET:
                return futex_wake(uaddr, flags, val, val3);
        case FUTEX_REQUEUE:
                return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0);
        case FUTEX_CMP_REQUEUE:
                return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0);
        case FUTEX_WAKE_OP:
                return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
        case FUTEX_LOCK_PI:
                return futex_lock_pi(uaddr, flags, timeout, 0);
        case FUTEX_UNLOCK_PI:
                return futex_unlock_pi(uaddr, flags);
        case FUTEX_TRYLOCK_PI:
                return futex_lock_pi(uaddr, flags, NULL, 1);
        case FUTEX_WAIT_REQUEUE_PI:
                val3 = FUTEX_BITSET_MATCH_ANY;
                return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3,
                                             uaddr2);
        case FUTEX_CMP_REQUEUE_PI:
                return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
        }
        return -ENOSYS;
}


SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
                struct timespec __user *, utime, u32 __user *, uaddr2,
                u32, val3)
{
        struct timespec ts;
        ktime_t t, *tp = NULL;
        u32 val2 = 0;
        int cmd = op & FUTEX_CMD_MASK;

        if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
                      cmd == FUTEX_WAIT_BITSET ||
                      cmd == FUTEX_WAIT_REQUEUE_PI)) {
                if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
                        return -EFAULT;
                if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
                        return -EFAULT;
                if (!timespec_valid(&ts))
                        return -EINVAL;

                t = timespec_to_ktime(ts);
                if (cmd == FUTEX_WAIT)
                        t = ktime_add_safe(ktime_get(), t);
                tp = &t;
        }
        /*
         * requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
         * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
         */
        if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
            cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
                val2 = (u32) (unsigned long) utime;

        return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}

static void __init futex_detect_cmpxchg(void)
{
#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
        u32 curval;

        /*
         * This will fail and we want it. Some arch implementations do
         * runtime detection of the futex_atomic_cmpxchg_inatomic()
         * functionality. We want to know that before we call in any
         * of the complex code paths. Also we want to prevent
         * registration of robust lists in that case. NULL is
         * guaranteed to fault and we get -EFAULT on functional
         * implementation, the non-functional ones will return
         * -ENOSYS.
         */
        if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
                futex_cmpxchg_enabled = 1;
#endif
}

static int __init futex_init(void)
{
        unsigned int futex_shift;
        unsigned long i;

#if CONFIG_BASE_SMALL
        futex_hashsize = 16;
#else
        futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
#endif

        futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
                                               futex_hashsize, 0,
                                               futex_hashsize < 256 ? HASH_SMALL : 0,
                                               &futex_shift, NULL,
                                               futex_hashsize, futex_hashsize);
        futex_hashsize = 1UL << futex_shift;

        futex_detect_cmpxchg();

        for (i = 0; i < futex_hashsize; i++) {
                atomic_set(&futex_queues[i].waiters, 0);
                plist_head_init(&futex_queues[i].chain);
                spin_lock_init(&futex_queues[i].lock);
        }

        return 0;
}
core_initcall(futex_init);