x86/amd-iommu: Add per IOMMU reference counting

[linux/fpc-iii.git] / fs / dcache.c

/*
 * fs/dcache.c
 *
 * Complete reimplementation
 * (C) 1997 Thomas Schoebel-Theuer,
 * with heavy changes by Linus Torvalds
 */

/*
 * Notes on the allocation strategy:
 *
 * The dcache is a master of the icache - whenever a dcache entry
 * exists, the inode will always exist. "iput()" is done either when
 * the dcache entry is deleted or garbage collected.
 */

#include <linux/syscalls.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/fsnotify.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/hash.h>
#include <linux/cache.h>
#include <linux/module.h>
#include <linux/mount.h>
#include <linux/file.h>
#include <asm/uaccess.h>
#include <linux/security.h>
#include <linux/seqlock.h>
#include <linux/swap.h>
#include <linux/bootmem.h>
#include <linux/fs_struct.h>
#include <linux/hardirq.h>
#include "internal.h"

int sysctl_vfs_cache_pressure __read_mostly = 100;
EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);

 __cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lock);
__cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);

EXPORT_SYMBOL(dcache_lock);

static struct kmem_cache *dentry_cache __read_mostly;

#define DNAME_INLINE_LEN (sizeof(struct dentry)-offsetof(struct dentry,d_iname))

/*
 * This is the single most critical data structure when it comes
 * to the dcache: the hashtable for lookups. Somebody should try
 * to make this good - I've just made it work.
 *
 * This hash-function tries to avoid losing too many bits of hash
 * information, yet avoid using a prime hash-size or similar.
 */
#define D_HASHBITS     d_hash_shift
#define D_HASHMASK     d_hash_mask

static unsigned int d_hash_mask __read_mostly;
static unsigned int d_hash_shift __read_mostly;
static struct hlist_head *dentry_hashtable __read_mostly;

/* Statistics gathering. */
struct dentry_stat_t dentry_stat = {
        .age_limit = 45,
};

static void __d_free(struct dentry *dentry)
{
        WARN_ON(!list_empty(&dentry->d_alias));
        if (dname_external(dentry))
                kfree(dentry->d_name.name);
        kmem_cache_free(dentry_cache, dentry); 
}

static void d_callback(struct rcu_head *head)
{
        struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu);
        __d_free(dentry);
}

/*
 * no dcache_lock, please.  The caller must decrement dentry_stat.nr_dentry
 * inside dcache_lock.
 */
static void d_free(struct dentry *dentry)
{
        if (dentry->d_op && dentry->d_op->d_release)
                dentry->d_op->d_release(dentry);
        /* if dentry was never inserted into hash, immediate free is OK */
        if (hlist_unhashed(&dentry->d_hash))
                __d_free(dentry);
        else
                call_rcu(&dentry->d_u.d_rcu, d_callback);
}

/*
 * Release the dentry's inode, using the filesystem
 * d_iput() operation if defined.
 */
static void dentry_iput(struct dentry * dentry)
        __releases(dentry->d_lock)
        __releases(dcache_lock)
{
        struct inode *inode = dentry->d_inode;
        if (inode) {
                dentry->d_inode = NULL;
                list_del_init(&dentry->d_alias);
                spin_unlock(&dentry->d_lock);
                spin_unlock(&dcache_lock);
                if (!inode->i_nlink)
                        fsnotify_inoderemove(inode);
                if (dentry->d_op && dentry->d_op->d_iput)
                        dentry->d_op->d_iput(dentry, inode);
                else
                        iput(inode);
        } else {
                spin_unlock(&dentry->d_lock);
                spin_unlock(&dcache_lock);
        }
}

/*
 * dentry_lru_(add|add_tail|del|del_init) must be called with dcache_lock held.
 */
static void dentry_lru_add(struct dentry *dentry)
{
        list_add(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
        dentry->d_sb->s_nr_dentry_unused++;
        dentry_stat.nr_unused++;
}

static void dentry_lru_add_tail(struct dentry *dentry)
{
        list_add_tail(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
        dentry->d_sb->s_nr_dentry_unused++;
        dentry_stat.nr_unused++;
}

static void dentry_lru_del(struct dentry *dentry)
{
        if (!list_empty(&dentry->d_lru)) {
                list_del(&dentry->d_lru);
                dentry->d_sb->s_nr_dentry_unused--;
                dentry_stat.nr_unused--;
        }
}

static void dentry_lru_del_init(struct dentry *dentry)
{
        if (likely(!list_empty(&dentry->d_lru))) {
                list_del_init(&dentry->d_lru);
                dentry->d_sb->s_nr_dentry_unused--;
                dentry_stat.nr_unused--;
        }
}

/**
 * d_kill - kill dentry and return parent
 * @dentry: dentry to kill
 *
 * The dentry must already be unhashed and removed from the LRU.
 *
 * If this is the root of the dentry tree, return NULL.
 */
static struct dentry *d_kill(struct dentry *dentry)
        __releases(dentry->d_lock)
        __releases(dcache_lock)
{
        struct dentry *parent;

        list_del(&dentry->d_u.d_child);
        dentry_stat.nr_dentry--;        /* For d_free, below */
        /*drops the locks, at that point nobody can reach this dentry */
        dentry_iput(dentry);
        if (IS_ROOT(dentry))
                parent = NULL;
        else
                parent = dentry->d_parent;
        d_free(dentry);
        return parent;
}

/* 
 * This is dput
 *
 * This is complicated by the fact that we do not want to put
 * dentries that are no longer on any hash chain on the unused
 * list: we'd much rather just get rid of them immediately.
 *
 * However, that implies that we have to traverse the dentry
 * tree upwards to the parents which might _also_ now be
 * scheduled for deletion (it may have been only waiting for
 * its last child to go away).
 *
 * This tail recursion is done by hand as we don't want to depend
 * on the compiler to always get this right (gcc generally doesn't).
 * Real recursion would eat up our stack space.
 */

/*
 * dput - release a dentry
 * @dentry: dentry to release 
 *
 * Release a dentry. This will drop the usage count and if appropriate
 * call the dentry unlink method as well as removing it from the queues and
 * releasing its resources. If the parent dentries were scheduled for release
 * they too may now get deleted.
 *
 * no dcache lock, please.
 */

void dput(struct dentry *dentry)
{
        if (!dentry)
                return;

repeat:
        if (atomic_read(&dentry->d_count) == 1)
                might_sleep();
        if (!atomic_dec_and_lock(&dentry->d_count, &dcache_lock))
                return;

        spin_lock(&dentry->d_lock);
        if (atomic_read(&dentry->d_count)) {
                spin_unlock(&dentry->d_lock);
                spin_unlock(&dcache_lock);
                return;
        }

        /*
         * AV: ->d_delete() is _NOT_ allowed to block now.
         */
        if (dentry->d_op && dentry->d_op->d_delete) {
                if (dentry->d_op->d_delete(dentry))
                        goto unhash_it;
        }
        /* Unreachable? Get rid of it */
        if (d_unhashed(dentry))
                goto kill_it;
        if (list_empty(&dentry->d_lru)) {
                dentry->d_flags |= DCACHE_REFERENCED;
                dentry_lru_add(dentry);
        }
        spin_unlock(&dentry->d_lock);
        spin_unlock(&dcache_lock);
        return;

unhash_it:
        __d_drop(dentry);
kill_it:
        /* if dentry was on the d_lru list delete it from there */
        dentry_lru_del(dentry);
        dentry = d_kill(dentry);
        if (dentry)
                goto repeat;
}

/**
 * d_invalidate - invalidate a dentry
 * @dentry: dentry to invalidate
 *
 * Try to invalidate the dentry if it turns out to be
 * possible. If there are other dentries that can be
 * reached through this one we can't delete it and we
 * return -EBUSY. On success we return 0.
 *
 * no dcache lock.
 */
 
int d_invalidate(struct dentry * dentry)
{
        /*
         * If it's already been dropped, return OK.
         */
        spin_lock(&dcache_lock);
        if (d_unhashed(dentry)) {
                spin_unlock(&dcache_lock);
                return 0;
        }
        /*
         * Check whether to do a partial shrink_dcache
         * to get rid of unused child entries.
         */
        if (!list_empty(&dentry->d_subdirs)) {
                spin_unlock(&dcache_lock);
                shrink_dcache_parent(dentry);
                spin_lock(&dcache_lock);
        }

        /*
         * Somebody else still using it?
         *
         * If it's a directory, we can't drop it
         * for fear of somebody re-populating it
         * with children (even though dropping it
         * would make it unreachable from the root,
         * we might still populate it if it was a
         * working directory or similar).
         */
        spin_lock(&dentry->d_lock);
        if (atomic_read(&dentry->d_count) > 1) {
                if (dentry->d_inode && S_ISDIR(dentry->d_inode->i_mode)) {
                        spin_unlock(&dentry->d_lock);
                        spin_unlock(&dcache_lock);
                        return -EBUSY;
                }
        }

        __d_drop(dentry);
        spin_unlock(&dentry->d_lock);
        spin_unlock(&dcache_lock);
        return 0;
}

/* This should be called _only_ with dcache_lock held */

static inline struct dentry * __dget_locked(struct dentry *dentry)
{
        atomic_inc(&dentry->d_count);
        dentry_lru_del_init(dentry);
        return dentry;
}

struct dentry * dget_locked(struct dentry *dentry)
{
        return __dget_locked(dentry);
}

/**
 * d_find_alias - grab a hashed alias of inode
 * @inode: inode in question
 * @want_discon:  flag, used by d_splice_alias, to request
 *          that only a DISCONNECTED alias be returned.
 *
 * If inode has a hashed alias, or is a directory and has any alias,
 * acquire the reference to alias and return it. Otherwise return NULL.
 * Notice that if inode is a directory there can be only one alias and
 * it can be unhashed only if it has no children, or if it is the root
 * of a filesystem.
 *
 * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer
 * any other hashed alias over that one unless @want_discon is set,
 * in which case only return an IS_ROOT, DCACHE_DISCONNECTED alias.
 */

static struct dentry * __d_find_alias(struct inode *inode, int want_discon)
{
        struct list_head *head, *next, *tmp;
        struct dentry *alias, *discon_alias=NULL;

        head = &inode->i_dentry;
        next = inode->i_dentry.next;
        while (next != head) {
                tmp = next;
                next = tmp->next;
                prefetch(next);
                alias = list_entry(tmp, struct dentry, d_alias);
                if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
                        if (IS_ROOT(alias) &&
                            (alias->d_flags & DCACHE_DISCONNECTED))
                                discon_alias = alias;
                        else if (!want_discon) {
                                __dget_locked(alias);
                                return alias;
                        }
                }
        }
        if (discon_alias)
                __dget_locked(discon_alias);
        return discon_alias;
}

struct dentry * d_find_alias(struct inode *inode)
{
        struct dentry *de = NULL;

        if (!list_empty(&inode->i_dentry)) {
                spin_lock(&dcache_lock);
                de = __d_find_alias(inode, 0);
                spin_unlock(&dcache_lock);
        }
        return de;
}

/*
 *      Try to kill dentries associated with this inode.
 * WARNING: you must own a reference to inode.
 */
void d_prune_aliases(struct inode *inode)
{
        struct dentry *dentry;
restart:
        spin_lock(&dcache_lock);
        list_for_each_entry(dentry, &inode->i_dentry, d_alias) {
                spin_lock(&dentry->d_lock);
                if (!atomic_read(&dentry->d_count)) {
                        __dget_locked(dentry);
                        __d_drop(dentry);
                        spin_unlock(&dentry->d_lock);
                        spin_unlock(&dcache_lock);
                        dput(dentry);
                        goto restart;
                }
                spin_unlock(&dentry->d_lock);
        }
        spin_unlock(&dcache_lock);
}

/*
 * Throw away a dentry - free the inode, dput the parent.  This requires that
 * the LRU list has already been removed.
 *
 * Try to prune ancestors as well.  This is necessary to prevent
 * quadratic behavior of shrink_dcache_parent(), but is also expected
 * to be beneficial in reducing dentry cache fragmentation.
 */
static void prune_one_dentry(struct dentry * dentry)
        __releases(dentry->d_lock)
        __releases(dcache_lock)
        __acquires(dcache_lock)
{
        __d_drop(dentry);
        dentry = d_kill(dentry);

        /*
         * Prune ancestors.  Locking is simpler than in dput(),
         * because dcache_lock needs to be taken anyway.
         */
        spin_lock(&dcache_lock);
        while (dentry) {
                if (!atomic_dec_and_lock(&dentry->d_count, &dentry->d_lock))
                        return;

                if (dentry->d_op && dentry->d_op->d_delete)
                        dentry->d_op->d_delete(dentry);
                dentry_lru_del_init(dentry);
                __d_drop(dentry);
                dentry = d_kill(dentry);
                spin_lock(&dcache_lock);
        }
}

/*
 * Shrink the dentry LRU on a given superblock.
 * @sb   : superblock to shrink dentry LRU.
 * @count: If count is NULL, we prune all dentries on superblock.
 * @flags: If flags is non-zero, we need to do special processing based on
 * which flags are set. This means we don't need to maintain multiple
 * similar copies of this loop.
 */
static void __shrink_dcache_sb(struct super_block *sb, int *count, int flags)
{
        LIST_HEAD(referenced);
        LIST_HEAD(tmp);
        struct dentry *dentry;
        int cnt = 0;

        BUG_ON(!sb);
        BUG_ON((flags & DCACHE_REFERENCED) && count == NULL);
        spin_lock(&dcache_lock);
        if (count != NULL)
                /* called from prune_dcache() and shrink_dcache_parent() */
                cnt = *count;
restart:
        if (count == NULL)
                list_splice_init(&sb->s_dentry_lru, &tmp);
        else {
                while (!list_empty(&sb->s_dentry_lru)) {
                        dentry = list_entry(sb->s_dentry_lru.prev,
                                        struct dentry, d_lru);
                        BUG_ON(dentry->d_sb != sb);

                        spin_lock(&dentry->d_lock);
                        /*
                         * If we are honouring the DCACHE_REFERENCED flag and
                         * the dentry has this flag set, don't free it. Clear
                         * the flag and put it back on the LRU.
                         */
                        if ((flags & DCACHE_REFERENCED)
                                && (dentry->d_flags & DCACHE_REFERENCED)) {
                                dentry->d_flags &= ~DCACHE_REFERENCED;
                                list_move(&dentry->d_lru, &referenced);
                                spin_unlock(&dentry->d_lock);
                        } else {
                                list_move_tail(&dentry->d_lru, &tmp);
                                spin_unlock(&dentry->d_lock);
                                cnt--;
                                if (!cnt)
                                        break;
                        }
                        cond_resched_lock(&dcache_lock);
                }
        }
        while (!list_empty(&tmp)) {
                dentry = list_entry(tmp.prev, struct dentry, d_lru);
                dentry_lru_del_init(dentry);
                spin_lock(&dentry->d_lock);
                /*
                 * We found an inuse dentry which was not removed from
                 * the LRU because of laziness during lookup.  Do not free
                 * it - just keep it off the LRU list.
                 */
                if (atomic_read(&dentry->d_count)) {
                        spin_unlock(&dentry->d_lock);
                        continue;
                }
                prune_one_dentry(dentry);
                /* dentry->d_lock was dropped in prune_one_dentry() */
                cond_resched_lock(&dcache_lock);
        }
        if (count == NULL && !list_empty(&sb->s_dentry_lru))
                goto restart;
        if (count != NULL)
                *count = cnt;
        if (!list_empty(&referenced))
                list_splice(&referenced, &sb->s_dentry_lru);
        spin_unlock(&dcache_lock);
}

/**
 * prune_dcache - shrink the dcache
 * @count: number of entries to try to free
 *
 * Shrink the dcache. This is done when we need more memory, or simply when we
 * need to unmount something (at which point we need to unuse all dentries).
 *
 * This function may fail to free any resources if all the dentries are in use.
 */
static void prune_dcache(int count)
{
        struct super_block *sb;
        int w_count;
        int unused = dentry_stat.nr_unused;
        int prune_ratio;
        int pruned;

        if (unused == 0 || count == 0)
                return;
        spin_lock(&dcache_lock);
restart:
        if (count >= unused)
                prune_ratio = 1;
        else
                prune_ratio = unused / count;
        spin_lock(&sb_lock);
        list_for_each_entry(sb, &super_blocks, s_list) {
                if (sb->s_nr_dentry_unused == 0)
                        continue;
                sb->s_count++;
                /* Now, we reclaim unused dentrins with fairness.
                 * We reclaim them same percentage from each superblock.
                 * We calculate number of dentries to scan on this sb
                 * as follows, but the implementation is arranged to avoid
                 * overflows:
                 * number of dentries to scan on this sb =
                 * count * (number of dentries on this sb /
                 * number of dentries in the machine)
                 */
                spin_unlock(&sb_lock);
                if (prune_ratio != 1)
                        w_count = (sb->s_nr_dentry_unused / prune_ratio) + 1;
                else
                        w_count = sb->s_nr_dentry_unused;
                pruned = w_count;
                /*
                 * We need to be sure this filesystem isn't being unmounted,
                 * otherwise we could race with generic_shutdown_super(), and
                 * end up holding a reference to an inode while the filesystem
                 * is unmounted.  So we try to get s_umount, and make sure
                 * s_root isn't NULL.
                 */
                if (down_read_trylock(&sb->s_umount)) {
                        if ((sb->s_root != NULL) &&
                            (!list_empty(&sb->s_dentry_lru))) {
                                spin_unlock(&dcache_lock);
                                __shrink_dcache_sb(sb, &w_count,
                                                DCACHE_REFERENCED);
                                pruned -= w_count;
                                spin_lock(&dcache_lock);
                        }
                        up_read(&sb->s_umount);
                }
                spin_lock(&sb_lock);
                count -= pruned;
                /*
                 * restart only when sb is no longer on the list and
                 * we have more work to do.
                 */
                if (__put_super_and_need_restart(sb) && count > 0) {
                        spin_unlock(&sb_lock);
                        goto restart;
                }
        }
        spin_unlock(&sb_lock);
        spin_unlock(&dcache_lock);
}

/**
 * shrink_dcache_sb - shrink dcache for a superblock
 * @sb: superblock
 *
 * Shrink the dcache for the specified super block. This
 * is used to free the dcache before unmounting a file
 * system
 */
void shrink_dcache_sb(struct super_block * sb)
{
        __shrink_dcache_sb(sb, NULL, 0);
}

/*
 * destroy a single subtree of dentries for unmount
 * - see the comments on shrink_dcache_for_umount() for a description of the
 *   locking
 */
static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
{
        struct dentry *parent;
        unsigned detached = 0;

        BUG_ON(!IS_ROOT(dentry));

        /* detach this root from the system */
        spin_lock(&dcache_lock);
        dentry_lru_del_init(dentry);
        __d_drop(dentry);
        spin_unlock(&dcache_lock);

        for (;;) {
                /* descend to the first leaf in the current subtree */
                while (!list_empty(&dentry->d_subdirs)) {
                        struct dentry *loop;

                        /* this is a branch with children - detach all of them
                         * from the system in one go */
                        spin_lock(&dcache_lock);
                        list_for_each_entry(loop, &dentry->d_subdirs,
                                            d_u.d_child) {
                                dentry_lru_del_init(loop);
                                __d_drop(loop);
                                cond_resched_lock(&dcache_lock);
                        }
                        spin_unlock(&dcache_lock);

                        /* move to the first child */
                        dentry = list_entry(dentry->d_subdirs.next,
                                            struct dentry, d_u.d_child);
                }

                /* consume the dentries from this leaf up through its parents
                 * until we find one with children or run out altogether */
                do {
                        struct inode *inode;

                        if (atomic_read(&dentry->d_count) != 0) {
                                printk(KERN_ERR
                                       "BUG: Dentry %p{i=%lx,n=%s}"
                                       " still in use (%d)"
                                       " [unmount of %s %s]\n",
                                       dentry,
                                       dentry->d_inode ?
                                       dentry->d_inode->i_ino : 0UL,
                                       dentry->d_name.name,
                                       atomic_read(&dentry->d_count),
                                       dentry->d_sb->s_type->name,
                                       dentry->d_sb->s_id);
                                BUG();
                        }

                        if (IS_ROOT(dentry))
                                parent = NULL;
                        else {
                                parent = dentry->d_parent;
                                atomic_dec(&parent->d_count);
                        }

                        list_del(&dentry->d_u.d_child);
                        detached++;

                        inode = dentry->d_inode;
                        if (inode) {
                                dentry->d_inode = NULL;
                                list_del_init(&dentry->d_alias);
                                if (dentry->d_op && dentry->d_op->d_iput)
                                        dentry->d_op->d_iput(dentry, inode);
                                else
                                        iput(inode);
                        }

                        d_free(dentry);

                        /* finished when we fall off the top of the tree,
                         * otherwise we ascend to the parent and move to the
                         * next sibling if there is one */
                        if (!parent)
                                goto out;

                        dentry = parent;

                } while (list_empty(&dentry->d_subdirs));

                dentry = list_entry(dentry->d_subdirs.next,
                                    struct dentry, d_u.d_child);
        }
out:
        /* several dentries were freed, need to correct nr_dentry */
        spin_lock(&dcache_lock);
        dentry_stat.nr_dentry -= detached;
        spin_unlock(&dcache_lock);
}

/*
 * destroy the dentries attached to a superblock on unmounting
 * - we don't need to use dentry->d_lock, and only need dcache_lock when
 *   removing the dentry from the system lists and hashes because:
 *   - the superblock is detached from all mountings and open files, so the
 *     dentry trees will not be rearranged by the VFS
 *   - s_umount is write-locked, so the memory pressure shrinker will ignore
 *     any dentries belonging to this superblock that it comes across
 *   - the filesystem itself is no longer permitted to rearrange the dentries
 *     in this superblock
 */
void shrink_dcache_for_umount(struct super_block *sb)
{
        struct dentry *dentry;

        if (down_read_trylock(&sb->s_umount))
                BUG();

        dentry = sb->s_root;
        sb->s_root = NULL;
        atomic_dec(&dentry->d_count);
        shrink_dcache_for_umount_subtree(dentry);

        while (!hlist_empty(&sb->s_anon)) {
                dentry = hlist_entry(sb->s_anon.first, struct dentry, d_hash);
                shrink_dcache_for_umount_subtree(dentry);
        }
}

/*
 * Search for at least 1 mount point in the dentry's subdirs.
 * We descend to the next level whenever the d_subdirs
 * list is non-empty and continue searching.
 */
 
/**
 * have_submounts - check for mounts over a dentry
 * @parent: dentry to check.
 *
 * Return true if the parent or its subdirectories contain
 * a mount point
 */
 
int have_submounts(struct dentry *parent)
{
        struct dentry *this_parent = parent;
        struct list_head *next;

        spin_lock(&dcache_lock);
        if (d_mountpoint(parent))
                goto positive;
repeat:
        next = this_parent->d_subdirs.next;
resume:
        while (next != &this_parent->d_subdirs) {
                struct list_head *tmp = next;
                struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
                next = tmp->next;
                /* Have we found a mount point ? */
                if (d_mountpoint(dentry))
                        goto positive;
                if (!list_empty(&dentry->d_subdirs)) {
                        this_parent = dentry;
                        goto repeat;
                }
        }
        /*
         * All done at this level ... ascend and resume the search.
         */
        if (this_parent != parent) {
                next = this_parent->d_u.d_child.next;
                this_parent = this_parent->d_parent;
                goto resume;
        }
        spin_unlock(&dcache_lock);
        return 0; /* No mount points found in tree */
positive:
        spin_unlock(&dcache_lock);
        return 1;
}

/*
 * Search the dentry child list for the specified parent,
 * and move any unused dentries to the end of the unused
 * list for prune_dcache(). We descend to the next level
 * whenever the d_subdirs list is non-empty and continue
 * searching.
 *
 * It returns zero iff there are no unused children,
 * otherwise  it returns the number of children moved to
 * the end of the unused list. This may not be the total
 * number of unused children, because select_parent can
 * drop the lock and return early due to latency
 * constraints.
 */
static int select_parent(struct dentry * parent)
{
        struct dentry *this_parent = parent;
        struct list_head *next;
        int found = 0;

        spin_lock(&dcache_lock);
repeat:
        next = this_parent->d_subdirs.next;
resume:
        while (next != &this_parent->d_subdirs) {
                struct list_head *tmp = next;
                struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
                next = tmp->next;

                dentry_lru_del_init(dentry);
                /* 
                 * move only zero ref count dentries to the end 
                 * of the unused list for prune_dcache
                 */
                if (!atomic_read(&dentry->d_count)) {
                        dentry_lru_add_tail(dentry);
                        found++;
                }

                /*
                 * We can return to the caller if we have found some (this
                 * ensures forward progress). We'll be coming back to find
                 * the rest.
                 */
                if (found && need_resched())
                        goto out;

                /*
                 * Descend a level if the d_subdirs list is non-empty.
                 */
                if (!list_empty(&dentry->d_subdirs)) {
                        this_parent = dentry;
                        goto repeat;
                }
        }
        /*
         * All done at this level ... ascend and resume the search.
         */
        if (this_parent != parent) {
                next = this_parent->d_u.d_child.next;
                this_parent = this_parent->d_parent;
                goto resume;
        }
out:
        spin_unlock(&dcache_lock);
        return found;
}

/**
 * shrink_dcache_parent - prune dcache
 * @parent: parent of entries to prune
 *
 * Prune the dcache to remove unused children of the parent dentry.
 */
 
void shrink_dcache_parent(struct dentry * parent)
{
        struct super_block *sb = parent->d_sb;
        int found;

        while ((found = select_parent(parent)) != 0)
                __shrink_dcache_sb(sb, &found, 0);
}

/*
 * Scan `nr' dentries and return the number which remain.
 *
 * We need to avoid reentering the filesystem if the caller is performing a
 * GFP_NOFS allocation attempt.  One example deadlock is:
 *
 * ext2_new_block->getblk->GFP->shrink_dcache_memory->prune_dcache->
 * prune_one_dentry->dput->dentry_iput->iput->inode->i_sb->s_op->put_inode->
 * ext2_discard_prealloc->ext2_free_blocks->lock_super->DEADLOCK.
 *
 * In this case we return -1 to tell the caller that we baled.
 */
static int shrink_dcache_memory(int nr, gfp_t gfp_mask)
{
        if (nr) {
                if (!(gfp_mask & __GFP_FS))
                        return -1;
                prune_dcache(nr);
        }
        return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
}

static struct shrinker dcache_shrinker = {
        .shrink = shrink_dcache_memory,
        .seeks = DEFAULT_SEEKS,
};

/**
 * d_alloc      -       allocate a dcache entry
 * @parent: parent of entry to allocate
 * @name: qstr of the name
 *
 * Allocates a dentry. It returns %NULL if there is insufficient memory
 * available. On a success the dentry is returned. The name passed in is
 * copied and the copy passed in may be reused after this call.
 */
 
struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
{
        struct dentry *dentry;
        char *dname;

        dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
        if (!dentry)
                return NULL;

        if (name->len > DNAME_INLINE_LEN-1) {
                dname = kmalloc(name->len + 1, GFP_KERNEL);
                if (!dname) {
                        kmem_cache_free(dentry_cache, dentry); 
                        return NULL;
                }
        } else  {
                dname = dentry->d_iname;
        }       
        dentry->d_name.name = dname;

        dentry->d_name.len = name->len;
        dentry->d_name.hash = name->hash;
        memcpy(dname, name->name, name->len);
        dname[name->len] = 0;

        atomic_set(&dentry->d_count, 1);
        dentry->d_flags = DCACHE_UNHASHED;
        spin_lock_init(&dentry->d_lock);
        dentry->d_inode = NULL;
        dentry->d_parent = NULL;
        dentry->d_sb = NULL;
        dentry->d_op = NULL;
        dentry->d_fsdata = NULL;
        dentry->d_mounted = 0;
        INIT_HLIST_NODE(&dentry->d_hash);
        INIT_LIST_HEAD(&dentry->d_lru);
        INIT_LIST_HEAD(&dentry->d_subdirs);
        INIT_LIST_HEAD(&dentry->d_alias);

        if (parent) {
                dentry->d_parent = dget(parent);
                dentry->d_sb = parent->d_sb;
        } else {
                INIT_LIST_HEAD(&dentry->d_u.d_child);
        }

        spin_lock(&dcache_lock);
        if (parent)
                list_add(&dentry->d_u.d_child, &parent->d_subdirs);
        dentry_stat.nr_dentry++;
        spin_unlock(&dcache_lock);

        return dentry;
}

struct dentry *d_alloc_name(struct dentry *parent, const char *name)
{
        struct qstr q;

        q.name = name;
        q.len = strlen(name);
        q.hash = full_name_hash(q.name, q.len);
        return d_alloc(parent, &q);
}

/* the caller must hold dcache_lock */
static void __d_instantiate(struct dentry *dentry, struct inode *inode)
{
        if (inode)
                list_add(&dentry->d_alias, &inode->i_dentry);
        dentry->d_inode = inode;
        fsnotify_d_instantiate(dentry, inode);
}

/**
 * d_instantiate - fill in inode information for a dentry
 * @entry: dentry to complete
 * @inode: inode to attach to this dentry
 *
 * Fill in inode information in the entry.
 *
 * This turns negative dentries into productive full members
 * of society.
 *
 * NOTE! This assumes that the inode count has been incremented
 * (or otherwise set) by the caller to indicate that it is now
 * in use by the dcache.
 */
 
void d_instantiate(struct dentry *entry, struct inode * inode)
{
        BUG_ON(!list_empty(&entry->d_alias));
        spin_lock(&dcache_lock);
        __d_instantiate(entry, inode);
        spin_unlock(&dcache_lock);
        security_d_instantiate(entry, inode);
}

/**
 * d_instantiate_unique - instantiate a non-aliased dentry
 * @entry: dentry to instantiate
 * @inode: inode to attach to this dentry
 *
 * Fill in inode information in the entry. On success, it returns NULL.
 * If an unhashed alias of "entry" already exists, then we return the
 * aliased dentry instead and drop one reference to inode.
 *
 * Note that in order to avoid conflicts with rename() etc, the caller
 * had better be holding the parent directory semaphore.
 *
 * This also assumes that the inode count has been incremented
 * (or otherwise set) by the caller to indicate that it is now
 * in use by the dcache.
 */
static struct dentry *__d_instantiate_unique(struct dentry *entry,
                                             struct inode *inode)
{
        struct dentry *alias;
        int len = entry->d_name.len;
        const char *name = entry->d_name.name;
        unsigned int hash = entry->d_name.hash;

        if (!inode) {
                __d_instantiate(entry, NULL);
                return NULL;
        }

        list_for_each_entry(alias, &inode->i_dentry, d_alias) {
                struct qstr *qstr = &alias->d_name;

                if (qstr->hash != hash)
                        continue;
                if (alias->d_parent != entry->d_parent)
                        continue;
                if (qstr->len != len)
                        continue;
                if (memcmp(qstr->name, name, len))
                        continue;
                dget_locked(alias);
                return alias;
        }

        __d_instantiate(entry, inode);
        return NULL;
}

struct dentry *d_instantiate_unique(struct dentry *entry, struct inode *inode)
{
        struct dentry *result;

        BUG_ON(!list_empty(&entry->d_alias));

        spin_lock(&dcache_lock);
        result = __d_instantiate_unique(entry, inode);
        spin_unlock(&dcache_lock);

        if (!result) {
                security_d_instantiate(entry, inode);
                return NULL;
        }

        BUG_ON(!d_unhashed(result));
        iput(inode);
        return result;
}

EXPORT_SYMBOL(d_instantiate_unique);

/**
 * d_alloc_root - allocate root dentry
 * @root_inode: inode to allocate the root for
 *
 * Allocate a root ("/") dentry for the inode given. The inode is
 * instantiated and returned. %NULL is returned if there is insufficient
 * memory or the inode passed is %NULL.
 */
 
struct dentry * d_alloc_root(struct inode * root_inode)
{
        struct dentry *res = NULL;

        if (root_inode) {
                static const struct qstr name = { .name = "/", .len = 1 };

                res = d_alloc(NULL, &name);
                if (res) {
                        res->d_sb = root_inode->i_sb;
                        res->d_parent = res;
                        d_instantiate(res, root_inode);
                }
        }
        return res;
}

static inline struct hlist_head *d_hash(struct dentry *parent,
                                        unsigned long hash)
{
        hash += ((unsigned long) parent ^ GOLDEN_RATIO_PRIME) / L1_CACHE_BYTES;
        hash = hash ^ ((hash ^ GOLDEN_RATIO_PRIME) >> D_HASHBITS);
        return dentry_hashtable + (hash & D_HASHMASK);
}

/**
 * d_obtain_alias - find or allocate a dentry for a given inode
 * @inode: inode to allocate the dentry for
 *
 * Obtain a dentry for an inode resulting from NFS filehandle conversion or
 * similar open by handle operations.  The returned dentry may be anonymous,
 * or may have a full name (if the inode was already in the cache).
 *
 * When called on a directory inode, we must ensure that the inode only ever
 * has one dentry.  If a dentry is found, that is returned instead of
 * allocating a new one.
 *
 * On successful return, the reference to the inode has been transferred
 * to the dentry.  In case of an error the reference on the inode is released.
 * To make it easier to use in export operations a %NULL or IS_ERR inode may
 * be passed in and will be the error will be propagate to the return value,
 * with a %NULL @inode replaced by ERR_PTR(-ESTALE).
 */
struct dentry *d_obtain_alias(struct inode *inode)
{
        static const struct qstr anonstring = { .name = "" };
        struct dentry *tmp;
        struct dentry *res;

        if (!inode)
                return ERR_PTR(-ESTALE);
        if (IS_ERR(inode))
                return ERR_CAST(inode);

        res = d_find_alias(inode);
        if (res)
                goto out_iput;

        tmp = d_alloc(NULL, &anonstring);
        if (!tmp) {
                res = ERR_PTR(-ENOMEM);
                goto out_iput;
        }
        tmp->d_parent = tmp; /* make sure dput doesn't croak */

        spin_lock(&dcache_lock);
        res = __d_find_alias(inode, 0);
        if (res) {
                spin_unlock(&dcache_lock);
                dput(tmp);
                goto out_iput;
        }

        /* attach a disconnected dentry */
        spin_lock(&tmp->d_lock);
        tmp->d_sb = inode->i_sb;
        tmp->d_inode = inode;
        tmp->d_flags |= DCACHE_DISCONNECTED;
        tmp->d_flags &= ~DCACHE_UNHASHED;
        list_add(&tmp->d_alias, &inode->i_dentry);
        hlist_add_head(&tmp->d_hash, &inode->i_sb->s_anon);
        spin_unlock(&tmp->d_lock);

        spin_unlock(&dcache_lock);
        return tmp;

 out_iput:
        iput(inode);
        return res;
}
EXPORT_SYMBOL(d_obtain_alias);

/**
 * d_splice_alias - splice a disconnected dentry into the tree if one exists
 * @inode:  the inode which may have a disconnected dentry
 * @dentry: a negative dentry which we want to point to the inode.
 *
 * If inode is a directory and has a 'disconnected' dentry (i.e. IS_ROOT and
 * DCACHE_DISCONNECTED), then d_move that in place of the given dentry
 * and return it, else simply d_add the inode to the dentry and return NULL.
 *
 * This is needed in the lookup routine of any filesystem that is exportable
 * (via knfsd) so that we can build dcache paths to directories effectively.
 *
 * If a dentry was found and moved, then it is returned.  Otherwise NULL
 * is returned.  This matches the expected return value of ->lookup.
 *
 */
struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
{
        struct dentry *new = NULL;

        if (inode && S_ISDIR(inode->i_mode)) {
                spin_lock(&dcache_lock);
                new = __d_find_alias(inode, 1);
                if (new) {
                        BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));
                        spin_unlock(&dcache_lock);
                        security_d_instantiate(new, inode);
                        d_rehash(dentry);
                        d_move(new, dentry);
                        iput(inode);
                } else {
                        /* already taking dcache_lock, so d_add() by hand */
                        __d_instantiate(dentry, inode);
                        spin_unlock(&dcache_lock);
                        security_d_instantiate(dentry, inode);
                        d_rehash(dentry);
                }
        } else
                d_add(dentry, inode);
        return new;
}

/**
 * d_add_ci - lookup or allocate new dentry with case-exact name
 * @inode:  the inode case-insensitive lookup has found
 * @dentry: the negative dentry that was passed to the parent's lookup func
 * @name:   the case-exact name to be associated with the returned dentry
 *
 * This is to avoid filling the dcache with case-insensitive names to the
 * same inode, only the actual correct case is stored in the dcache for
 * case-insensitive filesystems.
 *
 * For a case-insensitive lookup match and if the the case-exact dentry
 * already exists in in the dcache, use it and return it.
 *
 * If no entry exists with the exact case name, allocate new dentry with
 * the exact case, and return the spliced entry.
 */
struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
                        struct qstr *name)
{
        int error;
        struct dentry *found;
        struct dentry *new;

        /*
         * First check if a dentry matching the name already exists,
         * if not go ahead and create it now.
         */
        found = d_hash_and_lookup(dentry->d_parent, name);
        if (!found) {
                new = d_alloc(dentry->d_parent, name);
                if (!new) {
                        error = -ENOMEM;
                        goto err_out;
                }

                found = d_splice_alias(inode, new);
                if (found) {
                        dput(new);
                        return found;
                }
                return new;
        }

        /*
         * If a matching dentry exists, and it's not negative use it.
         *
         * Decrement the reference count to balance the iget() done
         * earlier on.
         */
        if (found->d_inode) {
                if (unlikely(found->d_inode != inode)) {
                        /* This can't happen because bad inodes are unhashed. */
                        BUG_ON(!is_bad_inode(inode));
                        BUG_ON(!is_bad_inode(found->d_inode));
                }
                iput(inode);
                return found;
        }

        /*
         * Negative dentry: instantiate it unless the inode is a directory and
         * already has a dentry.
         */
        spin_lock(&dcache_lock);
        if (!S_ISDIR(inode->i_mode) || list_empty(&inode->i_dentry)) {
                __d_instantiate(found, inode);
                spin_unlock(&dcache_lock);
                security_d_instantiate(found, inode);
                return found;
        }

        /*
         * In case a directory already has a (disconnected) entry grab a
         * reference to it, move it in place and use it.
         */
        new = list_entry(inode->i_dentry.next, struct dentry, d_alias);
        dget_locked(new);
        spin_unlock(&dcache_lock);
        security_d_instantiate(found, inode);
        d_move(new, found);
        iput(inode);
        dput(found);
        return new;

err_out:
        iput(inode);
        return ERR_PTR(error);
}

/**
 * d_lookup - search for a dentry
 * @parent: parent dentry
 * @name: qstr of name we wish to find
 *
 * Searches the children of the parent dentry for the name in question. If
 * the dentry is found its reference count is incremented and the dentry
 * is returned. The caller must use dput to free the entry when it has
 * finished using it. %NULL is returned on failure.
 *
 * __d_lookup is dcache_lock free. The hash list is protected using RCU.
 * Memory barriers are used while updating and doing lockless traversal. 
 * To avoid races with d_move while rename is happening, d_lock is used.
 *
 * Overflows in memcmp(), while d_move, are avoided by keeping the length
 * and name pointer in one structure pointed by d_qstr.
 *
 * rcu_read_lock() and rcu_read_unlock() are used to disable preemption while
 * lookup is going on.
 *
 * The dentry unused LRU is not updated even if lookup finds the required dentry
 * in there. It is updated in places such as prune_dcache, shrink_dcache_sb,
 * select_parent and __dget_locked. This laziness saves lookup from dcache_lock
 * acquisition.
 *
 * d_lookup() is protected against the concurrent renames in some unrelated
 * directory using the seqlockt_t rename_lock.
 */

struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
{
        struct dentry * dentry = NULL;
        unsigned long seq;

        do {
                seq = read_seqbegin(&rename_lock);
                dentry = __d_lookup(parent, name);
                if (dentry)
                        break;
        } while (read_seqretry(&rename_lock, seq));
        return dentry;
}

struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
{
        unsigned int len = name->len;
        unsigned int hash = name->hash;
        const unsigned char *str = name->name;
        struct hlist_head *head = d_hash(parent,hash);
        struct dentry *found = NULL;
        struct hlist_node *node;
        struct dentry *dentry;

        rcu_read_lock();
        
        hlist_for_each_entry_rcu(dentry, node, head, d_hash) {
                struct qstr *qstr;

                if (dentry->d_name.hash != hash)
                        continue;
                if (dentry->d_parent != parent)
                        continue;

                spin_lock(&dentry->d_lock);

                /*
                 * Recheck the dentry after taking the lock - d_move may have
                 * changed things.  Don't bother checking the hash because we're
                 * about to compare the whole name anyway.
                 */
                if (dentry->d_parent != parent)
                        goto next;

                /* non-existing due to RCU? */
                if (d_unhashed(dentry))
                        goto next;

                /*
                 * It is safe to compare names since d_move() cannot
                 * change the qstr (protected by d_lock).
                 */
                qstr = &dentry->d_name;
                if (parent->d_op && parent->d_op->d_compare) {
                        if (parent->d_op->d_compare(parent, qstr, name))
                                goto next;
                } else {
                        if (qstr->len != len)
                                goto next;
                        if (memcmp(qstr->name, str, len))
                                goto next;
                }

                atomic_inc(&dentry->d_count);
                found = dentry;
                spin_unlock(&dentry->d_lock);
                break;
next:
                spin_unlock(&dentry->d_lock);
        }
        rcu_read_unlock();

        return found;
}

/**
 * d_hash_and_lookup - hash the qstr then search for a dentry
 * @dir: Directory to search in
 * @name: qstr of name we wish to find
 *
 * On hash failure or on lookup failure NULL is returned.
 */
struct dentry *d_hash_and_lookup(struct dentry *dir, struct qstr *name)
{
        struct dentry *dentry = NULL;

        /*
         * Check for a fs-specific hash function. Note that we must
         * calculate the standard hash first, as the d_op->d_hash()
         * routine may choose to leave the hash value unchanged.
         */
        name->hash = full_name_hash(name->name, name->len);
        if (dir->d_op && dir->d_op->d_hash) {
                if (dir->d_op->d_hash(dir, name) < 0)
                        goto out;
        }
        dentry = d_lookup(dir, name);
out:
        return dentry;
}

/**
 * d_validate - verify dentry provided from insecure source
 * @dentry: The dentry alleged to be valid child of @dparent
 * @dparent: The parent dentry (known to be valid)
 *
 * An insecure source has sent us a dentry, here we verify it and dget() it.
 * This is used by ncpfs in its readdir implementation.
 * Zero is returned in the dentry is invalid.
 */
 
int d_validate(struct dentry *dentry, struct dentry *dparent)
{
        struct hlist_head *base;
        struct hlist_node *lhp;

        /* Check whether the ptr might be valid at all.. */
        if (!kmem_ptr_validate(dentry_cache, dentry))
                goto out;

        if (dentry->d_parent != dparent)
                goto out;

        spin_lock(&dcache_lock);
        base = d_hash(dparent, dentry->d_name.hash);
        hlist_for_each(lhp,base) { 
                /* hlist_for_each_entry_rcu() not required for d_hash list
                 * as it is parsed under dcache_lock
                 */
                if (dentry == hlist_entry(lhp, struct dentry, d_hash)) {
                        __dget_locked(dentry);
                        spin_unlock(&dcache_lock);
                        return 1;
                }
        }
        spin_unlock(&dcache_lock);
out:
        return 0;
}

/*
 * When a file is deleted, we have two options:
 * - turn this dentry into a negative dentry
 * - unhash this dentry and free it.
 *
 * Usually, we want to just turn this into
 * a negative dentry, but if anybody else is
 * currently using the dentry or the inode
 * we can't do that and we fall back on removing
 * it from the hash queues and waiting for
 * it to be deleted later when it has no users
 */
 
/**
 * d_delete - delete a dentry
 * @dentry: The dentry to delete
 *
 * Turn the dentry into a negative dentry if possible, otherwise
 * remove it from the hash queues so it can be deleted later
 */
 
void d_delete(struct dentry * dentry)
{
        int isdir = 0;
        /*
         * Are we the only user?
         */
        spin_lock(&dcache_lock);
        spin_lock(&dentry->d_lock);
        isdir = S_ISDIR(dentry->d_inode->i_mode);
        if (atomic_read(&dentry->d_count) == 1) {
                dentry_iput(dentry);
                fsnotify_nameremove(dentry, isdir);
                return;
        }

        if (!d_unhashed(dentry))
                __d_drop(dentry);

        spin_unlock(&dentry->d_lock);
        spin_unlock(&dcache_lock);

        fsnotify_nameremove(dentry, isdir);
}

static void __d_rehash(struct dentry * entry, struct hlist_head *list)
{

        entry->d_flags &= ~DCACHE_UNHASHED;
        hlist_add_head_rcu(&entry->d_hash, list);
}

static void _d_rehash(struct dentry * entry)
{
        __d_rehash(entry, d_hash(entry->d_parent, entry->d_name.hash));
}

/**
 * d_rehash     - add an entry back to the hash
 * @entry: dentry to add to the hash
 *
 * Adds a dentry to the hash according to its name.
 */
 
void d_rehash(struct dentry * entry)
{
        spin_lock(&dcache_lock);
        spin_lock(&entry->d_lock);
        _d_rehash(entry);
        spin_unlock(&entry->d_lock);
        spin_unlock(&dcache_lock);
}

/*
 * When switching names, the actual string doesn't strictly have to
 * be preserved in the target - because we're dropping the target
 * anyway. As such, we can just do a simple memcpy() to copy over
 * the new name before we switch.
 *
 * Note that we have to be a lot more careful about getting the hash
 * switched - we have to switch the hash value properly even if it
 * then no longer matches the actual (corrupted) string of the target.
 * The hash value has to match the hash queue that the dentry is on..
 */
static void switch_names(struct dentry *dentry, struct dentry *target)
{
        if (dname_external(target)) {
                if (dname_external(dentry)) {
                        /*
                         * Both external: swap the pointers
                         */
                        swap(target->d_name.name, dentry->d_name.name);
                } else {
                        /*
                         * dentry:internal, target:external.  Steal target's
                         * storage and make target internal.
                         */
                        memcpy(target->d_iname, dentry->d_name.name,
                                        dentry->d_name.len + 1);
                        dentry->d_name.name = target->d_name.name;
                        target->d_name.name = target->d_iname;
                }
        } else {
                if (dname_external(dentry)) {
                        /*
                         * dentry:external, target:internal.  Give dentry's
                         * storage to target and make dentry internal
                         */
                        memcpy(dentry->d_iname, target->d_name.name,
                                        target->d_name.len + 1);
                        target->d_name.name = dentry->d_name.name;
                        dentry->d_name.name = dentry->d_iname;
                } else {
                        /*
                         * Both are internal.  Just copy target to dentry
                         */
                        memcpy(dentry->d_iname, target->d_name.name,
                                        target->d_name.len + 1);
                        dentry->d_name.len = target->d_name.len;
                        return;
                }
        }
        swap(dentry->d_name.len, target->d_name.len);
}

/*
 * We cannibalize "target" when moving dentry on top of it,
 * because it's going to be thrown away anyway. We could be more
 * polite about it, though.
 *
 * This forceful removal will result in ugly /proc output if
 * somebody holds a file open that got deleted due to a rename.
 * We could be nicer about the deleted file, and let it show
 * up under the name it had before it was deleted rather than
 * under the original name of the file that was moved on top of it.
 */
 
/*
 * d_move_locked - move a dentry
 * @dentry: entry to move
 * @target: new dentry
 *
 * Update the dcache to reflect the move of a file name. Negative
 * dcache entries should not be moved in this way.
 */
static void d_move_locked(struct dentry * dentry, struct dentry * target)
{
        struct hlist_head *list;

        if (!dentry->d_inode)
                printk(KERN_WARNING "VFS: moving negative dcache entry\n");

        write_seqlock(&rename_lock);
        /*
         * XXXX: do we really need to take target->d_lock?
         */
        if (target < dentry) {
                spin_lock(&target->d_lock);
                spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        } else {
                spin_lock(&dentry->d_lock);
                spin_lock_nested(&target->d_lock, DENTRY_D_LOCK_NESTED);
        }

        /* Move the dentry to the target hash queue, if on different bucket */
        if (d_unhashed(dentry))
                goto already_unhashed;

        hlist_del_rcu(&dentry->d_hash);

already_unhashed:
        list = d_hash(target->d_parent, target->d_name.hash);
        __d_rehash(dentry, list);

        /* Unhash the target: dput() will then get rid of it */
        __d_drop(target);

        list_del(&dentry->d_u.d_child);
        list_del(&target->d_u.d_child);

        /* Switch the names.. */
        switch_names(dentry, target);
        swap(dentry->d_name.hash, target->d_name.hash);

        /* ... and switch the parents */
        if (IS_ROOT(dentry)) {
                dentry->d_parent = target->d_parent;
                target->d_parent = target;
                INIT_LIST_HEAD(&target->d_u.d_child);
        } else {
                swap(dentry->d_parent, target->d_parent);

                /* And add them back to the (new) parent lists */
                list_add(&target->d_u.d_child, &target->d_parent->d_subdirs);
        }

        list_add(&dentry->d_u.d_child, &dentry->d_parent->d_subdirs);
        spin_unlock(&target->d_lock);
        fsnotify_d_move(dentry);
        spin_unlock(&dentry->d_lock);
        write_sequnlock(&rename_lock);
}

/**
 * d_move - move a dentry
 * @dentry: entry to move
 * @target: new dentry
 *
 * Update the dcache to reflect the move of a file name. Negative
 * dcache entries should not be moved in this way.
 */

void d_move(struct dentry * dentry, struct dentry * target)
{
        spin_lock(&dcache_lock);
        d_move_locked(dentry, target);
        spin_unlock(&dcache_lock);
}

/**
 * d_ancestor - search for an ancestor
 * @p1: ancestor dentry
 * @p2: child dentry
 *
 * Returns the ancestor dentry of p2 which is a child of p1, if p1 is
 * an ancestor of p2, else NULL.
 */
struct dentry *d_ancestor(struct dentry *p1, struct dentry *p2)
{
        struct dentry *p;

        for (p = p2; !IS_ROOT(p); p = p->d_parent) {
                if (p->d_parent == p1)
                        return p;
        }
        return NULL;
}

/*
 * This helper attempts to cope with remotely renamed directories
 *
 * It assumes that the caller is already holding
 * dentry->d_parent->d_inode->i_mutex and the dcache_lock
 *
 * Note: If ever the locking in lock_rename() changes, then please
 * remember to update this too...
 */
static struct dentry *__d_unalias(struct dentry *dentry, struct dentry *alias)
        __releases(dcache_lock)
{
        struct mutex *m1 = NULL, *m2 = NULL;
        struct dentry *ret;

        /* If alias and dentry share a parent, then no extra locks required */
        if (alias->d_parent == dentry->d_parent)
                goto out_unalias;

        /* Check for loops */
        ret = ERR_PTR(-ELOOP);
        if (d_ancestor(alias, dentry))
                goto out_err;

        /* See lock_rename() */
        ret = ERR_PTR(-EBUSY);
        if (!mutex_trylock(&dentry->d_sb->s_vfs_rename_mutex))
                goto out_err;
        m1 = &dentry->d_sb->s_vfs_rename_mutex;
        if (!mutex_trylock(&alias->d_parent->d_inode->i_mutex))
                goto out_err;
        m2 = &alias->d_parent->d_inode->i_mutex;
out_unalias:
        d_move_locked(alias, dentry);
        ret = alias;
out_err:
        spin_unlock(&dcache_lock);
        if (m2)
                mutex_unlock(m2);
        if (m1)
                mutex_unlock(m1);
        return ret;
}

/*
 * Prepare an anonymous dentry for life in the superblock's dentry tree as a
 * named dentry in place of the dentry to be replaced.
 */
static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
{
        struct dentry *dparent, *aparent;

        switch_names(dentry, anon);
        swap(dentry->d_name.hash, anon->d_name.hash);

        dparent = dentry->d_parent;
        aparent = anon->d_parent;

        dentry->d_parent = (aparent == anon) ? dentry : aparent;
        list_del(&dentry->d_u.d_child);
        if (!IS_ROOT(dentry))
                list_add(&dentry->d_u.d_child, &dentry->d_parent->d_subdirs);
        else
                INIT_LIST_HEAD(&dentry->d_u.d_child);

        anon->d_parent = (dparent == dentry) ? anon : dparent;
        list_del(&anon->d_u.d_child);
        if (!IS_ROOT(anon))
                list_add(&anon->d_u.d_child, &anon->d_parent->d_subdirs);
        else
                INIT_LIST_HEAD(&anon->d_u.d_child);

        anon->d_flags &= ~DCACHE_DISCONNECTED;
}

/**
 * d_materialise_unique - introduce an inode into the tree
 * @dentry: candidate dentry
 * @inode: inode to bind to the dentry, to which aliases may be attached
 *
 * Introduces an dentry into the tree, substituting an extant disconnected
 * root directory alias in its place if there is one
 */
struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
{
        struct dentry *actual;

        BUG_ON(!d_unhashed(dentry));

        spin_lock(&dcache_lock);

        if (!inode) {
                actual = dentry;
                __d_instantiate(dentry, NULL);
                goto found_lock;
        }

        if (S_ISDIR(inode->i_mode)) {
                struct dentry *alias;

                /* Does an aliased dentry already exist? */
                alias = __d_find_alias(inode, 0);
                if (alias) {
                        actual = alias;
                        /* Is this an anonymous mountpoint that we could splice
                         * into our tree? */
                        if (IS_ROOT(alias)) {
                                spin_lock(&alias->d_lock);
                                __d_materialise_dentry(dentry, alias);
                                __d_drop(alias);
                                goto found;
                        }
                        /* Nope, but we must(!) avoid directory aliasing */
                        actual = __d_unalias(dentry, alias);
                        if (IS_ERR(actual))
                                dput(alias);
                        goto out_nolock;
                }
        }

        /* Add a unique reference */
        actual = __d_instantiate_unique(dentry, inode);
        if (!actual)
                actual = dentry;
        else if (unlikely(!d_unhashed(actual)))
                goto shouldnt_be_hashed;

found_lock:
        spin_lock(&actual->d_lock);
found:
        _d_rehash(actual);
        spin_unlock(&actual->d_lock);
        spin_unlock(&dcache_lock);
out_nolock:
        if (actual == dentry) {
                security_d_instantiate(dentry, inode);
                return NULL;
        }

        iput(inode);
        return actual;

shouldnt_be_hashed:
        spin_unlock(&dcache_lock);
        BUG();
}

static int prepend(char **buffer, int *buflen, const char *str, int namelen)
{
        *buflen -= namelen;
        if (*buflen < 0)
                return -ENAMETOOLONG;
        *buffer -= namelen;
        memcpy(*buffer, str, namelen);
        return 0;
}

static int prepend_name(char **buffer, int *buflen, struct qstr *name)
{
        return prepend(buffer, buflen, name->name, name->len);
}

/**
 * __d_path - return the path of a dentry
 * @path: the dentry/vfsmount to report
 * @root: root vfsmnt/dentry (may be modified by this function)
 * @buffer: buffer to return value in
 * @buflen: buffer length
 *
 * Convert a dentry into an ASCII path name. If the entry has been deleted
 * the string " (deleted)" is appended. Note that this is ambiguous.
 *
 * Returns a pointer into the buffer or an error code if the
 * path was too long.
 *
 * "buflen" should be positive. Caller holds the dcache_lock.
 *
 * If path is not reachable from the supplied root, then the value of
 * root is changed (without modifying refcounts).
 */
char *__d_path(const struct path *path, struct path *root,
               char *buffer, int buflen)
{
        struct dentry *dentry = path->dentry;
        struct vfsmount *vfsmnt = path->mnt;
        char *end = buffer + buflen;
        char *retval;

        spin_lock(&vfsmount_lock);
        prepend(&end, &buflen, "\0", 1);
        if (d_unlinked(dentry) &&
                (prepend(&end, &buflen, " (deleted)", 10) != 0))
                        goto Elong;

        if (buflen < 1)
                goto Elong;
        /* Get '/' right */
        retval = end-1;
        *retval = '/';

        for (;;) {
                struct dentry * parent;

                if (dentry == root->dentry && vfsmnt == root->mnt)
                        break;
                if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
                        /* Global root? */
                        if (vfsmnt->mnt_parent == vfsmnt) {
                                goto global_root;
                        }
                        dentry = vfsmnt->mnt_mountpoint;
                        vfsmnt = vfsmnt->mnt_parent;
                        continue;
                }
                parent = dentry->d_parent;
                prefetch(parent);
                if ((prepend_name(&end, &buflen, &dentry->d_name) != 0) ||
                    (prepend(&end, &buflen, "/", 1) != 0))
                        goto Elong;
                retval = end;
                dentry = parent;
        }

out:
        spin_unlock(&vfsmount_lock);
        return retval;

global_root:
        retval += 1;    /* hit the slash */
        if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
                goto Elong;
        root->mnt = vfsmnt;
        root->dentry = dentry;
        goto out;

Elong:
        retval = ERR_PTR(-ENAMETOOLONG);
        goto out;
}

/**
 * d_path - return the path of a dentry
 * @path: path to report
 * @buf: buffer to return value in
 * @buflen: buffer length
 *
 * Convert a dentry into an ASCII path name. If the entry has been deleted
 * the string " (deleted)" is appended. Note that this is ambiguous.
 *
 * Returns a pointer into the buffer or an error code if the path was
 * too long. Note: Callers should use the returned pointer, not the passed
 * in buffer, to use the name! The implementation often starts at an offset
 * into the buffer, and may leave 0 bytes at the start.
 *
 * "buflen" should be positive.
 */
char *d_path(const struct path *path, char *buf, int buflen)
{
        char *res;
        struct path root;
        struct path tmp;

        /*
         * We have various synthetic filesystems that never get mounted.  On
         * these filesystems dentries are never used for lookup purposes, and
         * thus don't need to be hashed.  They also don't need a name until a
         * user wants to identify the object in /proc/pid/fd/.  The little hack
         * below allows us to generate a name for these objects on demand:
         */
        if (path->dentry->d_op && path->dentry->d_op->d_dname)
                return path->dentry->d_op->d_dname(path->dentry, buf, buflen);

        read_lock(&current->fs->lock);
        root = current->fs->root;
        path_get(&root);
        read_unlock(&current->fs->lock);
        spin_lock(&dcache_lock);
        tmp = root;
        res = __d_path(path, &tmp, buf, buflen);
        spin_unlock(&dcache_lock);
        path_put(&root);
        return res;
}

/*
 * Helper function for dentry_operations.d_dname() members
 */
char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
                        const char *fmt, ...)
{
        va_list args;
        char temp[64];
        int sz;

        va_start(args, fmt);
        sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
        va_end(args);

        if (sz > sizeof(temp) || sz > buflen)
                return ERR_PTR(-ENAMETOOLONG);

        buffer += buflen - sz;
        return memcpy(buffer, temp, sz);
}

/*
 * Write full pathname from the root of the filesystem into the buffer.
 */
char *dentry_path(struct dentry *dentry, char *buf, int buflen)
{
        char *end = buf + buflen;
        char *retval;

        spin_lock(&dcache_lock);
        prepend(&end, &buflen, "\0", 1);
        if (d_unlinked(dentry) &&
                (prepend(&end, &buflen, "//deleted", 9) != 0))
                        goto Elong;
        if (buflen < 1)
                goto Elong;
        /* Get '/' right */
        retval = end-1;
        *retval = '/';

        while (!IS_ROOT(dentry)) {
                struct dentry *parent = dentry->d_parent;

                prefetch(parent);
                if ((prepend_name(&end, &buflen, &dentry->d_name) != 0) ||
                    (prepend(&end, &buflen, "/", 1) != 0))
                        goto Elong;

                retval = end;
                dentry = parent;
        }
        spin_unlock(&dcache_lock);
        return retval;
Elong:
        spin_unlock(&dcache_lock);
        return ERR_PTR(-ENAMETOOLONG);
}

/*
 * NOTE! The user-level library version returns a
 * character pointer. The kernel system call just
 * returns the length of the buffer filled (which
 * includes the ending '\0' character), or a negative
 * error value. So libc would do something like
 *
 *      char *getcwd(char * buf, size_t size)
 *      {
 *              int retval;
 *
 *              retval = sys_getcwd(buf, size);
 *              if (retval >= 0)
 *                      return buf;
 *              errno = -retval;
 *              return NULL;
 *      }
 */
SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
{
        int error;
        struct path pwd, root;
        char *page = (char *) __get_free_page(GFP_USER);

        if (!page)
                return -ENOMEM;

        read_lock(&current->fs->lock);
        pwd = current->fs->pwd;
        path_get(&pwd);
        root = current->fs->root;
        path_get(&root);
        read_unlock(&current->fs->lock);

        error = -ENOENT;
        spin_lock(&dcache_lock);
        if (!d_unlinked(pwd.dentry)) {
                unsigned long len;
                struct path tmp = root;
                char * cwd;

                cwd = __d_path(&pwd, &tmp, page, PAGE_SIZE);
                spin_unlock(&dcache_lock);

                error = PTR_ERR(cwd);
                if (IS_ERR(cwd))
                        goto out;

                error = -ERANGE;
                len = PAGE_SIZE + page - cwd;
                if (len <= size) {
                        error = len;
                        if (copy_to_user(buf, cwd, len))
                                error = -EFAULT;
                }
        } else
                spin_unlock(&dcache_lock);

out:
        path_put(&pwd);
        path_put(&root);
        free_page((unsigned long) page);
        return error;
}

/*
 * Test whether new_dentry is a subdirectory of old_dentry.
 *
 * Trivially implemented using the dcache structure
 */

/**
 * is_subdir - is new dentry a subdirectory of old_dentry
 * @new_dentry: new dentry
 * @old_dentry: old dentry
 *
 * Returns 1 if new_dentry is a subdirectory of the parent (at any depth).
 * Returns 0 otherwise.
 * Caller must ensure that "new_dentry" is pinned before calling is_subdir()
 */
  
int is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
{
        int result;
        unsigned long seq;

        if (new_dentry == old_dentry)
                return 1;

        /*
         * Need rcu_readlock to protect against the d_parent trashing
         * due to d_move
         */
        rcu_read_lock();
        do {
                /* for restarting inner loop in case of seq retry */
                seq = read_seqbegin(&rename_lock);
                if (d_ancestor(old_dentry, new_dentry))
                        result = 1;
                else
                        result = 0;
        } while (read_seqretry(&rename_lock, seq));
        rcu_read_unlock();

        return result;
}

void d_genocide(struct dentry *root)
{
        struct dentry *this_parent = root;
        struct list_head *next;

        spin_lock(&dcache_lock);
repeat:
        next = this_parent->d_subdirs.next;
resume:
        while (next != &this_parent->d_subdirs) {
                struct list_head *tmp = next;
                struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
                next = tmp->next;
                if (d_unhashed(dentry)||!dentry->d_inode)
                        continue;
                if (!list_empty(&dentry->d_subdirs)) {
                        this_parent = dentry;
                        goto repeat;
                }
                atomic_dec(&dentry->d_count);
        }
        if (this_parent != root) {
                next = this_parent->d_u.d_child.next;
                atomic_dec(&this_parent->d_count);
                this_parent = this_parent->d_parent;
                goto resume;
        }
        spin_unlock(&dcache_lock);
}

/**
 * find_inode_number - check for dentry with name
 * @dir: directory to check
 * @name: Name to find.
 *
 * Check whether a dentry already exists for the given name,
 * and return the inode number if it has an inode. Otherwise
 * 0 is returned.
 *
 * This routine is used to post-process directory listings for
 * filesystems using synthetic inode numbers, and is necessary
 * to keep getcwd() working.
 */
 
ino_t find_inode_number(struct dentry *dir, struct qstr *name)
{
        struct dentry * dentry;
        ino_t ino = 0;

        dentry = d_hash_and_lookup(dir, name);
        if (dentry) {
                if (dentry->d_inode)
                        ino = dentry->d_inode->i_ino;
                dput(dentry);
        }
        return ino;
}

static __initdata unsigned long dhash_entries;
static int __init set_dhash_entries(char *str)
{
        if (!str)
                return 0;
        dhash_entries = simple_strtoul(str, &str, 0);
        return 1;
}
__setup("dhash_entries=", set_dhash_entries);

static void __init dcache_init_early(void)
{
        int loop;

        /* If hashes are distributed across NUMA nodes, defer
         * hash allocation until vmalloc space is available.
         */
        if (hashdist)
                return;

        dentry_hashtable =
                alloc_large_system_hash("Dentry cache",
                                        sizeof(struct hlist_head),
                                        dhash_entries,
                                        13,
                                        HASH_EARLY,
                                        &d_hash_shift,
                                        &d_hash_mask,
                                        0);

        for (loop = 0; loop < (1 << d_hash_shift); loop++)
                INIT_HLIST_HEAD(&dentry_hashtable[loop]);
}

static void __init dcache_init(void)
{
        int loop;

        /* 
         * A constructor could be added for stable state like the lists,
         * but it is probably not worth it because of the cache nature
         * of the dcache. 
         */
        dentry_cache = KMEM_CACHE(dentry,
                SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD);
        
        register_shrinker(&dcache_shrinker);

        /* Hash may have been set up in dcache_init_early */
        if (!hashdist)
                return;

        dentry_hashtable =
                alloc_large_system_hash("Dentry cache",
                                        sizeof(struct hlist_head),
                                        dhash_entries,
                                        13,
                                        0,
                                        &d_hash_shift,
                                        &d_hash_mask,
                                        0);

        for (loop = 0; loop < (1 << d_hash_shift); loop++)
                INIT_HLIST_HEAD(&dentry_hashtable[loop]);
}

/* SLAB cache for __getname() consumers */
struct kmem_cache *names_cachep __read_mostly;

EXPORT_SYMBOL(d_genocide);

void __init vfs_caches_init_early(void)
{
        dcache_init_early();
        inode_init_early();
}

void __init vfs_caches_init(unsigned long mempages)
{
        unsigned long reserve;

        /* Base hash sizes on available memory, with a reserve equal to
           150% of current kernel size */

        reserve = min((mempages - nr_free_pages()) * 3/2, mempages - 1);
        mempages -= reserve;

        names_cachep = kmem_cache_create("names_cache", PATH_MAX, 0,
                        SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);

        dcache_init();
        inode_init();
        files_init(mempages);
        mnt_init();
        bdev_cache_init();
        chrdev_init();
}

EXPORT_SYMBOL(d_alloc);
EXPORT_SYMBOL(d_alloc_root);
EXPORT_SYMBOL(d_delete);
EXPORT_SYMBOL(d_find_alias);
EXPORT_SYMBOL(d_instantiate);
EXPORT_SYMBOL(d_invalidate);
EXPORT_SYMBOL(d_lookup);
EXPORT_SYMBOL(d_move);
EXPORT_SYMBOL_GPL(d_materialise_unique);
EXPORT_SYMBOL(d_path);
EXPORT_SYMBOL(d_prune_aliases);
EXPORT_SYMBOL(d_rehash);
EXPORT_SYMBOL(d_splice_alias);
EXPORT_SYMBOL(d_add_ci);
EXPORT_SYMBOL(d_validate);
EXPORT_SYMBOL(dget_locked);
EXPORT_SYMBOL(dput);
EXPORT_SYMBOL(find_inode_number);
EXPORT_SYMBOL(have_submounts);
EXPORT_SYMBOL(names_cachep);
EXPORT_SYMBOL(shrink_dcache_parent);
EXPORT_SYMBOL(shrink_dcache_sb);