Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux

[mirror_ubuntu-zesty-kernel.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/export.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 <linux/bit_spinlock.h>
#include <linux/rculist_bl.h>
#include <linux/prefetch.h>
#include <linux/ratelimit.h>
#include "internal.h"
#include "mount.h"

/*
 * Usage:
 * dcache->d_inode->i_lock protects:
 *   - i_dentry, d_alias, d_inode of aliases
 * dcache_hash_bucket lock protects:
 *   - the dcache hash table
 * s_anon bl list spinlock protects:
 *   - the s_anon list (see __d_drop)
 * dcache_lru_lock protects:
 *   - the dcache lru lists and counters
 * d_lock protects:
 *   - d_flags
 *   - d_name
 *   - d_lru
 *   - d_count
 *   - d_unhashed()
 *   - d_parent and d_subdirs
 *   - childrens' d_child and d_parent
 *   - d_alias, d_inode
 *
 * Ordering:
 * dentry->d_inode->i_lock
 *   dentry->d_lock
 *     dcache_lru_lock
 *     dcache_hash_bucket lock
 *     s_anon lock
 *
 * If there is an ancestor relationship:
 * dentry->d_parent->...->d_parent->d_lock
 *   ...
 *     dentry->d_parent->d_lock
 *       dentry->d_lock
 *
 * If no ancestor relationship:
 * if (dentry1 < dentry2)
 *   dentry1->d_lock
 *     dentry2->d_lock
 */
int sysctl_vfs_cache_pressure __read_mostly = 100;
EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);

static __cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lru_lock);
__cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);

EXPORT_SYMBOL(rename_lock);

static struct kmem_cache *dentry_cache __read_mostly;

/**
 * read_seqbegin_or_lock - begin a sequence number check or locking block
 * lock: sequence lock
 * seq : sequence number to be checked
 *
 * First try it once optimistically without taking the lock. If that fails,
 * take the lock. The sequence number is also used as a marker for deciding
 * whether to be a reader (even) or writer (odd).
 * N.B. seq must be initialized to an even number to begin with.
 */
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
        if (!(*seq & 1))        /* Even */
                *seq = read_seqbegin(lock);
        else                    /* Odd */
                write_seqlock(lock);
}

static inline int need_seqretry(seqlock_t *lock, int seq)
{
        return !(seq & 1) && read_seqretry(lock, seq);
}

static inline void done_seqretry(seqlock_t *lock, int seq)
{
        if (seq & 1)
                write_sequnlock(lock);
}

/*
 * 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_bl_head *dentry_hashtable __read_mostly;

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

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

static DEFINE_PER_CPU(unsigned int, nr_dentry);

#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
static int get_nr_dentry(void)
{
        int i;
        int sum = 0;
        for_each_possible_cpu(i)
                sum += per_cpu(nr_dentry, i);
        return sum < 0 ? 0 : sum;
}

int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
                   size_t *lenp, loff_t *ppos)
{
        dentry_stat.nr_dentry = get_nr_dentry();
        return proc_dointvec(table, write, buffer, lenp, ppos);
}
#endif

/*
 * Compare 2 name strings, return 0 if they match, otherwise non-zero.
 * The strings are both count bytes long, and count is non-zero.
 */
#ifdef CONFIG_DCACHE_WORD_ACCESS

#include <asm/word-at-a-time.h>
/*
 * NOTE! 'cs' and 'scount' come from a dentry, so it has a
 * aligned allocation for this particular component. We don't
 * strictly need the load_unaligned_zeropad() safety, but it
 * doesn't hurt either.
 *
 * In contrast, 'ct' and 'tcount' can be from a pathname, and do
 * need the careful unaligned handling.
 */
static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
{
        unsigned long a,b,mask;

        for (;;) {
                a = *(unsigned long *)cs;
                b = load_unaligned_zeropad(ct);
                if (tcount < sizeof(unsigned long))
                        break;
                if (unlikely(a != b))
                        return 1;
                cs += sizeof(unsigned long);
                ct += sizeof(unsigned long);
                tcount -= sizeof(unsigned long);
                if (!tcount)
                        return 0;
        }
        mask = ~(~0ul << tcount*8);
        return unlikely(!!((a ^ b) & mask));
}

#else

static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
{
        do {
                if (*cs != *ct)
                        return 1;
                cs++;
                ct++;
                tcount--;
        } while (tcount);
        return 0;
}

#endif

static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
{
        const unsigned char *cs;
        /*
         * Be careful about RCU walk racing with rename:
         * use ACCESS_ONCE to fetch the name pointer.
         *
         * NOTE! Even if a rename will mean that the length
         * was not loaded atomically, we don't care. The
         * RCU walk will check the sequence count eventually,
         * and catch it. And we won't overrun the buffer,
         * because we're reading the name pointer atomically,
         * and a dentry name is guaranteed to be properly
         * terminated with a NUL byte.
         *
         * End result: even if 'len' is wrong, we'll exit
         * early because the data cannot match (there can
         * be no NUL in the ct/tcount data)
         */
        cs = ACCESS_ONCE(dentry->d_name.name);
        smp_read_barrier_depends();
        return dentry_string_cmp(cs, ct, tcount);
}

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

        WARN_ON(!hlist_unhashed(&dentry->d_alias));
        if (dname_external(dentry))
                kfree(dentry->d_name.name);
        kmem_cache_free(dentry_cache, dentry); 
}

/*
 * no locks, please.
 */
static void d_free(struct dentry *dentry)
{
        BUG_ON((int)dentry->d_lockref.count > 0);
        this_cpu_dec(nr_dentry);
        if (dentry->d_op && dentry->d_op->d_release)
                dentry->d_op->d_release(dentry);

        /* if dentry was never visible to RCU, immediate free is OK */
        if (!(dentry->d_flags & DCACHE_RCUACCESS))
                __d_free(&dentry->d_u.d_rcu);
        else
                call_rcu(&dentry->d_u.d_rcu, __d_free);
}

/**
 * dentry_rcuwalk_barrier - invalidate in-progress rcu-walk lookups
 * @dentry: the target dentry
 * After this call, in-progress rcu-walk path lookup will fail. This
 * should be called after unhashing, and after changing d_inode (if
 * the dentry has not already been unhashed).
 */
static inline void dentry_rcuwalk_barrier(struct dentry *dentry)
{
        assert_spin_locked(&dentry->d_lock);
        /* Go through a barrier */
        write_seqcount_barrier(&dentry->d_seq);
}

/*
 * Release the dentry's inode, using the filesystem
 * d_iput() operation if defined. Dentry has no refcount
 * and is unhashed.
 */
static void dentry_iput(struct dentry * dentry)
        __releases(dentry->d_lock)
        __releases(dentry->d_inode->i_lock)
{
        struct inode *inode = dentry->d_inode;
        if (inode) {
                dentry->d_inode = NULL;
                hlist_del_init(&dentry->d_alias);
                spin_unlock(&dentry->d_lock);
                spin_unlock(&inode->i_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);
        }
}

/*
 * Release the dentry's inode, using the filesystem
 * d_iput() operation if defined. dentry remains in-use.
 */
static void dentry_unlink_inode(struct dentry * dentry)
        __releases(dentry->d_lock)
        __releases(dentry->d_inode->i_lock)
{
        struct inode *inode = dentry->d_inode;
        dentry->d_inode = NULL;
        hlist_del_init(&dentry->d_alias);
        dentry_rcuwalk_barrier(dentry);
        spin_unlock(&dentry->d_lock);
        spin_unlock(&inode->i_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);
}

/*
 * dentry_lru_(add|del|prune|move_tail) must be called with d_lock held.
 */
static void dentry_lru_add(struct dentry *dentry)
{
        if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST))) {
                spin_lock(&dcache_lru_lock);
                dentry->d_flags |= DCACHE_LRU_LIST;
                list_add(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
                dentry->d_sb->s_nr_dentry_unused++;
                dentry_stat.nr_unused++;
                spin_unlock(&dcache_lru_lock);
        }
}

static void __dentry_lru_del(struct dentry *dentry)
{
        list_del_init(&dentry->d_lru);
        dentry->d_flags &= ~(DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
        dentry->d_sb->s_nr_dentry_unused--;
        dentry_stat.nr_unused--;
}

/*
 * Remove a dentry with references from the LRU.
 */
static void dentry_lru_del(struct dentry *dentry)
{
        if (!list_empty(&dentry->d_lru)) {
                spin_lock(&dcache_lru_lock);
                __dentry_lru_del(dentry);
                spin_unlock(&dcache_lru_lock);
        }
}

static void dentry_lru_move_list(struct dentry *dentry, struct list_head *list)
{
        spin_lock(&dcache_lru_lock);
        if (list_empty(&dentry->d_lru)) {
                dentry->d_flags |= DCACHE_LRU_LIST;
                list_add_tail(&dentry->d_lru, list);
                dentry->d_sb->s_nr_dentry_unused++;
                dentry_stat.nr_unused++;
        } else {
                list_move_tail(&dentry->d_lru, list);
        }
        spin_unlock(&dcache_lru_lock);
}

/**
 * d_kill - kill dentry and return parent
 * @dentry: dentry to kill
 * @parent: parent dentry
 *
 * The dentry must already be unhashed and removed from the LRU.
 *
 * If this is the root of the dentry tree, return NULL.
 *
 * dentry->d_lock and parent->d_lock must be held by caller, and are dropped by
 * d_kill.
 */
static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent)
        __releases(dentry->d_lock)
        __releases(parent->d_lock)
        __releases(dentry->d_inode->i_lock)
{
        list_del(&dentry->d_u.d_child);
        /*
         * Inform try_to_ascend() that we are no longer attached to the
         * dentry tree
         */
        dentry->d_flags |= DCACHE_DENTRY_KILLED;
        if (parent)
                spin_unlock(&parent->d_lock);
        dentry_iput(dentry);
        /*
         * dentry_iput drops the locks, at which point nobody (except
         * transient RCU lookups) can reach this dentry.
         */
        d_free(dentry);
        return parent;
}

/*
 * Unhash a dentry without inserting an RCU walk barrier or checking that
 * dentry->d_lock is locked.  The caller must take care of that, if
 * appropriate.
 */
static void __d_shrink(struct dentry *dentry)
{
        if (!d_unhashed(dentry)) {
                struct hlist_bl_head *b;
                if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
                        b = &dentry->d_sb->s_anon;
                else
                        b = d_hash(dentry->d_parent, dentry->d_name.hash);

                hlist_bl_lock(b);
                __hlist_bl_del(&dentry->d_hash);
                dentry->d_hash.pprev = NULL;
                hlist_bl_unlock(b);
        }
}

/**
 * d_drop - drop a dentry
 * @dentry: dentry to drop
 *
 * d_drop() unhashes the entry from the parent dentry hashes, so that it won't
 * be found through a VFS lookup any more. Note that this is different from
 * deleting the dentry - d_delete will try to mark the dentry negative if
 * possible, giving a successful _negative_ lookup, while d_drop will
 * just make the cache lookup fail.
 *
 * d_drop() is used mainly for stuff that wants to invalidate a dentry for some
 * reason (NFS timeouts or autofs deletes).
 *
 * __d_drop requires dentry->d_lock.
 */
void __d_drop(struct dentry *dentry)
{
        if (!d_unhashed(dentry)) {
                __d_shrink(dentry);
                dentry_rcuwalk_barrier(dentry);
        }
}
EXPORT_SYMBOL(__d_drop);

void d_drop(struct dentry *dentry)
{
        spin_lock(&dentry->d_lock);
        __d_drop(dentry);
        spin_unlock(&dentry->d_lock);
}
EXPORT_SYMBOL(d_drop);

/*
 * Finish off a dentry we've decided to kill.
 * dentry->d_lock must be held, returns with it unlocked.
 * If ref is non-zero, then decrement the refcount too.
 * Returns dentry requiring refcount drop, or NULL if we're done.
 */
static inline struct dentry *dentry_kill(struct dentry *dentry)
        __releases(dentry->d_lock)
{
        struct inode *inode;
        struct dentry *parent;

        inode = dentry->d_inode;
        if (inode && !spin_trylock(&inode->i_lock)) {
relock:
                spin_unlock(&dentry->d_lock);
                cpu_relax();
                return dentry; /* try again with same dentry */
        }
        if (IS_ROOT(dentry))
                parent = NULL;
        else
                parent = dentry->d_parent;
        if (parent && !spin_trylock(&parent->d_lock)) {
                if (inode)
                        spin_unlock(&inode->i_lock);
                goto relock;
        }

        /*
         * The dentry is now unrecoverably dead to the world.
         */
        lockref_mark_dead(&dentry->d_lockref);

        /*
         * inform the fs via d_prune that this dentry is about to be
         * unhashed and destroyed.
         */
        if ((dentry->d_flags & DCACHE_OP_PRUNE) && !d_unhashed(dentry))
                dentry->d_op->d_prune(dentry);

        dentry_lru_del(dentry);
        /* if it was on the hash then remove it */
        __d_drop(dentry);
        return d_kill(dentry, 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.
 */
void dput(struct dentry *dentry)
{
        if (unlikely(!dentry))
                return;

repeat:
        if (lockref_put_or_lock(&dentry->d_lockref))
                return;

        /* Unreachable? Get rid of it */
        if (unlikely(d_unhashed(dentry)))
                goto kill_it;

        if (unlikely(dentry->d_flags & DCACHE_OP_DELETE)) {
                if (dentry->d_op->d_delete(dentry))
                        goto kill_it;
        }

        dentry->d_flags |= DCACHE_REFERENCED;
        dentry_lru_add(dentry);

        dentry->d_lockref.count--;
        spin_unlock(&dentry->d_lock);
        return;

kill_it:
        dentry = dentry_kill(dentry);
        if (dentry)
                goto repeat;
}
EXPORT_SYMBOL(dput);

/**
 * 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(&dentry->d_lock);
        if (d_unhashed(dentry)) {
                spin_unlock(&dentry->d_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(&dentry->d_lock);
                shrink_dcache_parent(dentry);
                spin_lock(&dentry->d_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).
         * We also need to leave mountpoints alone,
         * directory or not.
         */
        if (dentry->d_lockref.count > 1 && dentry->d_inode) {
                if (S_ISDIR(dentry->d_inode->i_mode) || d_mountpoint(dentry)) {
                        spin_unlock(&dentry->d_lock);
                        return -EBUSY;
                }
        }

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

/* This must be called with d_lock held */
static inline void __dget_dlock(struct dentry *dentry)
{
        dentry->d_lockref.count++;
}

static inline void __dget(struct dentry *dentry)
{
        lockref_get(&dentry->d_lockref);
}

struct dentry *dget_parent(struct dentry *dentry)
{
        int gotref;
        struct dentry *ret;

        /*
         * Do optimistic parent lookup without any
         * locking.
         */
        rcu_read_lock();
        ret = ACCESS_ONCE(dentry->d_parent);
        gotref = lockref_get_not_zero(&ret->d_lockref);
        rcu_read_unlock();
        if (likely(gotref)) {
                if (likely(ret == ACCESS_ONCE(dentry->d_parent)))
                        return ret;
                dput(ret);
        }

repeat:
        /*
         * Don't need rcu_dereference because we re-check it was correct under
         * the lock.
         */
        rcu_read_lock();
        ret = dentry->d_parent;
        spin_lock(&ret->d_lock);
        if (unlikely(ret != dentry->d_parent)) {
                spin_unlock(&ret->d_lock);
                rcu_read_unlock();
                goto repeat;
        }
        rcu_read_unlock();
        BUG_ON(!ret->d_lockref.count);
        ret->d_lockref.count++;
        spin_unlock(&ret->d_lock);
        return ret;
}
EXPORT_SYMBOL(dget_parent);

/**
 * 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 dentry *alias, *discon_alias;

again:
        discon_alias = NULL;
        hlist_for_each_entry(alias, &inode->i_dentry, d_alias) {
                spin_lock(&alias->d_lock);
                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_dlock(alias);
                                spin_unlock(&alias->d_lock);
                                return alias;
                        }
                }
                spin_unlock(&alias->d_lock);
        }
        if (discon_alias) {
                alias = discon_alias;
                spin_lock(&alias->d_lock);
                if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
                        if (IS_ROOT(alias) &&
                            (alias->d_flags & DCACHE_DISCONNECTED)) {
                                __dget_dlock(alias);
                                spin_unlock(&alias->d_lock);
                                return alias;
                        }
                }
                spin_unlock(&alias->d_lock);
                goto again;
        }
        return NULL;
}

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

        if (!hlist_empty(&inode->i_dentry)) {
                spin_lock(&inode->i_lock);
                de = __d_find_alias(inode, 0);
                spin_unlock(&inode->i_lock);
        }
        return de;
}
EXPORT_SYMBOL(d_find_alias);

/*
 *      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(&inode->i_lock);
        hlist_for_each_entry(dentry, &inode->i_dentry, d_alias) {
                spin_lock(&dentry->d_lock);
                if (!dentry->d_lockref.count) {
                        /*
                         * inform the fs via d_prune that this dentry
                         * is about to be unhashed and destroyed.
                         */
                        if ((dentry->d_flags & DCACHE_OP_PRUNE) &&
                            !d_unhashed(dentry))
                                dentry->d_op->d_prune(dentry);

                        __dget_dlock(dentry);
                        __d_drop(dentry);
                        spin_unlock(&dentry->d_lock);
                        spin_unlock(&inode->i_lock);
                        dput(dentry);
                        goto restart;
                }
                spin_unlock(&dentry->d_lock);
        }
        spin_unlock(&inode->i_lock);
}
EXPORT_SYMBOL(d_prune_aliases);

/*
 * Try to throw away a dentry - free the inode, dput the parent.
 * Requires dentry->d_lock is held, and dentry->d_count == 0.
 * Releases dentry->d_lock.
 *
 * This may fail if locks cannot be acquired no problem, just try again.
 */
static void try_prune_one_dentry(struct dentry *dentry)
        __releases(dentry->d_lock)
{
        struct dentry *parent;

        parent = dentry_kill(dentry);
        /*
         * If dentry_kill returns NULL, we have nothing more to do.
         * if it returns the same dentry, trylocks failed. In either
         * case, just loop again.
         *
         * Otherwise, we need to prune ancestors too. This is necessary
         * to prevent quadratic behavior of shrink_dcache_parent(), but
         * is also expected to be beneficial in reducing dentry cache
         * fragmentation.
         */
        if (!parent)
                return;
        if (parent == dentry)
                return;

        /* Prune ancestors. */
        dentry = parent;
        while (dentry) {
                if (lockref_put_or_lock(&dentry->d_lockref))
                        return;
                dentry = dentry_kill(dentry);
        }
}

static void shrink_dentry_list(struct list_head *list)
{
        struct dentry *dentry;

        rcu_read_lock();
        for (;;) {
                dentry = list_entry_rcu(list->prev, struct dentry, d_lru);
                if (&dentry->d_lru == list)
                        break; /* empty */
                spin_lock(&dentry->d_lock);
                if (dentry != list_entry(list->prev, struct dentry, d_lru)) {
                        spin_unlock(&dentry->d_lock);
                        continue;
                }

                /*
                 * 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 (dentry->d_lockref.count) {
                        dentry_lru_del(dentry);
                        spin_unlock(&dentry->d_lock);
                        continue;
                }

                rcu_read_unlock();

                try_prune_one_dentry(dentry);

                rcu_read_lock();
        }
        rcu_read_unlock();
}

/**
 * prune_dcache_sb - shrink the dcache
 * @sb: superblock
 * @count: number of entries to try to free
 *
 * Attempt to shrink the superblock dcache LRU by @count entries. This is
 * done when we need more memory an called from the superblock shrinker
 * function.
 *
 * This function may fail to free any resources if all the dentries are in
 * use.
 */
void prune_dcache_sb(struct super_block *sb, int count)
{
        struct dentry *dentry;
        LIST_HEAD(referenced);
        LIST_HEAD(tmp);

relock:
        spin_lock(&dcache_lru_lock);
        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);

                if (!spin_trylock(&dentry->d_lock)) {
                        spin_unlock(&dcache_lru_lock);
                        cpu_relax();
                        goto relock;
                }

                if (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);
                        dentry->d_flags |= DCACHE_SHRINK_LIST;
                        spin_unlock(&dentry->d_lock);
                        if (!--count)
                                break;
                }
                cond_resched_lock(&dcache_lru_lock);
        }
        if (!list_empty(&referenced))
                list_splice(&referenced, &sb->s_dentry_lru);
        spin_unlock(&dcache_lru_lock);

        shrink_dentry_list(&tmp);
}

/**
 * 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)
{
        LIST_HEAD(tmp);

        spin_lock(&dcache_lru_lock);
        while (!list_empty(&sb->s_dentry_lru)) {
                list_splice_init(&sb->s_dentry_lru, &tmp);
                spin_unlock(&dcache_lru_lock);
                shrink_dentry_list(&tmp);
                spin_lock(&dcache_lru_lock);
        }
        spin_unlock(&dcache_lru_lock);
}
EXPORT_SYMBOL(shrink_dcache_sb);

/*
 * 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;

        BUG_ON(!IS_ROOT(dentry));

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

                        /*
                         * inform the fs that this dentry is about to be
                         * unhashed and destroyed.
                         */
                        if ((dentry->d_flags & DCACHE_OP_PRUNE) &&
                            !d_unhashed(dentry))
                                dentry->d_op->d_prune(dentry);

                        dentry_lru_del(dentry);
                        __d_shrink(dentry);

                        if (dentry->d_lockref.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,
                                       dentry->d_lockref.count,
                                       dentry->d_sb->s_type->name,
                                       dentry->d_sb->s_id);
                                BUG();
                        }

                        if (IS_ROOT(dentry)) {
                                parent = NULL;
                                list_del(&dentry->d_u.d_child);
                        } else {
                                parent = dentry->d_parent;
                                parent->d_lockref.count--;
                                list_del(&dentry->d_u.d_child);
                        }

                        inode = dentry->d_inode;
                        if (inode) {
                                dentry->d_inode = NULL;
                                hlist_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)
                                return;
                        dentry = parent;
                } while (list_empty(&dentry->d_subdirs));

                dentry = list_entry(dentry->d_subdirs.next,
                                    struct dentry, d_u.d_child);
        }
}

/*
 * destroy the dentries attached to a superblock on unmounting
 * - we don't need to use dentry->d_lock 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;
        dentry->d_lockref.count--;
        shrink_dcache_for_umount_subtree(dentry);

        while (!hlist_bl_empty(&sb->s_anon)) {
                dentry = hlist_bl_entry(hlist_bl_first(&sb->s_anon), struct dentry, d_hash);
                shrink_dcache_for_umount_subtree(dentry);
        }
}

/*
 * This tries to ascend one level of parenthood, but
 * we can race with renaming, so we need to re-check
 * the parenthood after dropping the lock and check
 * that the sequence number still matches.
 */
static struct dentry *try_to_ascend(struct dentry *old, unsigned seq)
{
        struct dentry *new = old->d_parent;

        rcu_read_lock();
        spin_unlock(&old->d_lock);
        spin_lock(&new->d_lock);

        /*
         * might go back up the wrong parent if we have had a rename
         * or deletion
         */
        if (new != old->d_parent ||
                 (old->d_flags & DCACHE_DENTRY_KILLED) ||
                 need_seqretry(&rename_lock, seq)) {
                spin_unlock(&new->d_lock);
                new = NULL;
        }
        rcu_read_unlock();
        return new;
}

/**
 * enum d_walk_ret - action to talke during tree walk
 * @D_WALK_CONTINUE:    contrinue walk
 * @D_WALK_QUIT:        quit walk
 * @D_WALK_NORETRY:     quit when retry is needed
 * @D_WALK_SKIP:        skip this dentry and its children
 */
enum d_walk_ret {
        D_WALK_CONTINUE,
        D_WALK_QUIT,
        D_WALK_NORETRY,
        D_WALK_SKIP,
};

/**
 * d_walk - walk the dentry tree
 * @parent:     start of walk
 * @data:       data passed to @enter() and @finish()
 * @enter:      callback when first entering the dentry
 * @finish:     callback when successfully finished the walk
 *
 * The @enter() and @finish() callbacks are called with d_lock held.
 */
static void d_walk(struct dentry *parent, void *data,
                   enum d_walk_ret (*enter)(void *, struct dentry *),
                   void (*finish)(void *))
{
        struct dentry *this_parent;
        struct list_head *next;
        unsigned seq = 0;
        enum d_walk_ret ret;
        bool retry = true;

again:
        read_seqbegin_or_lock(&rename_lock, &seq);
        this_parent = parent;
        spin_lock(&this_parent->d_lock);

        ret = enter(data, this_parent);
        switch (ret) {
        case D_WALK_CONTINUE:
                break;
        case D_WALK_QUIT:
        case D_WALK_SKIP:
                goto out_unlock;
        case D_WALK_NORETRY:
                retry = false;
                break;
        }
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;

                spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);

                ret = enter(data, dentry);
                switch (ret) {
                case D_WALK_CONTINUE:
                        break;
                case D_WALK_QUIT:
                        spin_unlock(&dentry->d_lock);
                        goto out_unlock;
                case D_WALK_NORETRY:
                        retry = false;
                        break;
                case D_WALK_SKIP:
                        spin_unlock(&dentry->d_lock);
                        continue;
                }

                if (!list_empty(&dentry->d_subdirs)) {
                        spin_unlock(&this_parent->d_lock);
                        spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);
                        this_parent = dentry;
                        spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
                        goto repeat;
                }
                spin_unlock(&dentry->d_lock);
        }
        /*
         * All done at this level ... ascend and resume the search.
         */
        if (this_parent != parent) {
                struct dentry *child = this_parent;
                this_parent = try_to_ascend(this_parent, seq);
                if (!this_parent)
                        goto rename_retry;
                next = child->d_u.d_child.next;
                goto resume;
        }
        if (need_seqretry(&rename_lock, seq)) {
                spin_unlock(&this_parent->d_lock);
                goto rename_retry;
        }
        if (finish)
                finish(data);

out_unlock:
        spin_unlock(&this_parent->d_lock);
        done_seqretry(&rename_lock, seq);
        return;

rename_retry:
        if (!retry)
                return;
        seq = 1;
        goto again;
}

/*
 * 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
 */

static enum d_walk_ret check_mount(void *data, struct dentry *dentry)
{
        int *ret = data;
        if (d_mountpoint(dentry)) {
                *ret = 1;
                return D_WALK_QUIT;
        }
        return D_WALK_CONTINUE;
}

int have_submounts(struct dentry *parent)
{
        int ret = 0;

        d_walk(parent, &ret, check_mount, NULL);

        return ret;
}
EXPORT_SYMBOL(have_submounts);

/*
 * Called by mount code to set a mountpoint and check if the mountpoint is
 * reachable (e.g. NFS can unhash a directory dentry and then the complete
 * subtree can become unreachable).
 *
 * Only one of check_submounts_and_drop() and d_set_mounted() must succeed.  For
 * this reason take rename_lock and d_lock on dentry and ancestors.
 */
int d_set_mounted(struct dentry *dentry)
{
        struct dentry *p;
        int ret = -ENOENT;
        write_seqlock(&rename_lock);
        for (p = dentry->d_parent; !IS_ROOT(p); p = p->d_parent) {
                /* Need exclusion wrt. check_submounts_and_drop() */
                spin_lock(&p->d_lock);
                if (unlikely(d_unhashed(p))) {
                        spin_unlock(&p->d_lock);
                        goto out;
                }
                spin_unlock(&p->d_lock);
        }
        spin_lock(&dentry->d_lock);
        if (!d_unlinked(dentry)) {
                dentry->d_flags |= DCACHE_MOUNTED;
                ret = 0;
        }
        spin_unlock(&dentry->d_lock);
out:
        write_sequnlock(&rename_lock);
        return ret;
}

/*
 * Search the dentry child list of 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.
 */

struct select_data {
        struct dentry *start;
        struct list_head dispose;
        int found;
};

static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
{
        struct select_data *data = _data;
        enum d_walk_ret ret = D_WALK_CONTINUE;

        if (data->start == dentry)
                goto out;

        /*
         * move only zero ref count dentries to the dispose list.
         *
         * Those which are presently on the shrink list, being processed
         * by shrink_dentry_list(), shouldn't be moved.  Otherwise the
         * loop in shrink_dcache_parent() might not make any progress
         * and loop forever.
         */
        if (dentry->d_lockref.count) {
                dentry_lru_del(dentry);
        } else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
                dentry_lru_move_list(dentry, &data->dispose);
                dentry->d_flags |= DCACHE_SHRINK_LIST;
                data->found++;
                ret = D_WALK_NORETRY;
        }
        /*
         * 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 (data->found && need_resched())
                ret = D_WALK_QUIT;
out:
        return ret;
}

/**
 * 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)
{
        for (;;) {
                struct select_data data;

                INIT_LIST_HEAD(&data.dispose);
                data.start = parent;
                data.found = 0;

                d_walk(parent, &data, select_collect, NULL);
                if (!data.found)
                        break;

                shrink_dentry_list(&data.dispose);
                cond_resched();
        }
}
EXPORT_SYMBOL(shrink_dcache_parent);

static enum d_walk_ret check_and_collect(void *_data, struct dentry *dentry)
{
        struct select_data *data = _data;

        if (d_mountpoint(dentry)) {
                data->found = -EBUSY;
                return D_WALK_QUIT;
        }

        return select_collect(_data, dentry);
}

static void check_and_drop(void *_data)
{
        struct select_data *data = _data;

        if (d_mountpoint(data->start))
                data->found = -EBUSY;
        if (!data->found)
                __d_drop(data->start);
}

/**
 * check_submounts_and_drop - prune dcache, check for submounts and drop
 *
 * All done as a single atomic operation relative to has_unlinked_ancestor().
 * Returns 0 if successfully unhashed @parent.  If there were submounts then
 * return -EBUSY.
 *
 * @dentry: dentry to prune and drop
 */
int check_submounts_and_drop(struct dentry *dentry)
{
        int ret = 0;

        /* Negative dentries can be dropped without further checks */
        if (!dentry->d_inode) {
                d_drop(dentry);
                goto out;
        }

        for (;;) {
                struct select_data data;

                INIT_LIST_HEAD(&data.dispose);
                data.start = dentry;
                data.found = 0;

                d_walk(dentry, &data, check_and_collect, check_and_drop);
                ret = data.found;

                if (!list_empty(&data.dispose))
                        shrink_dentry_list(&data.dispose);

                if (ret <= 0)
                        break;

                cond_resched();
        }

out:
        return ret;
}
EXPORT_SYMBOL(check_submounts_and_drop);

/**
 * __d_alloc    -       allocate a dcache entry
 * @sb: filesystem it will belong to
 * @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 super_block *sb, const struct qstr *name)
{
        struct dentry *dentry;
        char *dname;

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

        /*
         * We guarantee that the inline name is always NUL-terminated.
         * This way the memcpy() done by the name switching in rename
         * will still always have a NUL at the end, even if we might
         * be overwriting an internal NUL character
         */
        dentry->d_iname[DNAME_INLINE_LEN-1] = 0;
        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.len = name->len;
        dentry->d_name.hash = name->hash;
        memcpy(dname, name->name, name->len);
        dname[name->len] = 0;

        /* Make sure we always see the terminating NUL character */
        smp_wmb();
        dentry->d_name.name = dname;

        dentry->d_lockref.count = 1;
        dentry->d_flags = 0;
        spin_lock_init(&dentry->d_lock);
        seqcount_init(&dentry->d_seq);
        dentry->d_inode = NULL;
        dentry->d_parent = dentry;
        dentry->d_sb = sb;
        dentry->d_op = NULL;
        dentry->d_fsdata = NULL;
        INIT_HLIST_BL_NODE(&dentry->d_hash);
        INIT_LIST_HEAD(&dentry->d_lru);
        INIT_LIST_HEAD(&dentry->d_subdirs);
        INIT_HLIST_NODE(&dentry->d_alias);
        INIT_LIST_HEAD(&dentry->d_u.d_child);
        d_set_d_op(dentry, dentry->d_sb->s_d_op);

        this_cpu_inc(nr_dentry);

        return dentry;
}

/**
 * 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 = __d_alloc(parent->d_sb, name);
        if (!dentry)
                return NULL;

        spin_lock(&parent->d_lock);
        /*
         * don't need child lock because it is not subject
         * to concurrency here
         */
        __dget_dlock(parent);
        dentry->d_parent = parent;
        list_add(&dentry->d_u.d_child, &parent->d_subdirs);
        spin_unlock(&parent->d_lock);

        return dentry;
}
EXPORT_SYMBOL(d_alloc);

struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
{
        struct dentry *dentry = __d_alloc(sb, name);
        if (dentry)
                dentry->d_flags |= DCACHE_DISCONNECTED;
        return dentry;
}
EXPORT_SYMBOL(d_alloc_pseudo);

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);
}
EXPORT_SYMBOL(d_alloc_name);

void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
{
        WARN_ON_ONCE(dentry->d_op);
        WARN_ON_ONCE(dentry->d_flags & (DCACHE_OP_HASH  |
                                DCACHE_OP_COMPARE       |
                                DCACHE_OP_REVALIDATE    |
                                DCACHE_OP_WEAK_REVALIDATE       |
                                DCACHE_OP_DELETE ));
        dentry->d_op = op;
        if (!op)
                return;
        if (op->d_hash)
                dentry->d_flags |= DCACHE_OP_HASH;
        if (op->d_compare)
                dentry->d_flags |= DCACHE_OP_COMPARE;
        if (op->d_revalidate)
                dentry->d_flags |= DCACHE_OP_REVALIDATE;
        if (op->d_weak_revalidate)
                dentry->d_flags |= DCACHE_OP_WEAK_REVALIDATE;
        if (op->d_delete)
                dentry->d_flags |= DCACHE_OP_DELETE;
        if (op->d_prune)
                dentry->d_flags |= DCACHE_OP_PRUNE;

}
EXPORT_SYMBOL(d_set_d_op);

static void __d_instantiate(struct dentry *dentry, struct inode *inode)
{
        spin_lock(&dentry->d_lock);
        if (inode) {
                if (unlikely(IS_AUTOMOUNT(inode)))
                        dentry->d_flags |= DCACHE_NEED_AUTOMOUNT;
                hlist_add_head(&dentry->d_alias, &inode->i_dentry);
        }
        dentry->d_inode = inode;
        dentry_rcuwalk_barrier(dentry);
        spin_unlock(&dentry->d_lock);
        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(!hlist_unhashed(&entry->d_alias));
        if (inode)
                spin_lock(&inode->i_lock);
        __d_instantiate(entry, inode);
        if (inode)
                spin_unlock(&inode->i_lock);
        security_d_instantiate(entry, inode);
}
EXPORT_SYMBOL(d_instantiate);

/**
 * 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;
        }

        hlist_for_each_entry(alias, &inode->i_dentry, d_alias) {
                /*
                 * Don't need alias->d_lock here, because aliases with
                 * d_parent == entry->d_parent are not subject to name or
                 * parent changes, because the parent inode i_mutex is held.
                 */
                if (alias->d_name.hash != hash)
                        continue;
                if (alias->d_parent != entry->d_parent)
                        continue;
                if (alias->d_name.len != len)
                        continue;
                if (dentry_cmp(alias, name, len))
                        continue;
                __dget(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(!hlist_unhashed(&entry->d_alias));

        if (inode)
                spin_lock(&inode->i_lock);
        result = __d_instantiate_unique(entry, inode);
        if (inode)
                spin_unlock(&inode->i_lock);

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

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

EXPORT_SYMBOL(d_instantiate_unique);

struct dentry *d_make_root(struct inode *root_inode)
{
        struct dentry *res = NULL;

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

                res = __d_alloc(root_inode->i_sb, &name);
                if (res)
                        d_instantiate(res, root_inode);
                else
                        iput(root_inode);
        }
        return res;
}
EXPORT_SYMBOL(d_make_root);

static struct dentry * __d_find_any_alias(struct inode *inode)
{
        struct dentry *alias;

        if (hlist_empty(&inode->i_dentry))
                return NULL;
        alias = hlist_entry(inode->i_dentry.first, struct dentry, d_alias);
        __dget(alias);
        return alias;
}

/**
 * d_find_any_alias - find any alias for a given inode
 * @inode: inode to find an alias for
 *
 * If any aliases exist for the given inode, take and return a
 * reference for one of them.  If no aliases exist, return %NULL.
 */
struct dentry *d_find_any_alias(struct inode *inode)
{
        struct dentry *de;

        spin_lock(&inode->i_lock);
        de = __d_find_any_alias(inode);
        spin_unlock(&inode->i_lock);
        return de;
}
EXPORT_SYMBOL(d_find_any_alias);

/**
 * 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 = QSTR_INIT("/", 1);
        struct dentry *tmp;
        struct dentry *res;

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

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

        tmp = __d_alloc(inode->i_sb, &anonstring);
        if (!tmp) {
                res = ERR_PTR(-ENOMEM);
                goto out_iput;
        }

        spin_lock(&inode->i_lock);
        res = __d_find_any_alias(inode);
        if (res) {
                spin_unlock(&inode->i_lock);
                dput(tmp);
                goto out_iput;
        }

        /* attach a disconnected dentry */
        spin_lock(&tmp->d_lock);
        tmp->d_inode = inode;
        tmp->d_flags |= DCACHE_DISCONNECTED;
        hlist_add_head(&tmp->d_alias, &inode->i_dentry);
        hlist_bl_lock(&tmp->d_sb->s_anon);
        hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon);
        hlist_bl_unlock(&tmp->d_sb->s_anon);
        spin_unlock(&tmp->d_lock);
        spin_unlock(&inode->i_lock);
        security_d_instantiate(tmp, inode);

        return tmp;

 out_iput:
        if (res && !IS_ERR(res))
                security_d_instantiate(res, inode);
        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.
 *
 * Cluster filesystems may call this function with a negative, hashed dentry.
 * In that case, we know that the inode will be a regular file, and also this
 * will only occur during atomic_open. So we need to check for the dentry
 * being already hashed only in the final case.
 */
struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
{
        struct dentry *new = NULL;

        if (IS_ERR(inode))
                return ERR_CAST(inode);

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

/**
 * 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)
{
        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 (unlikely(IS_ERR(found)))
                goto err_out;
        if (!found) {
                new = d_alloc(dentry->d_parent, name);
                if (!new) {
                        found = ERR_PTR(-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.
         */
        new = d_splice_alias(inode, found);
        if (new) {
                dput(found);
                found = new;
        }
        return found;

err_out:
        iput(inode);
        return found;
}
EXPORT_SYMBOL(d_add_ci);

/*
 * Do the slow-case of the dentry name compare.
 *
 * Unlike the dentry_cmp() function, we need to atomically
 * load the name and length information, so that the
 * filesystem can rely on them, and can use the 'name' and
 * 'len' information without worrying about walking off the
 * end of memory etc.
 *
 * Thus the read_seqcount_retry() and the "duplicate" info
 * in arguments (the low-level filesystem should not look
 * at the dentry inode or name contents directly, since
 * rename can change them while we're in RCU mode).
 */
enum slow_d_compare {
        D_COMP_OK,
        D_COMP_NOMATCH,
        D_COMP_SEQRETRY,
};

static noinline enum slow_d_compare slow_dentry_cmp(
                const struct dentry *parent,
                struct dentry *dentry,
                unsigned int seq,
                const struct qstr *name)
{
        int tlen = dentry->d_name.len;
        const char *tname = dentry->d_name.name;

        if (read_seqcount_retry(&dentry->d_seq, seq)) {
                cpu_relax();
                return D_COMP_SEQRETRY;
        }
        if (parent->d_op->d_compare(parent, dentry, tlen, tname, name))
                return D_COMP_NOMATCH;
        return D_COMP_OK;
}

/**
 * __d_lookup_rcu - search for a dentry (racy, store-free)
 * @parent: parent dentry
 * @name: qstr of name we wish to find
 * @seqp: returns d_seq value at the point where the dentry was found
 * Returns: dentry, or NULL
 *
 * __d_lookup_rcu is the dcache lookup function for rcu-walk name
 * resolution (store-free path walking) design described in
 * Documentation/filesystems/path-lookup.txt.
 *
 * This is not to be used outside core vfs.
 *
 * __d_lookup_rcu must only be used in rcu-walk mode, ie. with vfsmount lock
 * held, and rcu_read_lock held. The returned dentry must not be stored into
 * without taking d_lock and checking d_seq sequence count against @seq
 * returned here.
 *
 * A refcount may be taken on the found dentry with the d_rcu_to_refcount
 * function.
 *
 * Alternatively, __d_lookup_rcu may be called again to look up the child of
 * the returned dentry, so long as its parent's seqlock is checked after the
 * child is looked up. Thus, an interlocking stepping of sequence lock checks
 * is formed, giving integrity down the path walk.
 *
 * NOTE! The caller *has* to check the resulting dentry against the sequence
 * number we've returned before using any of the resulting dentry state!
 */
struct dentry *__d_lookup_rcu(const struct dentry *parent,
                                const struct qstr *name,
                                unsigned *seqp)
{
        u64 hashlen = name->hash_len;
        const unsigned char *str = name->name;
        struct hlist_bl_head *b = d_hash(parent, hashlen_hash(hashlen));
        struct hlist_bl_node *node;
        struct dentry *dentry;

        /*
         * Note: There is significant duplication with __d_lookup_rcu which is
         * required to prevent single threaded performance regressions
         * especially on architectures where smp_rmb (in seqcounts) are costly.
         * Keep the two functions in sync.
         */

        /*
         * The hash list is protected using RCU.
         *
         * Carefully use d_seq when comparing a candidate dentry, to avoid
         * races with d_move().
         *
         * It is possible that concurrent renames can mess up our list
         * walk here and result in missing our dentry, resulting in the
         * false-negative result. d_lookup() protects against concurrent
         * renames using rename_lock seqlock.
         *
         * See Documentation/filesystems/path-lookup.txt for more details.
         */
        hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
                unsigned seq;

seqretry:
                /*
                 * The dentry sequence count protects us from concurrent
                 * renames, and thus protects parent and name fields.
                 *
                 * The caller must perform a seqcount check in order
                 * to do anything useful with the returned dentry.
                 *
                 * NOTE! We do a "raw" seqcount_begin here. That means that
                 * we don't wait for the sequence count to stabilize if it
                 * is in the middle of a sequence change. If we do the slow
                 * dentry compare, we will do seqretries until it is stable,
                 * and if we end up with a successful lookup, we actually
                 * want to exit RCU lookup anyway.
                 */
                seq = raw_seqcount_begin(&dentry->d_seq);
                if (dentry->d_parent != parent)
                        continue;
                if (d_unhashed(dentry))
                        continue;

                if (unlikely(parent->d_flags & DCACHE_OP_COMPARE)) {
                        if (dentry->d_name.hash != hashlen_hash(hashlen))
                                continue;
                        *seqp = seq;
                        switch (slow_dentry_cmp(parent, dentry, seq, name)) {
                        case D_COMP_OK:
                                return dentry;
                        case D_COMP_NOMATCH:
                                continue;
                        default:
                                goto seqretry;
                        }
                }

                if (dentry->d_name.hash_len != hashlen)
                        continue;
                *seqp = seq;
                if (!dentry_cmp(dentry, str, hashlen_len(hashlen)))
                        return dentry;
        }
        return NULL;
}

/**
 * d_lookup - search for a dentry
 * @parent: parent dentry
 * @name: qstr of name we wish to find
 * Returns: dentry, or NULL
 *
 * d_lookup 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 if the dentry does not exist.
 */
struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
{
        struct dentry *dentry;
        unsigned seq;

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

/**
 * __d_lookup - search for a dentry (racy)
 * @parent: parent dentry
 * @name: qstr of name we wish to find
 * Returns: dentry, or NULL
 *
 * __d_lookup is like d_lookup, however it may (rarely) return a
 * false-negative result due to unrelated rename activity.
 *
 * __d_lookup is slightly faster by avoiding rename_lock read seqlock,
 * however it must be used carefully, eg. with a following d_lookup in
 * the case of failure.
 *
 * __d_lookup callers must be commented.
 */
struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)
{
        unsigned int len = name->len;
        unsigned int hash = name->hash;
        const unsigned char *str = name->name;
        struct hlist_bl_head *b = d_hash(parent, hash);
        struct hlist_bl_node *node;
        struct dentry *found = NULL;
        struct dentry *dentry;

        /*
         * Note: There is significant duplication with __d_lookup_rcu which is
         * required to prevent single threaded performance regressions
         * especially on architectures where smp_rmb (in seqcounts) are costly.
         * Keep the two functions in sync.
         */

        /*
         * The hash list is protected using RCU.
         *
         * Take d_lock when comparing a candidate dentry, to avoid races
         * with d_move().
         *
         * It is possible that concurrent renames can mess up our list
         * walk here and result in missing our dentry, resulting in the
         * false-negative result. d_lookup() protects against concurrent
         * renames using rename_lock seqlock.
         *
         * See Documentation/filesystems/path-lookup.txt for more details.
         */
        rcu_read_lock();
        
        hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {

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

                spin_lock(&dentry->d_lock);
                if (dentry->d_parent != parent)
                        goto next;
                if (d_unhashed(dentry))
                        goto next;

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

                dentry->d_lockref.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 lookup failure NULL is returned; on bad name - ERR_PTR(-error)
 */
struct dentry *d_hash_and_lookup(struct dentry *dir, struct qstr *name)
{
        /*
         * 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_flags & DCACHE_OP_HASH) {
                int err = dir->d_op->d_hash(dir, name);
                if (unlikely(err < 0))
                        return ERR_PTR(err);
        }
        return d_lookup(dir, name);
}
EXPORT_SYMBOL(d_hash_and_lookup);

/**
 * d_validate - verify dentry provided from insecure source (deprecated)
 * @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.
 *
 * This function is slow for big directories, and deprecated, do not use it.
 */
int d_validate(struct dentry *dentry, struct dentry *dparent)
{
        struct dentry *child;

        spin_lock(&dparent->d_lock);
        list_for_each_entry(child, &dparent->d_subdirs, d_u.d_child) {
                if (dentry == child) {
                        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
                        __dget_dlock(dentry);
                        spin_unlock(&dentry->d_lock);
                        spin_unlock(&dparent->d_lock);
                        return 1;
                }
        }
        spin_unlock(&dparent->d_lock);

        return 0;
}
EXPORT_SYMBOL(d_validate);

/*
 * 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)
{
        struct inode *inode;
        int isdir = 0;
        /*
         * Are we the only user?
         */
again:
        spin_lock(&dentry->d_lock);
        inode = dentry->d_inode;
        isdir = S_ISDIR(inode->i_mode);
        if (dentry->d_lockref.count == 1) {
                if (!spin_trylock(&inode->i_lock)) {
                        spin_unlock(&dentry->d_lock);
                        cpu_relax();
                        goto again;
                }
                dentry->d_flags &= ~DCACHE_CANT_MOUNT;
                dentry_unlink_inode(dentry);
                fsnotify_nameremove(dentry, isdir);
                return;
        }

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

        spin_unlock(&dentry->d_lock);

        fsnotify_nameremove(dentry, isdir);
}
EXPORT_SYMBOL(d_delete);

static void __d_rehash(struct dentry * entry, struct hlist_bl_head *b)
{
        BUG_ON(!d_unhashed(entry));
        hlist_bl_lock(b);
        entry->d_flags |= DCACHE_RCUACCESS;
        hlist_bl_add_head_rcu(&entry->d_hash, b);
        hlist_bl_unlock(b);
}

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(&entry->d_lock);
        _d_rehash(entry);
        spin_unlock(&entry->d_lock);
}
EXPORT_SYMBOL(d_rehash);

/**
 * dentry_update_name_case - update case insensitive dentry with a new name
 * @dentry: dentry to be updated
 * @name: new name
 *
 * Update a case insensitive dentry with new case of name.
 *
 * dentry must have been returned by d_lookup with name @name. Old and new
 * name lengths must match (ie. no d_compare which allows mismatched name
 * lengths).
 *
 * Parent inode i_mutex must be held over d_lookup and into this call (to
 * keep renames and concurrent inserts, and readdir(2) away).
 */
void dentry_update_name_case(struct dentry *dentry, struct qstr *name)
{
        BUG_ON(!mutex_is_locked(&dentry->d_parent->d_inode->i_mutex));
        BUG_ON(dentry->d_name.len != name->len); /* d_lookup gives this */

        spin_lock(&dentry->d_lock);
        write_seqcount_begin(&dentry->d_seq);
        memcpy((unsigned char *)dentry->d_name.name, name->name, name->len);
        write_seqcount_end(&dentry->d_seq);
        spin_unlock(&dentry->d_lock);
}
EXPORT_SYMBOL(dentry_update_name_case);

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);
}

static void dentry_lock_for_move(struct dentry *dentry, struct dentry *target)
{
        /*
         * XXXX: do we really need to take target->d_lock?
         */
        if (IS_ROOT(dentry) || dentry->d_parent == target->d_parent)
                spin_lock(&target->d_parent->d_lock);
        else {
                if (d_ancestor(dentry->d_parent, target->d_parent)) {
                        spin_lock(&dentry->d_parent->d_lock);
                        spin_lock_nested(&target->d_parent->d_lock,
                                                DENTRY_D_LOCK_NESTED);
                } else {
                        spin_lock(&target->d_parent->d_lock);
                        spin_lock_nested(&dentry->d_parent->d_lock,
                                                DENTRY_D_LOCK_NESTED);
                }
        }
        if (target < dentry) {
                spin_lock_nested(&target->d_lock, 2);
                spin_lock_nested(&dentry->d_lock, 3);
        } else {
                spin_lock_nested(&dentry->d_lock, 2);
                spin_lock_nested(&target->d_lock, 3);
        }
}

static void dentry_unlock_parents_for_move(struct dentry *dentry,
                                        struct dentry *target)
{
        if (target->d_parent != dentry->d_parent)
                spin_unlock(&dentry->d_parent->d_lock);
        if (target->d_parent != target)
                spin_unlock(&target->d_parent->d_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..
 */
/*
 * __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. Caller must hold
 * rename_lock, the i_mutex of the source and target directories,
 * and the sb->s_vfs_rename_mutex if they differ. See lock_rename().
 */
static void __d_move(struct dentry * dentry, struct dentry * target)
{
        if (!dentry->d_inode)
                printk(KERN_WARNING "VFS: moving negative dcache entry\n");

        BUG_ON(d_ancestor(dentry, target));
        BUG_ON(d_ancestor(target, dentry));

        dentry_lock_for_move(dentry, target);

        write_seqcount_begin(&dentry->d_seq);
        write_seqcount_begin(&target->d_seq);

        /* __d_drop does write_seqcount_barrier, but they're OK to nest. */

        /*
         * Move the dentry to the target hash queue. Don't bother checking
         * for the same hash queue because of how unlikely it is.
         */
        __d_drop(dentry);
        __d_rehash(dentry, d_hash(target->d_parent, target->d_name.hash));

        /* 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);

        write_seqcount_end(&target->d_seq);
        write_seqcount_end(&dentry->d_seq);

        dentry_unlock_parents_for_move(dentry, target);
        spin_unlock(&target->d_lock);
        fsnotify_d_move(dentry);
        spin_unlock(&dentry->d_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. See the locking
 * requirements for __d_move.
 */
void d_move(struct dentry *dentry, struct dentry *target)
{
        write_seqlock(&rename_lock);
        __d_move(dentry, target);
        write_sequnlock(&rename_lock);
}
EXPORT_SYMBOL(d_move);

/**
 * 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, inode->i_lock and rename_lock
 *
 * Note: If ever the locking in lock_rename() changes, then please
 * remember to update this too...
 */
static struct dentry *__d_unalias(struct inode *inode,
                struct dentry *dentry, struct dentry *alias)
{
        struct mutex *m1 = NULL, *m2 = NULL;
        struct dentry *ret = ERR_PTR(-EBUSY);

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

        /* See lock_rename() */
        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:
        if (likely(!d_mountpoint(alias))) {
                __d_move(alias, dentry);
                ret = alias;
        }
out_err:
        spin_unlock(&inode->i_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.
 * returns with anon->d_lock held!
 */
static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
{
        struct dentry *dparent;

        dentry_lock_for_move(anon, dentry);

        write_seqcount_begin(&dentry->d_seq);
        write_seqcount_begin(&anon->d_seq);

        dparent = dentry->d_parent;

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

        dentry->d_parent = dentry;
        list_del_init(&dentry->d_u.d_child);
        anon->d_parent = dparent;
        list_move(&anon->d_u.d_child, &dparent->d_subdirs);

        write_seqcount_end(&dentry->d_seq);
        write_seqcount_end(&anon->d_seq);

        dentry_unlock_parents_for_move(anon, dentry);
        spin_unlock(&dentry->d_lock);

        /* anon->d_lock still locked, returns locked */
        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. Caller must hold the
 * i_mutex of the parent directory.
 */
struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
{
        struct dentry *actual;

        BUG_ON(!d_unhashed(dentry));

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

        spin_lock(&inode->i_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;
                        write_seqlock(&rename_lock);

                        if (d_ancestor(alias, dentry)) {
                                /* Check for loops */
                                actual = ERR_PTR(-ELOOP);
                                spin_unlock(&inode->i_lock);
                        } else if (IS_ROOT(alias)) {
                                /* Is this an anonymous mountpoint that we
                                 * could splice into our tree? */
                                __d_materialise_dentry(dentry, alias);
                                write_sequnlock(&rename_lock);
                                __d_drop(alias);
                                goto found;
                        } else {
                                /* Nope, but we must(!) avoid directory
                                 * aliasing. This drops inode->i_lock */
                                actual = __d_unalias(inode, dentry, alias);
                        }
                        write_sequnlock(&rename_lock);
                        if (IS_ERR(actual)) {
                                if (PTR_ERR(actual) == -ELOOP)
                                        pr_warn_ratelimited(
                                                "VFS: Lookup of '%s' in %s %s"
                                                " would have caused loop\n",
                                                dentry->d_name.name,
                                                inode->i_sb->s_type->name,
                                                inode->i_sb->s_id);
                                dput(alias);
                        }
                        goto out_nolock;
                }
        }

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

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

        iput(inode);
        return actual;
}
EXPORT_SYMBOL_GPL(d_materialise_unique);

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;
}

/**
 * prepend_name - prepend a pathname in front of current buffer pointer
 * buffer: buffer pointer
 * buflen: allocated length of the buffer
 * name:   name string and length qstr structure
 *
 * With RCU path tracing, it may race with d_move(). Use ACCESS_ONCE() to
 * make sure that either the old or the new name pointer and length are
 * fetched. However, there may be mismatch between length and pointer.
 * The length cannot be trusted, we need to copy it byte-by-byte until
 * the length is reached or a null byte is found. It also prepends "/" at
 * the beginning of the name. The sequence number check at the caller will
 * retry it again when a d_move() does happen. So any garbage in the buffer
 * due to mismatched pointer and length will be discarded.
 */
static int prepend_name(char **buffer, int *buflen, struct qstr *name)
{
        const char *dname = ACCESS_ONCE(name->name);
        u32 dlen = ACCESS_ONCE(name->len);
        char *p;

        if (*buflen < dlen + 1)
                return -ENAMETOOLONG;
        *buflen -= dlen + 1;
        p = *buffer -= dlen + 1;
        *p++ = '/';
        while (dlen--) {
                char c = *dname++;
                if (!c)
                        break;
                *p++ = c;
        }
        return 0;
}

/**
 * prepend_path - Prepend path string to a buffer
 * @path: the dentry/vfsmount to report
 * @root: root vfsmnt/dentry
 * @buffer: pointer to the end of the buffer
 * @buflen: pointer to buffer length
 *
 * The function tries to write out the pathname without taking any lock other
 * than the RCU read lock to make sure that dentries won't go away. It only
 * checks the sequence number of the global rename_lock as any change in the
 * dentry's d_seq will be preceded by changes in the rename_lock sequence
 * number. If the sequence number had been change, it will restart the whole
 * pathname back-tracing sequence again. It performs a total of 3 trials of
 * lockless back-tracing sequences before falling back to take the
 * rename_lock.
 */
static int prepend_path(const struct path *path,
                        const struct path *root,
                        char **buffer, int *buflen)
{
        struct dentry *dentry = path->dentry;
        struct vfsmount *vfsmnt = path->mnt;
        struct mount *mnt = real_mount(vfsmnt);
        int error = 0;
        unsigned seq = 0;
        char *bptr;
        int blen;

        rcu_read_lock();
restart:
        bptr = *buffer;
        blen = *buflen;
        read_seqbegin_or_lock(&rename_lock, &seq);
        while (dentry != root->dentry || vfsmnt != root->mnt) {
                struct dentry * parent;

                if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
                        /* Global root? */
                        if (mnt_has_parent(mnt)) {
                                dentry = mnt->mnt_mountpoint;
                                mnt = mnt->mnt_parent;
                                vfsmnt = &mnt->mnt;
                                continue;
                        }
                        /*
                         * Filesystems needing to implement special "root names"
                         * should do so with ->d_dname()
                         */
                        if (IS_ROOT(dentry) &&
                           (dentry->d_name.len != 1 ||
                            dentry->d_name.name[0] != '/')) {
                                WARN(1, "Root dentry has weird name <%.*s>\n",
                                     (int) dentry->d_name.len,
                                     dentry->d_name.name);
                        }
                        if (!error)
                                error = is_mounted(vfsmnt) ? 1 : 2;
                        break;
                }
                parent = dentry->d_parent;
                prefetch(parent);
                error = prepend_name(&bptr, &blen, &dentry->d_name);
                if (error)
                        break;

                dentry = parent;
        }
        if (!(seq & 1))
                rcu_read_unlock();
        if (need_seqretry(&rename_lock, seq)) {
                seq = 1;
                goto restart;
        }
        done_seqretry(&rename_lock, seq);

        if (error >= 0 && bptr == *buffer) {
                if (--blen < 0)
                        error = -ENAMETOOLONG;
                else
                        *--bptr = '/';
        }
        *buffer = bptr;
        *buflen = blen;
        return error;
}

/**
 * __d_path - return the path of a dentry
 * @path: the dentry/vfsmount to report
 * @root: root vfsmnt/dentry
 * @buf: buffer to return value in
 * @buflen: buffer length
 *
 * Convert a dentry into an ASCII path name.
 *
 * Returns a pointer into the buffer or an error code if the
 * path was too long.
 *
 * "buflen" should be positive.
 *
 * If the path is not reachable from the supplied root, return %NULL.
 */
char *__d_path(const struct path *path,
               const struct path *root,
               char *buf, int buflen)
{
        char *res = buf + buflen;
        int error;

        prepend(&res, &buflen, "\0", 1);
        br_read_lock(&vfsmount_lock);
        error = prepend_path(path, root, &res, &buflen);
        br_read_unlock(&vfsmount_lock);

        if (error < 0)
                return ERR_PTR(error);
        if (error > 0)
                return NULL;
        return res;
}

char *d_absolute_path(const struct path *path,
               char *buf, int buflen)
{
        struct path root = {};
        char *res = buf + buflen;
        int error;

        prepend(&res, &buflen, "\0", 1);
        br_read_lock(&vfsmount_lock);
        error = prepend_path(path, &root, &res, &buflen);
        br_read_unlock(&vfsmount_lock);

        if (error > 1)
                error = -EINVAL;
        if (error < 0)
                return ERR_PTR(error);
        return res;
}

/*
 * same as __d_path but appends "(deleted)" for unlinked files.
 */
static int path_with_deleted(const struct path *path,
                             const struct path *root,
                             char **buf, int *buflen)
{
        prepend(buf, buflen, "\0", 1);
        if (d_unlinked(path->dentry)) {
                int error = prepend(buf, buflen, " (deleted)", 10);
                if (error)
                        return error;
        }

        return prepend_path(path, root, buf, buflen);
}

static int prepend_unreachable(char **buffer, int *buflen)
{
        return prepend(buffer, buflen, "(unreachable)", 13);
}

/**
 * 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 = buf + buflen;
        struct path root;
        int error;

        /*
         * 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);

        get_fs_root(current->fs, &root);
        br_read_lock(&vfsmount_lock);
        error = path_with_deleted(path, &root, &res, &buflen);
        br_read_unlock(&vfsmount_lock);
        if (error < 0)
                res = ERR_PTR(error);
        path_put(&root);
        return res;
}
EXPORT_SYMBOL(d_path);

/*
 * 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);
}

char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
{
        char *end = buffer + buflen;
        /* these dentries are never renamed, so d_lock is not needed */
        if (prepend(&end, &buflen, " (deleted)", 11) ||
            prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
            prepend(&end, &buflen, "/", 1))  
                end = ERR_PTR(-ENAMETOOLONG);
        return end;
}

/*
 * Write full pathname from the root of the filesystem into the buffer.
 */
static char *__dentry_path(struct dentry *dentry, char *buf, int buflen)
{
        char *end, *retval;
        int len, seq = 0;
        int error = 0;

        rcu_read_lock();
restart:
        end = buf + buflen;
        len = buflen;
        prepend(&end, &len, "\0", 1);
        if (buflen < 1)
                goto Elong;
        /* Get '/' right */
        retval = end-1;
        *retval = '/';
        read_seqbegin_or_lock(&rename_lock, &seq);
        while (!IS_ROOT(dentry)) {
                struct dentry *parent = dentry->d_parent;
                int error;

                prefetch(parent);
                error = prepend_name(&end, &len, &dentry->d_name);
                if (error)
                        break;

                retval = end;
                dentry = parent;
        }
        if (!(seq & 1))
                rcu_read_unlock();
        if (need_seqretry(&rename_lock, seq)) {
                seq = 1;
                goto restart;
        }
        done_seqretry(&rename_lock, seq);
        if (error)
                goto Elong;
        return retval;
Elong:
        return ERR_PTR(-ENAMETOOLONG);
}

char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
{
        return __dentry_path(dentry, buf, buflen);
}
EXPORT_SYMBOL(dentry_path_raw);

char *dentry_path(struct dentry *dentry, char *buf, int buflen)
{
        char *p = NULL;
        char *retval;

        if (d_unlinked(dentry)) {
                p = buf + buflen;
                if (prepend(&p, &buflen, "//deleted", 10) != 0)
                        goto Elong;
                buflen++;
        }
        retval = __dentry_path(dentry, buf, buflen);
        if (!IS_ERR(retval) && p)
                *p = '/';       /* restore '/' overriden with '\0' */
        return retval;
Elong:
        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;

        get_fs_root_and_pwd(current->fs, &root, &pwd);

        error = -ENOENT;
        br_read_lock(&vfsmount_lock);
        if (!d_unlinked(pwd.dentry)) {
                unsigned long len;
                char *cwd = page + PAGE_SIZE;
                int buflen = PAGE_SIZE;

                prepend(&cwd, &buflen, "\0", 1);
                error = prepend_path(&pwd, &root, &cwd, &buflen);
                br_read_unlock(&vfsmount_lock);

                if (error < 0)
                        goto out;

                /* Unreachable from current root */
                if (error > 0) {
                        error = prepend_unreachable(&cwd, &buflen);
                        if (error)
                                goto out;
                }

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

        if (new_dentry == old_dentry)
                return 1;

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

        return result;
}

static enum d_walk_ret d_genocide_kill(void *data, struct dentry *dentry)
{
        struct dentry *root = data;
        if (dentry != root) {
                if (d_unhashed(dentry) || !dentry->d_inode)
                        return D_WALK_SKIP;

                if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
                        dentry->d_flags |= DCACHE_GENOCIDE;
                        dentry->d_lockref.count--;
                }
        }
        return D_WALK_CONTINUE;
}

void d_genocide(struct dentry *parent)
{
        d_walk(parent, parent, d_genocide_kill, NULL);
}

void d_tmpfile(struct dentry *dentry, struct inode *inode)
{
        inode_dec_link_count(inode);
        BUG_ON(dentry->d_name.name != dentry->d_iname ||
                !hlist_unhashed(&dentry->d_alias) ||
                !d_unlinked(dentry));
        spin_lock(&dentry->d_parent->d_lock);
        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
                                (unsigned long long)inode->i_ino);
        spin_unlock(&dentry->d_lock);
        spin_unlock(&dentry->d_parent->d_lock);
        d_instantiate(dentry, inode);
}
EXPORT_SYMBOL(d_tmpfile);

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)
{
        unsigned 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_bl_head),
                                        dhash_entries,
                                        13,
                                        HASH_EARLY,
                                        &d_hash_shift,
                                        &d_hash_mask,
                                        0,
                                        0);

        for (loop = 0; loop < (1U << d_hash_shift); loop++)
                INIT_HLIST_BL_HEAD(dentry_hashtable + loop);
}

static void __init dcache_init(void)
{
        unsigned 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);

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

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

        for (loop = 0; loop < (1U << d_hash_shift); loop++)
                INIT_HLIST_BL_HEAD(dentry_hashtable + loop);
}

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

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();
}