Merge tag 'filesystems_for_v4.20-rc1' of git://git.kernel.org/pub/scm/linux/kernel...

[linux.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/ratelimit.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/security.h>
#include <linux/seqlock.h>
#include <linux/bootmem.h>
#include <linux/bit_spinlock.h>
#include <linux/rculist_bl.h>
#include <linux/list_lru.h>
#include "internal.h"
#include "mount.h"

/*
 * Usage:
 * dcache->d_inode->i_lock protects:
 *   - i_dentry, d_u.d_alias, d_inode of aliases
 * dcache_hash_bucket lock protects:
 *   - the dcache hash table
 * s_roots bl list spinlock protects:
 *   - the s_roots list (see __d_drop)
 * dentry->d_sb->s_dentry_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_u.d_alias, d_inode
 *
 * Ordering:
 * dentry->d_inode->i_lock
 *   dentry->d_lock
 *     dentry->d_sb->s_dentry_lru_lock
 *     dcache_hash_bucket lock
 *     s_roots 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:
 * arbitrary, since it's serialized on rename_lock
 */
int sysctl_vfs_cache_pressure __read_mostly = 100;
EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);

__cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);

EXPORT_SYMBOL(rename_lock);

static struct kmem_cache *dentry_cache __read_mostly;

const struct qstr empty_name = QSTR_INIT("", 0);
EXPORT_SYMBOL(empty_name);
const struct qstr slash_name = QSTR_INIT("/", 1);
EXPORT_SYMBOL(slash_name);

/*
 * 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.
 */

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(unsigned int hash)
{
        return dentry_hashtable + (hash >> d_hash_shift);
}

#define IN_LOOKUP_SHIFT 10
static struct hlist_bl_head in_lookup_hashtable[1 << IN_LOOKUP_SHIFT];

static inline struct hlist_bl_head *in_lookup_hash(const struct dentry *parent,
                                        unsigned int hash)
{
        hash += (unsigned long) parent / L1_CACHE_BYTES;
        return in_lookup_hashtable + hash_32(hash, IN_LOOKUP_SHIFT);
}


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

static DEFINE_PER_CPU(long, nr_dentry);
static DEFINE_PER_CPU(long, nr_dentry_unused);

#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)

/*
 * Here we resort to our own counters instead of using generic per-cpu counters
 * for consistency with what the vfs inode code does. We are expected to harvest
 * better code and performance by having our own specialized counters.
 *
 * Please note that the loop is done over all possible CPUs, not over all online
 * CPUs. The reason for this is that we don't want to play games with CPUs going
 * on and off. If one of them goes off, we will just keep their counters.
 *
 * glommer: See cffbc8a for details, and if you ever intend to change this,
 * please update all vfs counters to match.
 */
static long get_nr_dentry(void)
{
        int i;
        long sum = 0;
        for_each_possible_cpu(i)
                sum += per_cpu(nr_dentry, i);
        return sum < 0 ? 0 : sum;
}

static long get_nr_dentry_unused(void)
{
        int i;
        long sum = 0;
        for_each_possible_cpu(i)
                sum += per_cpu(nr_dentry_unused, i);
        return sum < 0 ? 0 : sum;
}

int proc_nr_dentry(struct ctl_table *table, int write, void __user *buffer,
                   size_t *lenp, loff_t *ppos)
{
        dentry_stat.nr_dentry = get_nr_dentry();
        dentry_stat.nr_unused = get_nr_dentry_unused();
        return proc_doulongvec_minmax(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 = read_word_at_a_time(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 = bytemask_from_count(tcount);
        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)
{
        /*
         * Be careful about RCU walk racing with rename:
         * use 'READ_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)
         */
        const unsigned char *cs = READ_ONCE(dentry->d_name.name);

        return dentry_string_cmp(cs, ct, tcount);
}

struct external_name {
        union {
                atomic_t count;
                struct rcu_head head;
        } u;
        unsigned char name[];
};

static inline struct external_name *external_name(struct dentry *dentry)
{
        return container_of(dentry->d_name.name, struct external_name, name[0]);
}

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

        kmem_cache_free(dentry_cache, dentry); 
}

static void __d_free_external(struct rcu_head *head)
{
        struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
        kfree(external_name(dentry));
        kmem_cache_free(dentry_cache, dentry);
}

static inline int dname_external(const struct dentry *dentry)
{
        return dentry->d_name.name != dentry->d_iname;
}

void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry)
{
        spin_lock(&dentry->d_lock);
        if (unlikely(dname_external(dentry))) {
                struct external_name *p = external_name(dentry);
                atomic_inc(&p->u.count);
                spin_unlock(&dentry->d_lock);
                name->name = p->name;
        } else {
                memcpy(name->inline_name, dentry->d_iname,
                       dentry->d_name.len + 1);
                spin_unlock(&dentry->d_lock);
                name->name = name->inline_name;
        }
}
EXPORT_SYMBOL(take_dentry_name_snapshot);

void release_dentry_name_snapshot(struct name_snapshot *name)
{
        if (unlikely(name->name != name->inline_name)) {
                struct external_name *p;
                p = container_of(name->name, struct external_name, name[0]);
                if (unlikely(atomic_dec_and_test(&p->u.count)))
                        kfree_rcu(p, u.head);
        }
}
EXPORT_SYMBOL(release_dentry_name_snapshot);

static inline void __d_set_inode_and_type(struct dentry *dentry,
                                          struct inode *inode,
                                          unsigned type_flags)
{
        unsigned flags;

        dentry->d_inode = inode;
        flags = READ_ONCE(dentry->d_flags);
        flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
        flags |= type_flags;
        WRITE_ONCE(dentry->d_flags, flags);
}

static inline void __d_clear_type_and_inode(struct dentry *dentry)
{
        unsigned flags = READ_ONCE(dentry->d_flags);

        flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
        WRITE_ONCE(dentry->d_flags, flags);
        dentry->d_inode = NULL;
}

static void dentry_free(struct dentry *dentry)
{
        WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));
        if (unlikely(dname_external(dentry))) {
                struct external_name *p = external_name(dentry);
                if (likely(atomic_dec_and_test(&p->u.count))) {
                        call_rcu(&dentry->d_u.d_rcu, __d_free_external);
                        return;
                }
        }
        /* 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);
}

/*
 * Release the dentry's inode, using the filesystem
 * d_iput() operation if defined.
 */
static void dentry_unlink_inode(struct dentry * dentry)
        __releases(dentry->d_lock)
        __releases(dentry->d_inode->i_lock)
{
        struct inode *inode = dentry->d_inode;

        raw_write_seqcount_begin(&dentry->d_seq);
        __d_clear_type_and_inode(dentry);
        hlist_del_init(&dentry->d_u.d_alias);
        raw_write_seqcount_end(&dentry->d_seq);
        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);
}

/*
 * The DCACHE_LRU_LIST bit is set whenever the 'd_lru' entry
 * is in use - which includes both the "real" per-superblock
 * LRU list _and_ the DCACHE_SHRINK_LIST use.
 *
 * The DCACHE_SHRINK_LIST bit is set whenever the dentry is
 * on the shrink list (ie not on the superblock LRU list).
 *
 * The per-cpu "nr_dentry_unused" counters are updated with
 * the DCACHE_LRU_LIST bit.
 *
 * These helper functions make sure we always follow the
 * rules. d_lock must be held by the caller.
 */
#define D_FLAG_VERIFY(dentry,x) WARN_ON_ONCE(((dentry)->d_flags & (DCACHE_LRU_LIST | DCACHE_SHRINK_LIST)) != (x))
static void d_lru_add(struct dentry *dentry)
{
        D_FLAG_VERIFY(dentry, 0);
        dentry->d_flags |= DCACHE_LRU_LIST;
        this_cpu_inc(nr_dentry_unused);
        WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
}

static void d_lru_del(struct dentry *dentry)
{
        D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
        dentry->d_flags &= ~DCACHE_LRU_LIST;
        this_cpu_dec(nr_dentry_unused);
        WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
}

static void d_shrink_del(struct dentry *dentry)
{
        D_FLAG_VERIFY(dentry, DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
        list_del_init(&dentry->d_lru);
        dentry->d_flags &= ~(DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
        this_cpu_dec(nr_dentry_unused);
}

static void d_shrink_add(struct dentry *dentry, struct list_head *list)
{
        D_FLAG_VERIFY(dentry, 0);
        list_add(&dentry->d_lru, list);
        dentry->d_flags |= DCACHE_SHRINK_LIST | DCACHE_LRU_LIST;
        this_cpu_inc(nr_dentry_unused);
}

/*
 * These can only be called under the global LRU lock, ie during the
 * callback for freeing the LRU list. "isolate" removes it from the
 * LRU lists entirely, while shrink_move moves it to the indicated
 * private list.
 */
static void d_lru_isolate(struct list_lru_one *lru, struct dentry *dentry)
{
        D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
        dentry->d_flags &= ~DCACHE_LRU_LIST;
        this_cpu_dec(nr_dentry_unused);
        list_lru_isolate(lru, &dentry->d_lru);
}

static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
                              struct list_head *list)
{
        D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
        dentry->d_flags |= DCACHE_SHRINK_LIST;
        list_lru_isolate_move(lru, &dentry->d_lru, list);
}

/**
 * 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
 * ___d_drop doesn't mark dentry as "unhashed"
 *   (dentry->d_hash.pprev will be LIST_POISON2, not NULL).
 */
static void ___d_drop(struct dentry *dentry)
{
        struct hlist_bl_head *b;
        /*
         * Hashed dentries are normally on the dentry hashtable,
         * with the exception of those newly allocated by
         * d_obtain_root, which are always IS_ROOT:
         */
        if (unlikely(IS_ROOT(dentry)))
                b = &dentry->d_sb->s_roots;
        else
                b = d_hash(dentry->d_name.hash);

        hlist_bl_lock(b);
        __hlist_bl_del(&dentry->d_hash);
        hlist_bl_unlock(b);
}

void __d_drop(struct dentry *dentry)
{
        if (!d_unhashed(dentry)) {
                ___d_drop(dentry);
                dentry->d_hash.pprev = NULL;
                write_seqcount_invalidate(&dentry->d_seq);
        }
}
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);

static inline void dentry_unlist(struct dentry *dentry, struct dentry *parent)
{
        struct dentry *next;
        /*
         * Inform d_walk() and shrink_dentry_list() that we are no longer
         * attached to the dentry tree
         */
        dentry->d_flags |= DCACHE_DENTRY_KILLED;
        if (unlikely(list_empty(&dentry->d_child)))
                return;
        __list_del_entry(&dentry->d_child);
        /*
         * Cursors can move around the list of children.  While we'd been
         * a normal list member, it didn't matter - ->d_child.next would've
         * been updated.  However, from now on it won't be and for the
         * things like d_walk() it might end up with a nasty surprise.
         * Normally d_walk() doesn't care about cursors moving around -
         * ->d_lock on parent prevents that and since a cursor has no children
         * of its own, we get through it without ever unlocking the parent.
         * There is one exception, though - if we ascend from a child that
         * gets killed as soon as we unlock it, the next sibling is found
         * using the value left in its ->d_child.next.  And if _that_
         * pointed to a cursor, and cursor got moved (e.g. by lseek())
         * before d_walk() regains parent->d_lock, we'll end up skipping
         * everything the cursor had been moved past.
         *
         * Solution: make sure that the pointer left behind in ->d_child.next
         * points to something that won't be moving around.  I.e. skip the
         * cursors.
         */
        while (dentry->d_child.next != &parent->d_subdirs) {
                next = list_entry(dentry->d_child.next, struct dentry, d_child);
                if (likely(!(next->d_flags & DCACHE_DENTRY_CURSOR)))
                        break;
                dentry->d_child.next = next->d_child.next;
        }
}

static void __dentry_kill(struct dentry *dentry)
{
        struct dentry *parent = NULL;
        bool can_free = true;
        if (!IS_ROOT(dentry))
                parent = dentry->d_parent;

        /*
         * 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)
                dentry->d_op->d_prune(dentry);

        if (dentry->d_flags & DCACHE_LRU_LIST) {
                if (!(dentry->d_flags & DCACHE_SHRINK_LIST))
                        d_lru_del(dentry);
        }
        /* if it was on the hash then remove it */
        __d_drop(dentry);
        dentry_unlist(dentry, parent);
        if (parent)
                spin_unlock(&parent->d_lock);
        if (dentry->d_inode)
                dentry_unlink_inode(dentry);
        else
                spin_unlock(&dentry->d_lock);
        this_cpu_dec(nr_dentry);
        if (dentry->d_op && dentry->d_op->d_release)
                dentry->d_op->d_release(dentry);

        spin_lock(&dentry->d_lock);
        if (dentry->d_flags & DCACHE_SHRINK_LIST) {
                dentry->d_flags |= DCACHE_MAY_FREE;
                can_free = false;
        }
        spin_unlock(&dentry->d_lock);
        if (likely(can_free))
                dentry_free(dentry);
        cond_resched();
}

static struct dentry *__lock_parent(struct dentry *dentry)
{
        struct dentry *parent;
        rcu_read_lock();
        spin_unlock(&dentry->d_lock);
again:
        parent = READ_ONCE(dentry->d_parent);
        spin_lock(&parent->d_lock);
        /*
         * We can't blindly lock dentry until we are sure
         * that we won't violate the locking order.
         * Any changes of dentry->d_parent must have
         * been done with parent->d_lock held, so
         * spin_lock() above is enough of a barrier
         * for checking if it's still our child.
         */
        if (unlikely(parent != dentry->d_parent)) {
                spin_unlock(&parent->d_lock);
                goto again;
        }
        rcu_read_unlock();
        if (parent != dentry)
                spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        else
                parent = NULL;
        return parent;
}

static inline struct dentry *lock_parent(struct dentry *dentry)
{
        struct dentry *parent = dentry->d_parent;
        if (IS_ROOT(dentry))
                return NULL;
        if (likely(spin_trylock(&parent->d_lock)))
                return parent;
        return __lock_parent(dentry);
}

static inline bool retain_dentry(struct dentry *dentry)
{
        WARN_ON(d_in_lookup(dentry));

        /* Unreachable? Get rid of it */
        if (unlikely(d_unhashed(dentry)))
                return false;

        if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
                return false;

        if (unlikely(dentry->d_flags & DCACHE_OP_DELETE)) {
                if (dentry->d_op->d_delete(dentry))
                        return false;
        }
        /* retain; LRU fodder */
        dentry->d_lockref.count--;
        if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST)))
                d_lru_add(dentry);
        else if (unlikely(!(dentry->d_flags & DCACHE_REFERENCED)))
                dentry->d_flags |= DCACHE_REFERENCED;
        return true;
}

/*
 * Finish off a dentry we've decided to kill.
 * dentry->d_lock must be held, returns with it unlocked.
 * Returns dentry requiring refcount drop, or NULL if we're done.
 */
static struct dentry *dentry_kill(struct dentry *dentry)
        __releases(dentry->d_lock)
{
        struct inode *inode = dentry->d_inode;
        struct dentry *parent = NULL;

        if (inode && unlikely(!spin_trylock(&inode->i_lock)))
                goto slow_positive;

        if (!IS_ROOT(dentry)) {
                parent = dentry->d_parent;
                if (unlikely(!spin_trylock(&parent->d_lock))) {
                        parent = __lock_parent(dentry);
                        if (likely(inode || !dentry->d_inode))
                                goto got_locks;
                        /* negative that became positive */
                        if (parent)
                                spin_unlock(&parent->d_lock);
                        inode = dentry->d_inode;
                        goto slow_positive;
                }
        }
        __dentry_kill(dentry);
        return parent;

slow_positive:
        spin_unlock(&dentry->d_lock);
        spin_lock(&inode->i_lock);
        spin_lock(&dentry->d_lock);
        parent = lock_parent(dentry);
got_locks:
        if (unlikely(dentry->d_lockref.count != 1)) {
                dentry->d_lockref.count--;
        } else if (likely(!retain_dentry(dentry))) {
                __dentry_kill(dentry);
                return parent;
        }
        /* we are keeping it, after all */
        if (inode)
                spin_unlock(&inode->i_lock);
        if (parent)
                spin_unlock(&parent->d_lock);
        spin_unlock(&dentry->d_lock);
        return NULL;
}

/*
 * Try to do a lockless dput(), and return whether that was successful.
 *
 * If unsuccessful, we return false, having already taken the dentry lock.
 *
 * The caller needs to hold the RCU read lock, so that the dentry is
 * guaranteed to stay around even if the refcount goes down to zero!
 */
static inline bool fast_dput(struct dentry *dentry)
{
        int ret;
        unsigned int d_flags;

        /*
         * If we have a d_op->d_delete() operation, we sould not
         * let the dentry count go to zero, so use "put_or_lock".
         */
        if (unlikely(dentry->d_flags & DCACHE_OP_DELETE))
                return lockref_put_or_lock(&dentry->d_lockref);

        /*
         * .. otherwise, we can try to just decrement the
         * lockref optimistically.
         */
        ret = lockref_put_return(&dentry->d_lockref);

        /*
         * If the lockref_put_return() failed due to the lock being held
         * by somebody else, the fast path has failed. We will need to
         * get the lock, and then check the count again.
         */
        if (unlikely(ret < 0)) {
                spin_lock(&dentry->d_lock);
                if (dentry->d_lockref.count > 1) {
                        dentry->d_lockref.count--;
                        spin_unlock(&dentry->d_lock);
                        return true;
                }
                return false;
        }

        /*
         * If we weren't the last ref, we're done.
         */
        if (ret)
                return true;

        /*
         * Careful, careful. The reference count went down
         * to zero, but we don't hold the dentry lock, so
         * somebody else could get it again, and do another
         * dput(), and we need to not race with that.
         *
         * However, there is a very special and common case
         * where we don't care, because there is nothing to
         * do: the dentry is still hashed, it does not have
         * a 'delete' op, and it's referenced and already on
         * the LRU list.
         *
         * NOTE! Since we aren't locked, these values are
         * not "stable". However, it is sufficient that at
         * some point after we dropped the reference the
         * dentry was hashed and the flags had the proper
         * value. Other dentry users may have re-gotten
         * a reference to the dentry and change that, but
         * our work is done - we can leave the dentry
         * around with a zero refcount.
         */
        smp_rmb();
        d_flags = READ_ONCE(dentry->d_flags);
        d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_DISCONNECTED;

        /* Nothing to do? Dropping the reference was all we needed? */
        if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))
                return true;

        /*
         * Not the fast normal case? Get the lock. We've already decremented
         * the refcount, but we'll need to re-check the situation after
         * getting the lock.
         */
        spin_lock(&dentry->d_lock);

        /*
         * Did somebody else grab a reference to it in the meantime, and
         * we're no longer the last user after all? Alternatively, somebody
         * else could have killed it and marked it dead. Either way, we
         * don't need to do anything else.
         */
        if (dentry->d_lockref.count) {
                spin_unlock(&dentry->d_lock);
                return true;
        }

        /*
         * Re-get the reference we optimistically dropped. We hold the
         * lock, and we just tested that it was zero, so we can just
         * set it to 1.
         */
        dentry->d_lockref.count = 1;
        return false;
}


/* 
 * 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)
{
        while (dentry) {
                might_sleep();

                rcu_read_lock();
                if (likely(fast_dput(dentry))) {
                        rcu_read_unlock();
                        return;
                }

                /* Slow case: now with the dentry lock held */
                rcu_read_unlock();

                if (likely(retain_dentry(dentry))) {
                        spin_unlock(&dentry->d_lock);
                        return;
                }

                dentry = dentry_kill(dentry);
        }
}
EXPORT_SYMBOL(dput);


/* 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 = READ_ONCE(dentry->d_parent);
        gotref = lockref_get_not_zero(&ret->d_lockref);
        rcu_read_unlock();
        if (likely(gotref)) {
                if (likely(ret == READ_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);

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_u.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_find_alias - grab a hashed alias of inode
 * @inode: inode in question
 *
 * 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, or if the directory was renamed and d_revalidate
 * was the first vfs operation to notice.
 *
 * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer
 * any other hashed alias over that one.
 */
static struct dentry *__d_find_alias(struct inode *inode)
{
        struct dentry *alias;

        if (S_ISDIR(inode->i_mode))
                return __d_find_any_alias(inode);

        hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
                spin_lock(&alias->d_lock);
                if (!d_unhashed(alias)) {
                        __dget_dlock(alias);
                        spin_unlock(&alias->d_lock);
                        return alias;
                }
                spin_unlock(&alias->d_lock);
        }
        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);
                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_u.d_alias) {
                spin_lock(&dentry->d_lock);
                if (!dentry->d_lockref.count) {
                        struct dentry *parent = lock_parent(dentry);
                        if (likely(!dentry->d_lockref.count)) {
                                __dentry_kill(dentry);
                                dput(parent);
                                goto restart;
                        }
                        if (parent)
                                spin_unlock(&parent->d_lock);
                }
                spin_unlock(&dentry->d_lock);
        }
        spin_unlock(&inode->i_lock);
}
EXPORT_SYMBOL(d_prune_aliases);

/*
 * Lock a dentry from shrink list.
 * Called under rcu_read_lock() and dentry->d_lock; the former
 * guarantees that nothing we access will be freed under us.
 * Note that dentry is *not* protected from concurrent dentry_kill(),
 * d_delete(), etc.
 *
 * Return false if dentry has been disrupted or grabbed, leaving
 * the caller to kick it off-list.  Otherwise, return true and have
 * that dentry's inode and parent both locked.
 */
static bool shrink_lock_dentry(struct dentry *dentry)
{
        struct inode *inode;
        struct dentry *parent;

        if (dentry->d_lockref.count)
                return false;

        inode = dentry->d_inode;
        if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
                spin_unlock(&dentry->d_lock);
                spin_lock(&inode->i_lock);
                spin_lock(&dentry->d_lock);
                if (unlikely(dentry->d_lockref.count))
                        goto out;
                /* changed inode means that somebody had grabbed it */
                if (unlikely(inode != dentry->d_inode))
                        goto out;
        }

        parent = dentry->d_parent;
        if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
                return true;

        spin_unlock(&dentry->d_lock);
        spin_lock(&parent->d_lock);
        if (unlikely(parent != dentry->d_parent)) {
                spin_unlock(&parent->d_lock);
                spin_lock(&dentry->d_lock);
                goto out;
        }
        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        if (likely(!dentry->d_lockref.count))
                return true;
        spin_unlock(&parent->d_lock);
out:
        if (inode)
                spin_unlock(&inode->i_lock);
        return false;
}

static void shrink_dentry_list(struct list_head *list)
{
        while (!list_empty(list)) {
                struct dentry *dentry, *parent;

                dentry = list_entry(list->prev, struct dentry, d_lru);
                spin_lock(&dentry->d_lock);
                rcu_read_lock();
                if (!shrink_lock_dentry(dentry)) {
                        bool can_free = false;
                        rcu_read_unlock();
                        d_shrink_del(dentry);
                        if (dentry->d_lockref.count < 0)
                                can_free = dentry->d_flags & DCACHE_MAY_FREE;
                        spin_unlock(&dentry->d_lock);
                        if (can_free)
                                dentry_free(dentry);
                        continue;
                }
                rcu_read_unlock();
                d_shrink_del(dentry);
                parent = dentry->d_parent;
                __dentry_kill(dentry);
                if (parent == dentry)
                        continue;
                /*
                 * 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.
                 */
                dentry = parent;
                while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
                        dentry = dentry_kill(dentry);
        }
}

static enum lru_status dentry_lru_isolate(struct list_head *item,
                struct list_lru_one *lru, spinlock_t *lru_lock, void *arg)
{
        struct list_head *freeable = arg;
        struct dentry   *dentry = container_of(item, struct dentry, d_lru);


        /*
         * we are inverting the lru lock/dentry->d_lock here,
         * so use a trylock. If we fail to get the lock, just skip
         * it
         */
        if (!spin_trylock(&dentry->d_lock))
                return LRU_SKIP;

        /*
         * Referenced dentries are still in use. If they have active
         * counts, just remove them from the LRU. Otherwise give them
         * another pass through the LRU.
         */
        if (dentry->d_lockref.count) {
                d_lru_isolate(lru, dentry);
                spin_unlock(&dentry->d_lock);
                return LRU_REMOVED;
        }

        if (dentry->d_flags & DCACHE_REFERENCED) {
                dentry->d_flags &= ~DCACHE_REFERENCED;
                spin_unlock(&dentry->d_lock);

                /*
                 * The list move itself will be made by the common LRU code. At
                 * this point, we've dropped the dentry->d_lock but keep the
                 * lru lock. This is safe to do, since every list movement is
                 * protected by the lru lock even if both locks are held.
                 *
                 * This is guaranteed by the fact that all LRU management
                 * functions are intermediated by the LRU API calls like
                 * list_lru_add and list_lru_del. List movement in this file
                 * only ever occur through this functions or through callbacks
                 * like this one, that are called from the LRU API.
                 *
                 * The only exceptions to this are functions like
                 * shrink_dentry_list, and code that first checks for the
                 * DCACHE_SHRINK_LIST flag.  Those are guaranteed to be
                 * operating only with stack provided lists after they are
                 * properly isolated from the main list.  It is thus, always a
                 * local access.
                 */
                return LRU_ROTATE;
        }

        d_lru_shrink_move(lru, dentry, freeable);
        spin_unlock(&dentry->d_lock);

        return LRU_REMOVED;
}

/**
 * prune_dcache_sb - shrink the dcache
 * @sb: superblock
 * @sc: shrink control, passed to list_lru_shrink_walk()
 *
 * Attempt to shrink the superblock dcache LRU by @sc->nr_to_scan entries. This
 * is done when we need more memory and called from the superblock shrinker
 * function.
 *
 * This function may fail to free any resources if all the dentries are in
 * use.
 */
long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc)
{
        LIST_HEAD(dispose);
        long freed;

        freed = list_lru_shrink_walk(&sb->s_dentry_lru, sc,
                                     dentry_lru_isolate, &dispose);
        shrink_dentry_list(&dispose);
        return freed;
}

static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
                struct list_lru_one *lru, spinlock_t *lru_lock, void *arg)
{
        struct list_head *freeable = arg;
        struct dentry   *dentry = container_of(item, struct dentry, d_lru);

        /*
         * we are inverting the lru lock/dentry->d_lock here,
         * so use a trylock. If we fail to get the lock, just skip
         * it
         */
        if (!spin_trylock(&dentry->d_lock))
                return LRU_SKIP;

        d_lru_shrink_move(lru, dentry, freeable);
        spin_unlock(&dentry->d_lock);

        return LRU_REMOVED;
}


/**
 * 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)
{
        long freed;

        do {
                LIST_HEAD(dispose);

                freed = list_lru_walk(&sb->s_dentry_lru,
                        dentry_lru_isolate_shrink, &dispose, 1024);

                this_cpu_sub(nr_dentry_unused, freed);
                shrink_dentry_list(&dispose);
        } while (list_lru_count(&sb->s_dentry_lru) > 0);
}
EXPORT_SYMBOL(shrink_dcache_sb);

/**
 * 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
 *
 * The @enter() callbacks are called with d_lock held.
 */
static void d_walk(struct dentry *parent, void *data,
                   enum d_walk_ret (*enter)(void *, struct dentry *))
{
        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_child);
                next = tmp->next;

                if (unlikely(dentry->d_flags & DCACHE_DENTRY_CURSOR))
                        continue;

                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.
         */
        rcu_read_lock();
ascend:
        if (this_parent != parent) {
                struct dentry *child = this_parent;
                this_parent = child->d_parent;

                spin_unlock(&child->d_lock);
                spin_lock(&this_parent->d_lock);

                /* might go back up the wrong parent if we have had a rename. */
                if (need_seqretry(&rename_lock, seq))
                        goto rename_retry;
                /* go into the first sibling still alive */
                do {
                        next = child->d_child.next;
                        if (next == &this_parent->d_subdirs)
                                goto ascend;
                        child = list_entry(next, struct dentry, d_child);
                } while (unlikely(child->d_flags & DCACHE_DENTRY_KILLED));
                rcu_read_unlock();
                goto resume;
        }
        if (need_seqretry(&rename_lock, seq))
                goto rename_retry;
        rcu_read_unlock();

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

rename_retry:
        spin_unlock(&this_parent->d_lock);
        rcu_read_unlock();
        BUG_ON(seq & 1);
        if (!retry)
                return;
        seq = 1;
        goto again;
}

struct check_mount {
        struct vfsmount *mnt;
        unsigned int mounted;
};

static enum d_walk_ret path_check_mount(void *data, struct dentry *dentry)
{
        struct check_mount *info = data;
        struct path path = { .mnt = info->mnt, .dentry = dentry };

        if (likely(!d_mountpoint(dentry)))
                return D_WALK_CONTINUE;
        if (__path_is_mountpoint(&path)) {
                info->mounted = 1;
                return D_WALK_QUIT;
        }
        return D_WALK_CONTINUE;
}

/**
 * path_has_submounts - check for mounts over a dentry in the
 *                      current namespace.
 * @parent: path to check.
 *
 * Return true if the parent or its subdirectories contain
 * a mount point in the current namespace.
 */
int path_has_submounts(const struct path *parent)
{
        struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };

        read_seqlock_excl(&mount_lock);
        d_walk(parent->dentry, &data, path_check_mount);
        read_sequnlock_excl(&mount_lock);

        return data.mounted;
}
EXPORT_SYMBOL(path_has_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 d_invalidate() 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. d_invalidate() */
                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)) {
                ret = -EBUSY;
                if (!d_mountpoint(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;

        if (dentry->d_flags & DCACHE_SHRINK_LIST) {
                data->found++;
        } else {
                if (dentry->d_flags & DCACHE_LRU_LIST)
                        d_lru_del(dentry);
                if (!dentry->d_lockref.count) {
                        d_shrink_add(dentry, &data->dispose);
                        data->found++;
                }
        }
        /*
         * We can return to the caller if we have found some (this
         * ensures forward progress). We'll be coming back to find
         * the rest.
         */
        if (!list_empty(&data->dispose))
                ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
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);

                if (!list_empty(&data.dispose)) {
                        shrink_dentry_list(&data.dispose);
                        continue;
                }

                cond_resched();
                if (!data.found)
                        break;
        }
}
EXPORT_SYMBOL(shrink_dcache_parent);

static enum d_walk_ret umount_check(void *_data, struct dentry *dentry)
{
        /* it has busy descendents; complain about those instead */
        if (!list_empty(&dentry->d_subdirs))
                return D_WALK_CONTINUE;

        /* root with refcount 1 is fine */
        if (dentry == _data && dentry->d_lockref.count == 1)
                return D_WALK_CONTINUE;

        printk(KERN_ERR "BUG: Dentry %p{i=%lx,n=%pd} "
                        " still in use (%d) [unmount of %s %s]\n",
                       dentry,
                       dentry->d_inode ?
                       dentry->d_inode->i_ino : 0UL,
                       dentry,
                       dentry->d_lockref.count,
                       dentry->d_sb->s_type->name,
                       dentry->d_sb->s_id);
        WARN_ON(1);
        return D_WALK_CONTINUE;
}

static void do_one_tree(struct dentry *dentry)
{
        shrink_dcache_parent(dentry);
        d_walk(dentry, dentry, umount_check);
        d_drop(dentry);
        dput(dentry);
}

/*
 * destroy the dentries attached to a superblock on unmounting
 */
void shrink_dcache_for_umount(struct super_block *sb)
{
        struct dentry *dentry;

        WARN(down_read_trylock(&sb->s_umount), "s_umount should've been locked");

        dentry = sb->s_root;
        sb->s_root = NULL;
        do_one_tree(dentry);

        while (!hlist_bl_empty(&sb->s_roots)) {
                dentry = dget(hlist_bl_entry(hlist_bl_first(&sb->s_roots), struct dentry, d_hash));
                do_one_tree(dentry);
        }
}

static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
{
        struct dentry **victim = _data;
        if (d_mountpoint(dentry)) {
                __dget_dlock(dentry);
                *victim = dentry;
                return D_WALK_QUIT;
        }
        return D_WALK_CONTINUE;
}

/**
 * d_invalidate - detach submounts, prune dcache, and drop
 * @dentry: dentry to invalidate (aka detach, prune and drop)
 */
void d_invalidate(struct dentry *dentry)
{
        bool had_submounts = false;
        spin_lock(&dentry->d_lock);
        if (d_unhashed(dentry)) {
                spin_unlock(&dentry->d_lock);
                return;
        }
        __d_drop(dentry);
        spin_unlock(&dentry->d_lock);

        /* Negative dentries can be dropped without further checks */
        if (!dentry->d_inode)
                return;

        shrink_dcache_parent(dentry);
        for (;;) {
                struct dentry *victim = NULL;
                d_walk(dentry, &victim, find_submount);
                if (!victim) {
                        if (had_submounts)
                                shrink_dcache_parent(dentry);
                        return;
                }
                had_submounts = true;
                detach_mounts(victim);
                dput(victim);
        }
}
EXPORT_SYMBOL(d_invalidate);

/**
 * __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;
        int err;

        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 (unlikely(!name)) {
                name = &slash_name;
                dname = dentry->d_iname;
        } else if (name->len > DNAME_INLINE_LEN-1) {
                size_t size = offsetof(struct external_name, name[1]);
                struct external_name *p = kmalloc(size + name->len,
                                                  GFP_KERNEL_ACCOUNT |
                                                  __GFP_RECLAIMABLE);
                if (!p) {
                        kmem_cache_free(dentry_cache, dentry); 
                        return NULL;
                }
                atomic_set(&p->u.count, 1);
                dname = p->name;
        } 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_store_release(&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_u.d_alias);
        INIT_LIST_HEAD(&dentry->d_child);
        d_set_d_op(dentry, dentry->d_sb->s_d_op);

        if (dentry->d_op && dentry->d_op->d_init) {
                err = dentry->d_op->d_init(dentry);
                if (err) {
                        if (dname_external(dentry))
                                kfree(external_name(dentry));
                        kmem_cache_free(dentry_cache, dentry);
                        return NULL;
                }
        }

        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;
        dentry->d_flags |= DCACHE_RCUACCESS;
        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_child, &parent->d_subdirs);
        spin_unlock(&parent->d_lock);

        return dentry;
}
EXPORT_SYMBOL(d_alloc);

struct dentry *d_alloc_anon(struct super_block *sb)
{
        return __d_alloc(sb, NULL);
}
EXPORT_SYMBOL(d_alloc_anon);

struct dentry *d_alloc_cursor(struct dentry * parent)
{
        struct dentry *dentry = d_alloc_anon(parent->d_sb);
        if (dentry) {
                dentry->d_flags |= DCACHE_RCUACCESS | DCACHE_DENTRY_CURSOR;
                dentry->d_parent = dget(parent);
        }
        return dentry;
}

/**
 * d_alloc_pseudo - allocate a dentry (for lookup-less filesystems)
 * @sb: the superblock
 * @name: qstr of the name
 *
 * For a filesystem that just pins its dentries in memory and never
 * performs lookups at all, return an unhashed IS_ROOT dentry.
 */
struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
{
        return __d_alloc(sb, name);
}
EXPORT_SYMBOL(d_alloc_pseudo);

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

        q.name = name;
        q.hash_len = hashlen_string(parent, name);
        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        |
                                DCACHE_OP_REAL));
        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;
        if (op->d_real)
                dentry->d_flags |= DCACHE_OP_REAL;

}
EXPORT_SYMBOL(d_set_d_op);


/*
 * d_set_fallthru - Mark a dentry as falling through to a lower layer
 * @dentry - The dentry to mark
 *
 * Mark a dentry as falling through to the lower layer (as set with
 * d_pin_lower()).  This flag may be recorded on the medium.
 */
void d_set_fallthru(struct dentry *dentry)
{
        spin_lock(&dentry->d_lock);
        dentry->d_flags |= DCACHE_FALLTHRU;
        spin_unlock(&dentry->d_lock);
}
EXPORT_SYMBOL(d_set_fallthru);

static unsigned d_flags_for_inode(struct inode *inode)
{
        unsigned add_flags = DCACHE_REGULAR_TYPE;

        if (!inode)
                return DCACHE_MISS_TYPE;

        if (S_ISDIR(inode->i_mode)) {
                add_flags = DCACHE_DIRECTORY_TYPE;
                if (unlikely(!(inode->i_opflags & IOP_LOOKUP))) {
                        if (unlikely(!inode->i_op->lookup))
                                add_flags = DCACHE_AUTODIR_TYPE;
                        else
                                inode->i_opflags |= IOP_LOOKUP;
                }
                goto type_determined;
        }

        if (unlikely(!(inode->i_opflags & IOP_NOFOLLOW))) {
                if (unlikely(inode->i_op->get_link)) {
                        add_flags = DCACHE_SYMLINK_TYPE;
                        goto type_determined;
                }
                inode->i_opflags |= IOP_NOFOLLOW;
        }

        if (unlikely(!S_ISREG(inode->i_mode)))
                add_flags = DCACHE_SPECIAL_TYPE;

type_determined:
        if (unlikely(IS_AUTOMOUNT(inode)))
                add_flags |= DCACHE_NEED_AUTOMOUNT;
        return add_flags;
}

static void __d_instantiate(struct dentry *dentry, struct inode *inode)
{
        unsigned add_flags = d_flags_for_inode(inode);
        WARN_ON(d_in_lookup(dentry));

        spin_lock(&dentry->d_lock);
        hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
        raw_write_seqcount_begin(&dentry->d_seq);
        __d_set_inode_and_type(dentry, inode, add_flags);
        raw_write_seqcount_end(&dentry->d_seq);
        fsnotify_update_flags(dentry);
        spin_unlock(&dentry->d_lock);
}

/**
 * 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_u.d_alias));
        if (inode) {
                security_d_instantiate(entry, inode);
                spin_lock(&inode->i_lock);
                __d_instantiate(entry, inode);
                spin_unlock(&inode->i_lock);
        }
}
EXPORT_SYMBOL(d_instantiate);

/*
 * This should be equivalent to d_instantiate() + unlock_new_inode(),
 * with lockdep-related part of unlock_new_inode() done before
 * anything else.  Use that instead of open-coding d_instantiate()/
 * unlock_new_inode() combinations.
 */
void d_instantiate_new(struct dentry *entry, struct inode *inode)
{
        BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
        BUG_ON(!inode);
        lockdep_annotate_inode_mutex_key(inode);
        security_d_instantiate(entry, inode);
        spin_lock(&inode->i_lock);
        __d_instantiate(entry, inode);
        WARN_ON(!(inode->i_state & I_NEW));
        inode->i_state &= ~I_NEW & ~I_CREATING;
        smp_mb();
        wake_up_bit(&inode->i_state, __I_NEW);
        spin_unlock(&inode->i_lock);
}
EXPORT_SYMBOL(d_instantiate_new);

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

        if (root_inode) {
                res = d_alloc_anon(root_inode->i_sb);
                if (res) {
                        res->d_flags |= DCACHE_RCUACCESS;
                        d_instantiate(res, root_inode);
                } else {
                        iput(root_inode);
                }
        }
        return res;
}
EXPORT_SYMBOL(d_make_root);

static struct dentry *__d_instantiate_anon(struct dentry *dentry,
                                           struct inode *inode,
                                           bool disconnected)
{
        struct dentry *res;
        unsigned add_flags;

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

        /* attach a disconnected dentry */
        add_flags = d_flags_for_inode(inode);

        if (disconnected)
                add_flags |= DCACHE_DISCONNECTED;

        spin_lock(&dentry->d_lock);
        __d_set_inode_and_type(dentry, inode, add_flags);
        hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
        if (!disconnected) {
                hlist_bl_lock(&dentry->d_sb->s_roots);
                hlist_bl_add_head(&dentry->d_hash, &dentry->d_sb->s_roots);
                hlist_bl_unlock(&dentry->d_sb->s_roots);
        }
        spin_unlock(&dentry->d_lock);
        spin_unlock(&inode->i_lock);

        return dentry;

 out_iput:
        iput(inode);
        return res;
}

struct dentry *d_instantiate_anon(struct dentry *dentry, struct inode *inode)
{
        return __d_instantiate_anon(dentry, inode, true);
}
EXPORT_SYMBOL(d_instantiate_anon);

static struct dentry *__d_obtain_alias(struct inode *inode, bool disconnected)
{
        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_anon(inode->i_sb);
        if (!tmp) {
                res = ERR_PTR(-ENOMEM);
                goto out_iput;
        }

        return __d_instantiate_anon(tmp, inode, disconnected);

out_iput:
        iput(inode);
        return res;
}

/**
 * d_obtain_alias - find or allocate a DISCONNECTED 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 the error will be propagated to the return value,
 * with a %NULL @inode replaced by ERR_PTR(-ESTALE).
 */
struct dentry *d_obtain_alias(struct inode *inode)
{
        return __d_obtain_alias(inode, true);
}
EXPORT_SYMBOL(d_obtain_alias);

/**
 * d_obtain_root - find or allocate a dentry for a given inode
 * @inode: inode to allocate the dentry for
 *
 * Obtain an IS_ROOT dentry for the root of a filesystem.
 *
 * We must ensure that directory inodes only ever have 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.  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_root(struct inode *inode)
{
        return __d_obtain_alias(inode, false);
}
EXPORT_SYMBOL(d_obtain_root);

/**
 * 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, *res;

        /*
         * First check if a dentry matching the name already exists,
         * if not go ahead and create it now.
         */
        found = d_hash_and_lookup(dentry->d_parent, name);
        if (found) {
                iput(inode);
                return found;
        }
        if (d_in_lookup(dentry)) {
                found = d_alloc_parallel(dentry->d_parent, name,
                                        dentry->d_wait);
                if (IS_ERR(found) || !d_in_lookup(found)) {
                        iput(inode);
                        return found;
                }
        } else {
                found = d_alloc(dentry->d_parent, name);
                if (!found) {
                        iput(inode);
                        return ERR_PTR(-ENOMEM);
                } 
        }
        res = d_splice_alias(inode, found);
        if (res) {
                dput(found);
                return res;
        }
        return found;
}
EXPORT_SYMBOL(d_add_ci);


static inline bool d_same_name(const struct dentry *dentry,
                                const struct dentry *parent,
                                const struct qstr *name)
{
        if (likely(!(parent->d_flags & DCACHE_OP_COMPARE))) {
                if (dentry->d_name.len != name->len)
                        return false;
                return dentry_cmp(dentry, name->name, name->len) == 0;
        }
        return parent->d_op->d_compare(dentry,
                                       dentry->d_name.len, dentry->d_name.name,
                                       name) == 0;
}

/**
 * __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(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.
                 *
                 * Note that raw_seqcount_begin still *does* smp_rmb(), so
                 * we are still guaranteed NUL-termination of ->d_name.name.
                 */
                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)) {
                        int tlen;
                        const char *tname;
                        if (dentry->d_name.hash != hashlen_hash(hashlen))
                                continue;
                        tlen = dentry->d_name.len;
                        tname = dentry->d_name.name;
                        /* we want a consistent (name,len) pair */
                        if (read_seqcount_retry(&dentry->d_seq, seq)) {
                                cpu_relax();
                                goto seqretry;
                        }
                        if (parent->d_op->d_compare(dentry,
                                                    tlen, tname, name) != 0)
                                continue;
                } else {
                        if (dentry->d_name.hash_len != hashlen)
                                continue;
                        if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
                                continue;
                }
                *seqp = seq;
                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 hash = name->hash;
        struct hlist_bl_head *b = d_hash(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;

                if (!d_same_name(dentry, parent, name))
                        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(dir, 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);

/*
 * 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 = dentry->d_inode;
        int isdir = d_is_dir(dentry);

        spin_lock(&inode->i_lock);
        spin_lock(&dentry->d_lock);
        /*
         * Are we the only user?
         */
        if (dentry->d_lockref.count == 1) {
                dentry->d_flags &= ~DCACHE_CANT_MOUNT;
                dentry_unlink_inode(dentry);
        } else {
                __d_drop(dentry);
                spin_unlock(&dentry->d_lock);
                spin_unlock(&inode->i_lock);
        }
        fsnotify_nameremove(dentry, isdir);
}
EXPORT_SYMBOL(d_delete);

static void __d_rehash(struct dentry *entry)
{
        struct hlist_bl_head *b = d_hash(entry->d_name.hash);

        hlist_bl_lock(b);
        hlist_bl_add_head_rcu(&entry->d_hash, b);
        hlist_bl_unlock(b);
}

/**
 * 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);

static inline unsigned start_dir_add(struct inode *dir)
{

        for (;;) {
                unsigned n = dir->i_dir_seq;
                if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
                        return n;
                cpu_relax();
        }
}

static inline void end_dir_add(struct inode *dir, unsigned n)
{
        smp_store_release(&dir->i_dir_seq, n + 2);
}

static void d_wait_lookup(struct dentry *dentry)
{
        if (d_in_lookup(dentry)) {
                DECLARE_WAITQUEUE(wait, current);
                add_wait_queue(dentry->d_wait, &wait);
                do {
                        set_current_state(TASK_UNINTERRUPTIBLE);
                        spin_unlock(&dentry->d_lock);
                        schedule();
                        spin_lock(&dentry->d_lock);
                } while (d_in_lookup(dentry));
        }
}

struct dentry *d_alloc_parallel(struct dentry *parent,
                                const struct qstr *name,
                                wait_queue_head_t *wq)
{
        unsigned int hash = name->hash;
        struct hlist_bl_head *b = in_lookup_hash(parent, hash);
        struct hlist_bl_node *node;
        struct dentry *new = d_alloc(parent, name);
        struct dentry *dentry;
        unsigned seq, r_seq, d_seq;

        if (unlikely(!new))
                return ERR_PTR(-ENOMEM);

retry:
        rcu_read_lock();
        seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
        r_seq = read_seqbegin(&rename_lock);
        dentry = __d_lookup_rcu(parent, name, &d_seq);
        if (unlikely(dentry)) {
                if (!lockref_get_not_dead(&dentry->d_lockref)) {
                        rcu_read_unlock();
                        goto retry;
                }
                if (read_seqcount_retry(&dentry->d_seq, d_seq)) {
                        rcu_read_unlock();
                        dput(dentry);
                        goto retry;
                }
                rcu_read_unlock();
                dput(new);
                return dentry;
        }
        if (unlikely(read_seqretry(&rename_lock, r_seq))) {
                rcu_read_unlock();
                goto retry;
        }

        if (unlikely(seq & 1)) {
                rcu_read_unlock();
                goto retry;
        }

        hlist_bl_lock(b);
        if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
                hlist_bl_unlock(b);
                rcu_read_unlock();
                goto retry;
        }
        /*
         * No changes for the parent since the beginning of d_lookup().
         * Since all removals from the chain happen with hlist_bl_lock(),
         * any potential in-lookup matches are going to stay here until
         * we unlock the chain.  All fields are stable in everything
         * we encounter.
         */
        hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {
                if (dentry->d_name.hash != hash)
                        continue;
                if (dentry->d_parent != parent)
                        continue;
                if (!d_same_name(dentry, parent, name))
                        continue;
                hlist_bl_unlock(b);
                /* now we can try to grab a reference */
                if (!lockref_get_not_dead(&dentry->d_lockref)) {
                        rcu_read_unlock();
                        goto retry;
                }

                rcu_read_unlock();
                /*
                 * somebody is likely to be still doing lookup for it;
                 * wait for them to finish
                 */
                spin_lock(&dentry->d_lock);
                d_wait_lookup(dentry);
                /*
                 * it's not in-lookup anymore; in principle we should repeat
                 * everything from dcache lookup, but it's likely to be what
                 * d_lookup() would've found anyway.  If it is, just return it;
                 * otherwise we really have to repeat the whole thing.
                 */
                if (unlikely(dentry->d_name.hash != hash))
                        goto mismatch;
                if (unlikely(dentry->d_parent != parent))
                        goto mismatch;
                if (unlikely(d_unhashed(dentry)))
                        goto mismatch;
                if (unlikely(!d_same_name(dentry, parent, name)))
                        goto mismatch;
                /* OK, it *is* a hashed match; return it */
                spin_unlock(&dentry->d_lock);
                dput(new);
                return dentry;
        }
        rcu_read_unlock();
        /* we can't take ->d_lock here; it's OK, though. */
        new->d_flags |= DCACHE_PAR_LOOKUP;
        new->d_wait = wq;
        hlist_bl_add_head_rcu(&new->d_u.d_in_lookup_hash, b);
        hlist_bl_unlock(b);
        return new;
mismatch:
        spin_unlock(&dentry->d_lock);
        dput(dentry);
        goto retry;
}
EXPORT_SYMBOL(d_alloc_parallel);

void __d_lookup_done(struct dentry *dentry)
{
        struct hlist_bl_head *b = in_lookup_hash(dentry->d_parent,
                                                 dentry->d_name.hash);
        hlist_bl_lock(b);
        dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
        __hlist_bl_del(&dentry->d_u.d_in_lookup_hash);
        wake_up_all(dentry->d_wait);
        dentry->d_wait = NULL;
        hlist_bl_unlock(b);
        INIT_HLIST_NODE(&dentry->d_u.d_alias);
        INIT_LIST_HEAD(&dentry->d_lru);
}
EXPORT_SYMBOL(__d_lookup_done);

/* inode->i_lock held if inode is non-NULL */

static inline void __d_add(struct dentry *dentry, struct inode *inode)
{
        struct inode *dir = NULL;
        unsigned n;
        spin_lock(&dentry->d_lock);
        if (unlikely(d_in_lookup(dentry))) {
                dir = dentry->d_parent->d_inode;
                n = start_dir_add(dir);
                __d_lookup_done(dentry);
        }
        if (inode) {
                unsigned add_flags = d_flags_for_inode(inode);
                hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
                raw_write_seqcount_begin(&dentry->d_seq);
                __d_set_inode_and_type(dentry, inode, add_flags);
                raw_write_seqcount_end(&dentry->d_seq);
                fsnotify_update_flags(dentry);
        }
        __d_rehash(dentry);
        if (dir)
                end_dir_add(dir, n);
        spin_unlock(&dentry->d_lock);
        if (inode)
                spin_unlock(&inode->i_lock);
}

/**
 * d_add - add dentry to hash queues
 * @entry: dentry to add
 * @inode: The inode to attach to this dentry
 *
 * This adds the entry to the hash queues and initializes @inode.
 * The entry was actually filled in earlier during d_alloc().
 */

void d_add(struct dentry *entry, struct inode *inode)
{
        if (inode) {
                security_d_instantiate(entry, inode);
                spin_lock(&inode->i_lock);
        }
        __d_add(entry, inode);
}
EXPORT_SYMBOL(d_add);

/**
 * d_exact_alias - find and hash an exact unhashed alias
 * @entry: dentry to add
 * @inode: The inode to go with this dentry
 *
 * If an unhashed dentry with the same name/parent and desired
 * inode already exists, hash and return it.  Otherwise, return
 * NULL.
 *
 * Parent directory should be locked.
 */
struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
{
        struct dentry *alias;
        unsigned int hash = entry->d_name.hash;

        spin_lock(&inode->i_lock);
        hlist_for_each_entry(alias, &inode->i_dentry, d_u.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 (!d_same_name(alias, entry->d_parent, &entry->d_name))
                        continue;
                spin_lock(&alias->d_lock);
                if (!d_unhashed(alias)) {
                        spin_unlock(&alias->d_lock);
                        alias = NULL;
                } else {
                        __dget_dlock(alias);
                        __d_rehash(alias);
                        spin_unlock(&alias->d_lock);
                }
                spin_unlock(&inode->i_lock);
                return alias;
        }
        spin_unlock(&inode->i_lock);
        return NULL;
}
EXPORT_SYMBOL(d_exact_alias);

static void swap_names(struct dentry *dentry, struct dentry *target)
{
        if (unlikely(dname_external(target))) {
                if (unlikely(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 (unlikely(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.
                         */
                        unsigned int i;
                        BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
                        for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
                                swap(((long *) &dentry->d_iname)[i],
                                     ((long *) &target->d_iname)[i]);
                        }
                }
        }
        swap(dentry->d_name.hash_len, target->d_name.hash_len);
}

static void copy_name(struct dentry *dentry, struct dentry *target)
{
        struct external_name *old_name = NULL;
        if (unlikely(dname_external(dentry)))
                old_name = external_name(dentry);
        if (unlikely(dname_external(target))) {
                atomic_inc(&external_name(target)->u.count);
                dentry->d_name = target->d_name;
        } else {
                memcpy(dentry->d_iname, target->d_name.name,
                                target->d_name.len + 1);
                dentry->d_name.name = dentry->d_iname;
                dentry->d_name.hash_len = target->d_name.hash_len;
        }
        if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
                kfree_rcu(old_name, u.head);
}

/*
 * __d_move - move a dentry
 * @dentry: entry to move
 * @target: new dentry
 * @exchange: exchange the two dentries
 *
 * 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,
                     bool exchange)
{
        struct dentry *old_parent, *p;
        struct inode *dir = NULL;
        unsigned n;

        WARN_ON(!dentry->d_inode);
        if (WARN_ON(dentry == target))
                return;

        BUG_ON(d_ancestor(target, dentry));
        old_parent = dentry->d_parent;
        p = d_ancestor(old_parent, target);
        if (IS_ROOT(dentry)) {
                BUG_ON(p);
                spin_lock(&target->d_parent->d_lock);
        } else if (!p) {
                /* target is not a descendent of dentry->d_parent */
                spin_lock(&target->d_parent->d_lock);
                spin_lock_nested(&old_parent->d_lock, DENTRY_D_LOCK_NESTED);
        } else {
                BUG_ON(p == dentry);
                spin_lock(&old_parent->d_lock);
                if (p != target)
                        spin_lock_nested(&target->d_parent->d_lock,
                                        DENTRY_D_LOCK_NESTED);
        }
        spin_lock_nested(&dentry->d_lock, 2);
        spin_lock_nested(&target->d_lock, 3);

        if (unlikely(d_in_lookup(target))) {
                dir = target->d_parent->d_inode;
                n = start_dir_add(dir);
                __d_lookup_done(target);
        }

        write_seqcount_begin(&dentry->d_seq);
        write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);

        /* unhash both */
        if (!d_unhashed(dentry))
                ___d_drop(dentry);
        if (!d_unhashed(target))
                ___d_drop(target);

        /* ... and switch them in the tree */
        dentry->d_parent = target->d_parent;
        if (!exchange) {
                copy_name(dentry, target);
                target->d_hash.pprev = NULL;
                dentry->d_parent->d_lockref.count++;
                if (dentry == old_parent)
                        dentry->d_flags |= DCACHE_RCUACCESS;
                else
                        WARN_ON(!--old_parent->d_lockref.count);
        } else {
                target->d_parent = old_parent;
                swap_names(dentry, target);
                list_move(&target->d_child, &target->d_parent->d_subdirs);
                __d_rehash(target);
                fsnotify_update_flags(target);
        }
        list_move(&dentry->d_child, &dentry->d_parent->d_subdirs);
        __d_rehash(dentry);
        fsnotify_update_flags(dentry);

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

        if (dir)
                end_dir_add(dir, n);

        if (dentry->d_parent != old_parent)
                spin_unlock(&dentry->d_parent->d_lock);
        if (dentry != old_parent)
                spin_unlock(&old_parent->d_lock);
        spin_unlock(&target->d_lock);
        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, false);
        write_sequnlock(&rename_lock);
}
EXPORT_SYMBOL(d_move);

/*
 * d_exchange - exchange two dentries
 * @dentry1: first dentry
 * @dentry2: second dentry
 */
void d_exchange(struct dentry *dentry1, struct dentry *dentry2)
{
        write_seqlock(&rename_lock);

        WARN_ON(!dentry1->d_inode);
        WARN_ON(!dentry2->d_inode);
        WARN_ON(IS_ROOT(dentry1));
        WARN_ON(IS_ROOT(dentry2));

        __d_move(dentry1, dentry2, true);

        write_sequnlock(&rename_lock);
}

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

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

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

        /* 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 (!inode_trylock_shared(alias->d_parent->d_inode))
                goto out_err;
        m2 = &alias->d_parent->d_inode->i_rwsem;
out_unalias:
        __d_move(alias, dentry, false);
        ret = 0;
out_err:
        if (m2)
                up_read(m2);
        if (m1)
                mutex_unlock(m1);
        return ret;
}

/**
 * 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 an IS_ROOT alias, 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.
 *
 * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
 * we should error out: directories can't have multiple aliases.
 *
 * 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)
{
        if (IS_ERR(inode))
                return ERR_CAST(inode);

        BUG_ON(!d_unhashed(dentry));

        if (!inode)
                goto out;

        security_d_instantiate(dentry, inode);
        spin_lock(&inode->i_lock);
        if (S_ISDIR(inode->i_mode)) {
                struct dentry *new = __d_find_any_alias(inode);
                if (unlikely(new)) {
                        /* The reference to new ensures it remains an alias */
                        spin_unlock(&inode->i_lock);
                        write_seqlock(&rename_lock);
                        if (unlikely(d_ancestor(new, dentry))) {
                                write_sequnlock(&rename_lock);
                                dput(new);
                                new = ERR_PTR(-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);
                        } else if (!IS_ROOT(new)) {
                                struct dentry *old_parent = dget(new->d_parent);
                                int err = __d_unalias(inode, dentry, new);
                                write_sequnlock(&rename_lock);
                                if (err) {
                                        dput(new);
                                        new = ERR_PTR(err);
                                }
                                dput(old_parent);
                        } else {
                                __d_move(new, dentry, false);
                                write_sequnlock(&rename_lock);
                        }
                        iput(inode);
                        return new;
                }
        }
out:
        __d_add(dentry, inode);
        return NULL;
}
EXPORT_SYMBOL(d_splice_alias);

/*
 * 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 true if new_dentry is a subdirectory of the parent (at any depth).
 * Returns false otherwise.
 * Caller must ensure that "new_dentry" is pinned before calling is_subdir()
 */
  
bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
{
        bool result;
        unsigned seq;

        if (new_dentry == old_dentry)
                return true;

        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 = true;
                else
                        result = false;
                rcu_read_unlock();
        } while (read_seqretry(&rename_lock, seq));

        return result;
}
EXPORT_SYMBOL(is_subdir);

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);
}

EXPORT_SYMBOL(d_genocide);

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_u.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)
{
        /* 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 | HASH_ZERO,
                                        &d_hash_shift,
                                        NULL,
                                        0,
                                        0);
        d_hash_shift = 32 - d_hash_shift;
}

static void __init dcache_init(void)
{
        /*
         * 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_USERCOPY(dentry,
                SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD|SLAB_ACCOUNT,
                d_iname);

        /* 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,
                                        HASH_ZERO,
                                        &d_hash_shift,
                                        NULL,
                                        0,
                                        0);
        d_hash_shift = 32 - d_hash_shift;
}

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

void __init vfs_caches_init_early(void)
{
        int i;

        for (i = 0; i < ARRAY_SIZE(in_lookup_hashtable); i++)
                INIT_HLIST_BL_HEAD(&in_lookup_hashtable[i]);

        dcache_init_early();
        inode_init_early();
}

void __init vfs_caches_init(void)
{
        names_cachep = kmem_cache_create_usercopy("names_cache", PATH_MAX, 0,
                        SLAB_HWCACHE_ALIGN|SLAB_PANIC, 0, PATH_MAX, NULL);

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