initial commit with v2.6.9

[linux-2.6.9-moxart.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/config.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/smp_lock.h>
#include <linux/hash.h>
#include <linux/cache.h>
#include <linux/module.h>
#include <linux/mount.h>
#include <linux/file.h>
#include <asm/uaccess.h>
#include <linux/security.h>
#include <linux/seqlock.h>
#include <linux/swap.h>
#include <linux/bootmem.h>

/* #define DCACHE_DEBUG 1 */

int sysctl_vfs_cache_pressure = 100;

spinlock_t dcache_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
seqlock_t rename_lock __cacheline_aligned_in_smp = SEQLOCK_UNLOCKED;

EXPORT_SYMBOL(dcache_lock);

static kmem_cache_t *dentry_cache; 

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

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

static unsigned int d_hash_mask;
static unsigned int d_hash_shift;
static struct hlist_head *dentry_hashtable;
static LIST_HEAD(dentry_unused);

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

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

        if (dname_external(dentry))
                kfree(dentry->d_name.name);
        kmem_cache_free(dentry_cache, dentry); 
}

/*
 * no dcache_lock, please.  The caller must decrement dentry_stat.nr_dentry
 * inside dcache_lock.
 */
static void d_free(struct dentry *dentry)
{
        if (dentry->d_op && dentry->d_op->d_release)
                dentry->d_op->d_release(dentry);
        call_rcu(&dentry->d_rcu, d_callback);
}

/*
 * Release the dentry's inode, using the filesystem
 * d_iput() operation if defined.
 * Called with dcache_lock and per dentry lock held, drops both.
 */
static inline void dentry_iput(struct dentry * dentry)
{
        struct inode *inode = dentry->d_inode;
        if (inode) {
                dentry->d_inode = NULL;
                list_del_init(&dentry->d_alias);
                spin_unlock(&dentry->d_lock);
                spin_unlock(&dcache_lock);
                if (dentry->d_op && dentry->d_op->d_iput)
                        dentry->d_op->d_iput(dentry, inode);
                else
                        iput(inode);
        } else {
                spin_unlock(&dentry->d_lock);
                spin_unlock(&dcache_lock);
        }
}

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

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

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

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

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

unhash_it:
        __d_drop(dentry);

kill_it: {
                struct dentry *parent;

                /* If dentry was on d_lru list
                 * delete it from there
                 */
                if (!list_empty(&dentry->d_lru)) {
                        list_del(&dentry->d_lru);
                        dentry_stat.nr_unused--;
                }
                list_del(&dentry->d_child);
                dentry_stat.nr_dentry--;        /* For d_free, below */
                /*drops the locks, at that point nobody can reach this dentry */
                dentry_iput(dentry);
                parent = dentry->d_parent;
                d_free(dentry);
                if (dentry == parent)
                        return;
                dentry = parent;
                goto repeat;
        }
}

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

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

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

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

static inline struct dentry * __dget_locked(struct dentry *dentry)
{
        atomic_inc(&dentry->d_count);
        if (!list_empty(&dentry->d_lru)) {
                dentry_stat.nr_unused--;
                list_del_init(&dentry->d_lru);
        }
        return dentry;
}

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

/**
 * d_find_alias - grab a hashed alias of inode
 * @inode: inode in question
 *
 * If inode has a hashed 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.
 *
 * If the inode has a DCACHE_DISCONNECTED alias, then prefer
 * any other hashed alias over that one.
 */

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

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

struct dentry * d_find_alias(struct inode *inode)
{
        struct dentry *de;
        spin_lock(&dcache_lock);
        de = __d_find_alias(inode, 0);
        spin_unlock(&dcache_lock);
        return de;
}

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

/*
 * Throw away a dentry - free the inode, dput the parent.
 * This requires that the LRU list has already been
 * removed.
 * Called with dcache_lock, drops it and then regains.
 */
static inline void prune_one_dentry(struct dentry * dentry)
{
        struct dentry * parent;

        __d_drop(dentry);
        list_del(&dentry->d_child);
        dentry_stat.nr_dentry--;        /* For d_free, below */
        dentry_iput(dentry);
        parent = dentry->d_parent;
        d_free(dentry);
        if (parent != dentry)
                dput(parent);
        spin_lock(&dcache_lock);
}

/**
 * prune_dcache - shrink the dcache
 * @count: number of entries to try and free
 *
 * Shrink the dcache. This is done when we need
 * more memory, or simply when we need to unmount
 * something (at which point we need to unuse
 * all dentries).
 *
 * This function may fail to free any resources if
 * all the dentries are in use.
 */
 
static void prune_dcache(int count)
{
        spin_lock(&dcache_lock);
        for (; count ; count--) {
                struct dentry *dentry;
                struct list_head *tmp;

                tmp = dentry_unused.prev;
                if (tmp == &dentry_unused)
                        break;
                list_del_init(tmp);
                prefetch(dentry_unused.prev);
                dentry_stat.nr_unused--;
                dentry = list_entry(tmp, struct dentry, d_lru);

                spin_lock(&dentry->d_lock);
                /*
                 * We found an inuse dentry which was not removed from
                 * dentry_unused because of laziness during lookup.  Do not free
                 * it - just keep it off the dentry_unused list.
                 */
                if (atomic_read(&dentry->d_count)) {
                        spin_unlock(&dentry->d_lock);
                        continue;
                }
                /* If the dentry was recently referenced, don't free it. */
                if (dentry->d_flags & DCACHE_REFERENCED) {
                        dentry->d_flags &= ~DCACHE_REFERENCED;
                        list_add(&dentry->d_lru, &dentry_unused);
                        dentry_stat.nr_unused++;
                        spin_unlock(&dentry->d_lock);
                        continue;
                }
                prune_one_dentry(dentry);
        }
        spin_unlock(&dcache_lock);
}

/*
 * Shrink the dcache for the specified super block.
 * This allows us to unmount a device without disturbing
 * the dcache for the other devices.
 *
 * This implementation makes just two traversals of the
 * unused list.  On the first pass we move the selected
 * dentries to the most recent end, and on the second
 * pass we free them.  The second pass must restart after
 * each dput(), but since the target dentries are all at
 * the end, it's really just a single traversal.
 */

/**
 * 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)
{
        struct list_head *tmp, *next;
        struct dentry *dentry;

        /*
         * Pass one ... move the dentries for the specified
         * superblock to the most recent end of the unused list.
         */
        spin_lock(&dcache_lock);
        next = dentry_unused.next;
        while (next != &dentry_unused) {
                tmp = next;
                next = tmp->next;
                dentry = list_entry(tmp, struct dentry, d_lru);
                if (dentry->d_sb != sb)
                        continue;
                list_del(tmp);
                list_add(tmp, &dentry_unused);
        }

        /*
         * Pass two ... free the dentries for this superblock.
         */
repeat:
        next = dentry_unused.next;
        while (next != &dentry_unused) {
                tmp = next;
                next = tmp->next;
                dentry = list_entry(tmp, struct dentry, d_lru);
                if (dentry->d_sb != sb)
                        continue;
                dentry_stat.nr_unused--;
                list_del_init(tmp);
                spin_lock(&dentry->d_lock);
                if (atomic_read(&dentry->d_count)) {
                        spin_unlock(&dentry->d_lock);
                        continue;
                }
                prune_one_dentry(dentry);
                goto repeat;
        }
        spin_unlock(&dcache_lock);
}

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

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

/*
 * Search the dentry child list for the specified parent,
 * and move any unused dentries to the end of the unused
 * list for prune_dcache(). We descend to the next level
 * whenever the d_subdirs list is non-empty and continue
 * searching.
 */
static int select_parent(struct dentry * parent)
{
        struct dentry *this_parent = parent;
        struct list_head *next;
        int found = 0;

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

                if (!list_empty(&dentry->d_lru)) {
                        dentry_stat.nr_unused--;
                        list_del_init(&dentry->d_lru);
                }
                /* 
                 * move only zero ref count dentries to the end 
                 * of the unused list for prune_dcache
                 */
                if (!atomic_read(&dentry->d_count)) {
                        list_add(&dentry->d_lru, dentry_unused.prev);
                        dentry_stat.nr_unused++;
                        found++;
                }
                /*
                 * Descend a level if the d_subdirs list is non-empty.
                 */
                if (!list_empty(&dentry->d_subdirs)) {
                        this_parent = dentry;
#ifdef DCACHE_DEBUG
printk(KERN_DEBUG "select_parent: descending to %s/%s, found=%d\n",
dentry->d_parent->d_name.name, dentry->d_name.name, found);
#endif
                        goto repeat;
                }
        }
        /*
         * All done at this level ... ascend and resume the search.
         */
        if (this_parent != parent) {
                next = this_parent->d_child.next; 
                this_parent = this_parent->d_parent;
#ifdef DCACHE_DEBUG
printk(KERN_DEBUG "select_parent: ascending to %s/%s, found=%d\n",
this_parent->d_parent->d_name.name, this_parent->d_name.name, found);
#endif
                goto resume;
        }
        spin_unlock(&dcache_lock);
        return found;
}

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

        while ((found = select_parent(parent)) != 0)
                prune_dcache(found);
}

/**
 * shrink_dcache_anon - further prune the cache
 * @head: head of d_hash list of dentries to prune
 *
 * Prune the dentries that are anonymous
 *
 * parsing d_hash list does not hlist_for_each_rcu() as it
 * done under dcache_lock.
 *
 */
void shrink_dcache_anon(struct hlist_head *head)
{
        struct hlist_node *lp;
        int found;
        do {
                found = 0;
                spin_lock(&dcache_lock);
                hlist_for_each(lp, head) {
                        struct dentry *this = hlist_entry(lp, struct dentry, d_hash);
                        if (!list_empty(&this->d_lru)) {
                                dentry_stat.nr_unused--;
                                list_del_init(&this->d_lru);
                        }

                        /* 
                         * move only zero ref count dentries to the end 
                         * of the unused list for prune_dcache
                         */
                        if (!atomic_read(&this->d_count)) {
                                list_add_tail(&this->d_lru, &dentry_unused);
                                dentry_stat.nr_unused++;
                                found++;
                        }
                }
                spin_unlock(&dcache_lock);
                prune_dcache(found);
        } while(found);
}

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

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

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

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

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

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

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

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

        return dentry;
}

/**
 * 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)
{
        if (!list_empty(&entry->d_alias)) BUG();
        spin_lock(&dcache_lock);
        if (inode)
                list_add(&entry->d_alias, &inode->i_dentry);
        entry->d_inode = inode;
        spin_unlock(&dcache_lock);
        security_d_instantiate(entry, inode);
}

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

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

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

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

/**
 * d_alloc_anon - allocate an anonymous dentry
 * @inode: inode to allocate the dentry for
 *
 * This is similar to d_alloc_root.  It is used by filesystems when
 * creating a dentry for a given inode, often in the process of 
 * mapping a filehandle to a dentry.  The returned dentry may be
 * anonymous, or may have a full name (if the inode was already
 * in the cache).  The file system may need to make further
 * efforts to connect this dentry into the dcache properly.
 *
 * 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.  If %NULL is returned (indicating kmalloc failure),
 * the reference on the inode has not been released.
 */

struct dentry * d_alloc_anon(struct inode *inode)
{
        static const struct qstr anonstring = { .name = "" };
        struct dentry *tmp;
        struct dentry *res;

        if ((res = d_find_alias(inode))) {
                iput(inode);
                return res;
        }

        tmp = d_alloc(NULL, &anonstring);
        if (!tmp)
                return NULL;

        tmp->d_parent = tmp; /* make sure dput doesn't croak */
        
        spin_lock(&dcache_lock);
        res = __d_find_alias(inode, 0);
        if (!res) {
                /* attach a disconnected dentry */
                res = tmp;
                tmp = NULL;
                spin_lock(&res->d_lock);
                res->d_sb = inode->i_sb;
                res->d_parent = res;
                res->d_inode = inode;

                /*
                 * Set d_bucket to an "impossible" bucket address so
                 * that d_move() doesn't get a false positive
                 */
                res->d_bucket = NULL;
                res->d_flags |= DCACHE_DISCONNECTED;
                res->d_flags &= ~DCACHE_UNHASHED;
                list_add(&res->d_alias, &inode->i_dentry);
                hlist_add_head(&res->d_hash, &inode->i_sb->s_anon);
                spin_unlock(&res->d_lock);

                inode = NULL; /* don't drop reference */
        }
        spin_unlock(&dcache_lock);

        if (inode)
                iput(inode);
        if (tmp)
                dput(tmp);
        return res;
}


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

        if (inode) {
                spin_lock(&dcache_lock);
                new = __d_find_alias(inode, 1);
                if (new) {
                        BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));
                        spin_unlock(&dcache_lock);
                        security_d_instantiate(new, inode);
                        d_rehash(dentry);
                        d_move(new, dentry);
                        iput(inode);
                } else {
                        /* d_instantiate takes dcache_lock, so we do it by hand */
                        list_add(&dentry->d_alias, &inode->i_dentry);
                        dentry->d_inode = inode;
                        spin_unlock(&dcache_lock);
                        security_d_instantiate(dentry, inode);
                        d_rehash(dentry);
                }
        } else
                d_add(dentry, inode);
        return new;
}


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

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

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

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

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

                dentry = hlist_entry(node, struct dentry, d_hash);

                smp_rmb();

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

                spin_lock(&dentry->d_lock);

                /*
                 * If lookup ends up in a different bucket due to concurrent
                 * rename, fail it
                 */
                if (unlikely(dentry->d_bucket != head))
                        goto terminate;

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

                qstr = rcu_dereference(&dentry->d_name);
                if (parent->d_op && parent->d_op->d_compare) {
                        if (parent->d_op->d_compare(parent, qstr, name))
                                goto next;
                } else {
                        if (qstr->len != len)
                                goto next;
                        if (memcmp(qstr->name, str, len))
                                goto next;
                }

                if (!d_unhashed(dentry)) {
                        atomic_inc(&dentry->d_count);
                        found = dentry;
                }
terminate:
                spin_unlock(&dentry->d_lock);
                break;
next:
                spin_unlock(&dentry->d_lock);
        }
        rcu_read_unlock();

        return found;
}

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

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

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

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

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

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

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

/**
 * 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)
{
        struct hlist_head *list = d_hash(entry->d_parent, entry->d_name.hash);

        spin_lock(&dcache_lock);
        spin_lock(&entry->d_lock);
        entry->d_flags &= ~DCACHE_UNHASHED;
        spin_unlock(&entry->d_lock);
        entry->d_bucket = list;
        hlist_add_head_rcu(&entry->d_hash, list);
        spin_unlock(&dcache_lock);
}

#define do_switch(x,y) do { \
        __typeof__ (x) __tmp = x; \
        x = y; y = __tmp; } while (0)

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

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

void d_move(struct dentry * dentry, struct dentry * target)
{
        if (!dentry->d_inode)
                printk(KERN_WARNING "VFS: moving negative dcache entry\n");

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

        /* Move the dentry to the target hash queue, if on different bucket */
        if (dentry->d_flags & DCACHE_UNHASHED)
                goto already_unhashed;
        if (dentry->d_bucket != target->d_bucket) {
                hlist_del_rcu(&dentry->d_hash);
already_unhashed:
                dentry->d_bucket = target->d_bucket;
                hlist_add_head_rcu(&dentry->d_hash, target->d_bucket);
                dentry->d_flags &= ~DCACHE_UNHASHED;
        }

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

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

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

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

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

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

/**
 * d_path - return the path of a dentry
 * @dentry: dentry to report
 * @vfsmnt: vfsmnt to which the dentry belongs
 * @root: root dentry
 * @rootmnt: vfsmnt to which the root dentry belongs
 * @buffer: buffer to return value in
 * @buflen: buffer length
 *
 * Convert a dentry into an ASCII path name. If the entry has been deleted
 * the string " (deleted)" is appended. Note that this is ambiguous.
 *
 * Returns the buffer or an error code if the path was too long.
 *
 * "buflen" should be positive. Caller holds the dcache_lock.
 */
static char * __d_path( struct dentry *dentry, struct vfsmount *vfsmnt,
                        struct dentry *root, struct vfsmount *rootmnt,
                        char *buffer, int buflen)
{
        char * end = buffer+buflen;
        char * retval;
        int namelen;

        *--end = '\0';
        buflen--;
        if (!IS_ROOT(dentry) && d_unhashed(dentry)) {
                buflen -= 10;
                end -= 10;
                if (buflen < 0)
                        goto Elong;
                memcpy(end, " (deleted)", 10);
        }

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

        for (;;) {
                struct dentry * parent;

                if (dentry == root && vfsmnt == rootmnt)
                        break;
                if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
                        /* Global root? */
                        spin_lock(&vfsmount_lock);
                        if (vfsmnt->mnt_parent == vfsmnt) {
                                spin_unlock(&vfsmount_lock);
                                goto global_root;
                        }
                        dentry = vfsmnt->mnt_mountpoint;
                        vfsmnt = vfsmnt->mnt_parent;
                        spin_unlock(&vfsmount_lock);
                        continue;
                }
                parent = dentry->d_parent;
                prefetch(parent);
                namelen = dentry->d_name.len;
                buflen -= namelen + 1;
                if (buflen < 0)
                        goto Elong;
                end -= namelen;
                memcpy(end, dentry->d_name.name, namelen);
                *--end = '/';
                retval = end;
                dentry = parent;
        }

        return retval;

global_root:
        namelen = dentry->d_name.len;
        buflen -= namelen;
        if (buflen < 0)
                goto Elong;
        retval -= namelen-1;    /* hit the slash */
        memcpy(retval, dentry->d_name.name, namelen);
        return retval;
Elong:
        return ERR_PTR(-ENAMETOOLONG);
}

/* write full pathname into buffer and return start of pathname */
char * d_path(struct dentry *dentry, struct vfsmount *vfsmnt,
                                char *buf, int buflen)
{
        char *res;
        struct vfsmount *rootmnt;
        struct dentry *root;

        read_lock(&current->fs->lock);
        rootmnt = mntget(current->fs->rootmnt);
        root = dget(current->fs->root);
        read_unlock(&current->fs->lock);
        spin_lock(&dcache_lock);
        res = __d_path(dentry, vfsmnt, root, rootmnt, buf, buflen);
        spin_unlock(&dcache_lock);
        dput(root);
        mntput(rootmnt);
        return res;
}

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

        if (!page)
                return -ENOMEM;

        read_lock(&current->fs->lock);
        pwdmnt = mntget(current->fs->pwdmnt);
        pwd = dget(current->fs->pwd);
        rootmnt = mntget(current->fs->rootmnt);
        root = dget(current->fs->root);
        read_unlock(&current->fs->lock);

        error = -ENOENT;
        /* Has the current directory has been unlinked? */
        spin_lock(&dcache_lock);
        if (pwd->d_parent == pwd || !d_unhashed(pwd)) {
                unsigned long len;
                char * cwd;

                cwd = __d_path(pwd, pwdmnt, root, rootmnt, page, PAGE_SIZE);
                spin_unlock(&dcache_lock);

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

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

out:
        dput(pwd);
        mntput(pwdmnt);
        dput(root);
        mntput(rootmnt);
        free_page((unsigned long) page);
        return error;
}

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

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

        result = 0;
        /* need rcu_readlock to protect against the d_parent trashing due to
         * d_move
         */
        rcu_read_lock();
        do {
                /* for restarting inner loop in case of seq retry */
                new_dentry = saved;
                seq = read_seqbegin(&rename_lock);
                for (;;) {
                        if (new_dentry != old_dentry) {
                                struct dentry * parent = new_dentry->d_parent;
                                if (parent == new_dentry)
                                        break;
                                new_dentry = parent;
                                continue;
                        }
                        result = 1;
                        break;
                }
        } while (read_seqretry(&rename_lock, seq));
        rcu_read_unlock();

        return result;
}

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

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

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

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

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

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

static void __init dcache_init_early(void)
{
        int loop;

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

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

static void __init dcache_init(unsigned long mempages)
{
        /* 
         * 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_create("dentry_cache",
                                         sizeof(struct dentry),
                                         0,
                                         SLAB_RECLAIM_ACCOUNT|SLAB_PANIC,
                                         NULL, NULL);
        
        set_shrinker(DEFAULT_SEEKS, shrink_dcache_memory);
}

/* SLAB cache for __getname() consumers */
kmem_cache_t *names_cachep;

/* SLAB cache for file structures */
kmem_cache_t *filp_cachep;

EXPORT_SYMBOL(d_genocide);

extern void bdev_cache_init(void);
extern void chrdev_init(void);

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

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

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

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

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

        filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
                        SLAB_HWCACHE_ALIGN|SLAB_PANIC, filp_ctor, filp_dtor);

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

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