/*
   * fs/dcache.c
   *
   * Complete reimplementation
   * (C) 1997 Thomas Schoebel-Theuer,
   * with heavy changes by Linus Torvalds
   */
  
  /*
   * Notes on the allocation strategy:
   *
   * The dcache is a master of the icache - whenever a dcache entry
   * exists, the inode will always exist. "iput()" is done either when
   * the dcache entry is deleted or garbage collected.
   */

  #include <linux/syscalls.h>
  #include <linux/string.h>
  #include <linux/mm.h>
  #include <linux/fs.h>

  #include <linux/fsnotify.h>

  #include <linux/slab.h>
  #include <linux/init.h>

  #include <linux/hash.h>
  #include <linux/cache.h>
  #include <linux/module.h>
  #include <linux/mount.h>
  #include <linux/file.h>
  #include <asm/uaccess.h>
  #include <linux/security.h>
  #include <linux/seqlock.h>
  #include <linux/swap.h>
  #include <linux/bootmem.h>

  #include <linux/fs_struct.h>

  #include <linux/hardirq.h>

  #include <linux/bit_spinlock.h>
  #include <linux/rculist_bl.h>

  #include <linux/prefetch.h>

  #include "internal.h"

  /*
   * Usage:

   * dcache->d_inode->i_lock protects:
   *   - i_dentry, d_alias, d_inode of aliases

   * dcache_hash_bucket lock protects:
   *   - the dcache hash table
   * s_anon bl list spinlock protects:
   *   - the s_anon list (see __d_drop)

   * dcache_lru_lock protects:
   *   - the dcache lru lists and counters
   * d_lock protects:
   *   - d_flags
   *   - d_name
   *   - d_lru

   *   - d_count

   *   - d_unhashed()

   *   - d_parent and d_subdirs
   *   - childrens' d_child and d_parent

   *   - d_alias, d_inode

   *
   * Ordering:

   * dentry->d_inode->i_lock

   *   dentry->d_lock
   *     dcache_lru_lock

   *     dcache_hash_bucket lock
   *     s_anon lock

   *

   * If there is an ancestor relationship:
   * dentry->d_parent->...->d_parent->d_lock
   *   ...
   *     dentry->d_parent->d_lock
   *       dentry->d_lock
   *
   * If no ancestor relationship:

   * if (dentry1 < dentry2)
   *   dentry1->d_lock
   *     dentry2->d_lock
   */

  int sysctl_vfs_cache_pressure __read_mostly = 100;

  EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);

  static __cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lru_lock);

  __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);

  EXPORT_SYMBOL(rename_lock);

  static struct kmem_cache *dentry_cache __read_mostly;

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

  static unsigned int d_hash_mask __read_mostly;
  static unsigned int d_hash_shift __read_mostly;

  static struct hlist_bl_head *dentry_hashtable __read_mostly;

  static inline struct hlist_bl_head *d_hash(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);
  }

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

  static DEFINE_PER_CPU(unsigned int, nr_dentry);

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

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

  int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
  		   size_t *lenp, loff_t *ppos)
  {

  	dentry_stat.nr_dentry = get_nr_dentry();

  	return proc_dointvec(table, write, buffer, lenp, ppos);
  }
  #endif

  static void __d_free(struct rcu_head *head)

  {

  	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);

  	WARN_ON(!list_empty(&dentry->d_alias));

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

   * no locks, please.

   */
  static void d_free(struct dentry *dentry)
  {

  	BUG_ON(dentry->d_count);

  	this_cpu_dec(nr_dentry);

  	if (dentry->d_op && dentry->d_op->d_release)
  		dentry->d_op->d_release(dentry);

  	/* if dentry was never visible to RCU, immediate free is OK */
  	if (!(dentry->d_flags & DCACHE_RCUACCESS))

  		__d_free(&dentry->d_u.d_rcu);

  	else

  		call_rcu(&dentry->d_u.d_rcu, __d_free);

  }

  /**
   * dentry_rcuwalk_barrier - invalidate in-progress rcu-walk lookups

   * @dentry: the target dentry

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

  /*
   * Release the dentry's inode, using the filesystem

   * d_iput() operation if defined. Dentry has no refcount
   * and is unhashed.

   */

  static void dentry_iput(struct dentry * dentry)

  	__releases(dentry->d_lock)

  	__releases(dentry->d_inode->i_lock)

  {
  	struct inode *inode = dentry->d_inode;
  	if (inode) {
  		dentry->d_inode = NULL;
  		list_del_init(&dentry->d_alias);
  		spin_unlock(&dentry->d_lock);

  		spin_unlock(&inode->i_lock);

  		if (!inode->i_nlink)
  			fsnotify_inoderemove(inode);

  		if (dentry->d_op && dentry->d_op->d_iput)
  			dentry->d_op->d_iput(dentry, inode);
  		else
  			iput(inode);
  	} else {
  		spin_unlock(&dentry->d_lock);

  	}
  }

  /*

   * Release the dentry's inode, using the filesystem
   * d_iput() operation if defined. dentry remains in-use.
   */
  static void dentry_unlink_inode(struct dentry * dentry)
  	__releases(dentry->d_lock)

  	__releases(dentry->d_inode->i_lock)

  {
  	struct inode *inode = dentry->d_inode;
  	dentry->d_inode = NULL;
  	list_del_init(&dentry->d_alias);
  	dentry_rcuwalk_barrier(dentry);
  	spin_unlock(&dentry->d_lock);

  	spin_unlock(&inode->i_lock);

  	if (!inode->i_nlink)
  		fsnotify_inoderemove(inode);
  	if (dentry->d_op && dentry->d_op->d_iput)
  		dentry->d_op->d_iput(dentry, inode);
  	else
  		iput(inode);
  }
  
  /*

   * dentry_lru_(add|del|move_tail) must be called with d_lock held.

   */
  static void dentry_lru_add(struct dentry *dentry)
  {

  	if (list_empty(&dentry->d_lru)) {

  		spin_lock(&dcache_lru_lock);

  		list_add(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
  		dentry->d_sb->s_nr_dentry_unused++;

  		dentry_stat.nr_unused++;

  		spin_unlock(&dcache_lru_lock);

  	}

  }

  static void __dentry_lru_del(struct dentry *dentry)
  {
  	list_del_init(&dentry->d_lru);
  	dentry->d_sb->s_nr_dentry_unused--;
  	dentry_stat.nr_unused--;
  }

  static void dentry_lru_del(struct dentry *dentry)
  {
  	if (!list_empty(&dentry->d_lru)) {

  		spin_lock(&dcache_lru_lock);
  		__dentry_lru_del(dentry);
  		spin_unlock(&dcache_lru_lock);

  	}
  }

  static void dentry_lru_move_tail(struct dentry *dentry)

  {

  	spin_lock(&dcache_lru_lock);

  	if (list_empty(&dentry->d_lru)) {
  		list_add_tail(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
  		dentry->d_sb->s_nr_dentry_unused++;

  		dentry_stat.nr_unused++;

  	} else {
  		list_move_tail(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);

  	}

  	spin_unlock(&dcache_lru_lock);

  }

  /**
   * d_kill - kill dentry and return parent
   * @dentry: dentry to kill

   * @parent: parent dentry

   *

   * The dentry must already be unhashed and removed from the LRU.

   *
   * If this is the root of the dentry tree, return NULL.

   *

   * dentry->d_lock and parent->d_lock must be held by caller, and are dropped by
   * d_kill.

   */

  static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent)

  	__releases(dentry->d_lock)

  	__releases(parent->d_lock)

  	__releases(dentry->d_inode->i_lock)

  {

  	list_del(&dentry->d_u.d_child);

  	/*
  	 * Inform try_to_ascend() that we are no longer attached to the
  	 * dentry tree
  	 */
  	dentry->d_flags |= DCACHE_DISCONNECTED;

  	if (parent)
  		spin_unlock(&parent->d_lock);

  	dentry_iput(dentry);

  	/*
  	 * dentry_iput drops the locks, at which point nobody (except
  	 * transient RCU lookups) can reach this dentry.
  	 */

  	d_free(dentry);

  	return parent;

  }

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

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

  	if (!d_unhashed(dentry)) {

  		__d_shrink(dentry);

  		dentry_rcuwalk_barrier(dentry);

  	}
  }
  EXPORT_SYMBOL(__d_drop);
  
  void d_drop(struct dentry *dentry)
  {

  	spin_lock(&dentry->d_lock);
  	__d_drop(dentry);
  	spin_unlock(&dentry->d_lock);

  }
  EXPORT_SYMBOL(d_drop);

  /*

   * d_clear_need_lookup - drop a dentry from cache and clear the need lookup flag
   * @dentry: dentry to drop
   *
   * This is called when we do a lookup on a placeholder dentry that needed to be
   * looked up.  The dentry should have been hashed in order for it to be found by
   * the lookup code, but now needs to be unhashed while we do the actual lookup
   * and clear the DCACHE_NEED_LOOKUP flag.
   */
  void d_clear_need_lookup(struct dentry *dentry)
  {
  	spin_lock(&dentry->d_lock);
  	__d_drop(dentry);
  	dentry->d_flags &= ~DCACHE_NEED_LOOKUP;
  	spin_unlock(&dentry->d_lock);
  }
  EXPORT_SYMBOL(d_clear_need_lookup);
  
  /*

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

  	struct inode *inode;

  	struct dentry *parent;

  	inode = dentry->d_inode;
  	if (inode && !spin_trylock(&inode->i_lock)) {

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

  		if (inode)
  			spin_unlock(&inode->i_lock);

  		goto relock;
  	}

  	if (ref)
  		dentry->d_count--;
  	/* if dentry was on the d_lru list delete it from there */
  	dentry_lru_del(dentry);
  	/* if it was on the hash then remove it */
  	__d_drop(dentry);
  	return d_kill(dentry, parent);
  }

  /* 
   * This is dput
   *
   * This is complicated by the fact that we do not want to put
   * dentries that are no longer on any hash chain on the unused
   * list: we'd much rather just get rid of them immediately.
   *
   * However, that implies that we have to traverse the dentry
   * tree upwards to the parents which might _also_ now be
   * scheduled for deletion (it may have been only waiting for
   * its last child to go away).
   *
   * This tail recursion is done by hand as we don't want to depend
   * on the compiler to always get this right (gcc generally doesn't).
   * Real recursion would eat up our stack space.
   */
  
  /*
   * dput - release a dentry
   * @dentry: dentry to release 
   *
   * Release a dentry. This will drop the usage count and if appropriate
   * call the dentry unlink method as well as removing it from the queues and
   * releasing its resources. If the parent dentries were scheduled for release
   * they too may now get deleted.

   */

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

  	if (dentry->d_count == 1)

  		might_sleep();

  	spin_lock(&dentry->d_lock);

  	BUG_ON(!dentry->d_count);
  	if (dentry->d_count > 1) {
  		dentry->d_count--;

  		spin_unlock(&dentry->d_lock);

  		return;
  	}

  	if (dentry->d_flags & DCACHE_OP_DELETE) {

  		if (dentry->d_op->d_delete(dentry))

  			goto kill_it;

  	}

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

  	/*
  	 * If this dentry needs lookup, don't set the referenced flag so that it
  	 * is more likely to be cleaned up by the dcache shrinker in case of
  	 * memory pressure.
  	 */
  	if (!d_need_lookup(dentry))
  		dentry->d_flags |= DCACHE_REFERENCED;

  	dentry_lru_add(dentry);

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

  	return;

  kill_it:

  	dentry = dentry_kill(dentry, 1);

  	if (dentry)
  		goto repeat;

  }

  EXPORT_SYMBOL(dput);

  
  /**
   * d_invalidate - invalidate a dentry
   * @dentry: dentry to invalidate
   *
   * Try to invalidate the dentry if it turns out to be
   * possible. If there are other dentries that can be
   * reached through this one we can't delete it and we
   * return -EBUSY. On success we return 0.
   *
   * no dcache lock.
   */
   
  int d_invalidate(struct dentry * dentry)
  {
  	/*
  	 * If it's already been dropped, return OK.
  	 */

  	spin_lock(&dentry->d_lock);

  	if (d_unhashed(dentry)) {

  		spin_unlock(&dentry->d_lock);

  		return 0;
  	}
  	/*
  	 * Check whether to do a partial shrink_dcache
  	 * to get rid of unused child entries.
  	 */
  	if (!list_empty(&dentry->d_subdirs)) {

  		spin_unlock(&dentry->d_lock);

  		shrink_dcache_parent(dentry);

  		spin_lock(&dentry->d_lock);

  	}
  
  	/*
  	 * Somebody else still using it?
  	 *
  	 * If it's a directory, we can't drop it
  	 * for fear of somebody re-populating it
  	 * with children (even though dropping it
  	 * would make it unreachable from the root,
  	 * we might still populate it if it was a
  	 * working directory or similar).
  	 */

  	if (dentry->d_count > 1) {

  		if (dentry->d_inode && S_ISDIR(dentry->d_inode->i_mode)) {
  			spin_unlock(&dentry->d_lock);

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

  	return 0;
  }

  EXPORT_SYMBOL(d_invalidate);

  /* This must be called with d_lock held */

  static inline void __dget_dlock(struct dentry *dentry)

  {

  	dentry->d_count++;

  }

  static inline void __dget(struct dentry *dentry)

  {

  	spin_lock(&dentry->d_lock);

  	__dget_dlock(dentry);

  	spin_unlock(&dentry->d_lock);

  }

  struct dentry *dget_parent(struct dentry *dentry)
  {
  	struct dentry *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_count);
  	ret->d_count++;
  	spin_unlock(&ret->d_lock);

  	return ret;
  }
  EXPORT_SYMBOL(dget_parent);

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

   * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer

   * any other hashed alias over that one unless @want_discon is set,

   * in which case only return an IS_ROOT, DCACHE_DISCONNECTED alias.

   */

  static struct dentry *__d_find_alias(struct inode *inode, int want_discon)

  {

  	struct dentry *alias, *discon_alias;

  again:
  	discon_alias = NULL;
  	list_for_each_entry(alias, &inode->i_dentry, d_alias) {
  		spin_lock(&alias->d_lock);

   		if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {

  			if (IS_ROOT(alias) &&

  			    (alias->d_flags & DCACHE_DISCONNECTED)) {

  				discon_alias = alias;

  			} else if (!want_discon) {

  				__dget_dlock(alias);

  				spin_unlock(&alias->d_lock);
  				return alias;
  			}
  		}
  		spin_unlock(&alias->d_lock);
  	}
  	if (discon_alias) {
  		alias = discon_alias;
  		spin_lock(&alias->d_lock);
  		if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
  			if (IS_ROOT(alias) &&
  			    (alias->d_flags & DCACHE_DISCONNECTED)) {

  				__dget_dlock(alias);

  				spin_unlock(&alias->d_lock);

  				return alias;
  			}
  		}

  		spin_unlock(&alias->d_lock);
  		goto again;

  	}

  	return NULL;

  }

  struct dentry *d_find_alias(struct inode *inode)

  {

  	struct dentry *de = NULL;
  
  	if (!list_empty(&inode->i_dentry)) {

  		spin_lock(&inode->i_lock);

  		de = __d_find_alias(inode, 0);

  		spin_unlock(&inode->i_lock);

  	}

  	return de;
  }

  EXPORT_SYMBOL(d_find_alias);

  
  /*
   *	Try to kill dentries associated with this inode.
   * WARNING: you must own a reference to inode.
   */
  void d_prune_aliases(struct inode *inode)
  {

  	struct dentry *dentry;

  restart:

  	spin_lock(&inode->i_lock);

  	list_for_each_entry(dentry, &inode->i_dentry, d_alias) {

  		spin_lock(&dentry->d_lock);

  		if (!dentry->d_count) {

  			__dget_dlock(dentry);

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

  			spin_unlock(&inode->i_lock);

  			dput(dentry);
  			goto restart;
  		}
  		spin_unlock(&dentry->d_lock);
  	}

  	spin_unlock(&inode->i_lock);

  }

  EXPORT_SYMBOL(d_prune_aliases);

  
  /*

   * Try to throw away a dentry - free the inode, dput the parent.
   * Requires dentry->d_lock is held, and dentry->d_count == 0.
   * Releases dentry->d_lock.

   *

   * This may fail if locks cannot be acquired no problem, just try again.

   */

  static void try_prune_one_dentry(struct dentry *dentry)

  	__releases(dentry->d_lock)

  {

  	struct dentry *parent;

  	parent = dentry_kill(dentry, 0);

  	/*

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

  	 */

  	if (!parent)
  		return;
  	if (parent == dentry)
  		return;
  
  	/* Prune ancestors. */
  	dentry = parent;

  	while (dentry) {

  		spin_lock(&dentry->d_lock);

  		if (dentry->d_count > 1) {
  			dentry->d_count--;
  			spin_unlock(&dentry->d_lock);
  			return;
  		}

  		dentry = dentry_kill(dentry, 1);

  	}

  }

  static void shrink_dentry_list(struct list_head *list)

  {

  	struct dentry *dentry;

  	rcu_read_lock();
  	for (;;) {

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

  			continue;
  		}

  		/*
  		 * We found an inuse dentry which was not removed from

  		 * the LRU because of laziness during lookup.  Do not free
  		 * it - just keep it off the LRU list.

  		 */

  		if (dentry->d_count) {

  			dentry_lru_del(dentry);

  			spin_unlock(&dentry->d_lock);

  			continue;
  		}

  		rcu_read_unlock();

  
  		try_prune_one_dentry(dentry);

  		rcu_read_lock();

  	}

  	rcu_read_unlock();

  }
  
  /**
   * __shrink_dcache_sb - shrink the dentry LRU on a given superblock
   * @sb:		superblock to shrink dentry LRU.
   * @count:	number of entries to prune
   * @flags:	flags to control the dentry processing
   *
   * If flags contains DCACHE_REFERENCED reference dentries will not be pruned.
   */

  static void __shrink_dcache_sb(struct super_block *sb, int count, int flags)

  {

  	struct dentry *dentry;
  	LIST_HEAD(referenced);
  	LIST_HEAD(tmp);

  relock:
  	spin_lock(&dcache_lru_lock);

  	while (!list_empty(&sb->s_dentry_lru)) {
  		dentry = list_entry(sb->s_dentry_lru.prev,
  				struct dentry, d_lru);
  		BUG_ON(dentry->d_sb != sb);

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

  		/*
  		 * If we are honouring the DCACHE_REFERENCED flag and the
  		 * dentry has this flag set, don't free it.  Clear the flag
  		 * and put it back on the LRU.
  		 */

  		if (flags & DCACHE_REFERENCED &&
  				dentry->d_flags & DCACHE_REFERENCED) {
  			dentry->d_flags &= ~DCACHE_REFERENCED;
  			list_move(&dentry->d_lru, &referenced);

  			spin_unlock(&dentry->d_lock);

  		} else {
  			list_move_tail(&dentry->d_lru, &tmp);
  			spin_unlock(&dentry->d_lock);

  			if (!--count)

  				break;

  		}

  		cond_resched_lock(&dcache_lru_lock);

  	}

  	if (!list_empty(&referenced))
  		list_splice(&referenced, &sb->s_dentry_lru);

  	spin_unlock(&dcache_lru_lock);

  
  	shrink_dentry_list(&tmp);

  }
  
  /**

   * prune_dcache_sb - shrink the dcache

   * @sb: superblock

   * @nr_to_scan: number of entries to try to free

   *

   * Attempt to shrink the superblock dcache LRU by @nr_to_scan entries. This is
   * done when we need more memory an called from the superblock shrinker
   * function.

   *

   * This function may fail to free any resources if all the dentries are in
   * use.

   */

  void prune_dcache_sb(struct super_block *sb, int nr_to_scan)

  {

  	__shrink_dcache_sb(sb, nr_to_scan, DCACHE_REFERENCED);

  }

  /**
   * shrink_dcache_sb - shrink dcache for a superblock
   * @sb: superblock
   *

   * Shrink the dcache for the specified super block. This is used to free
   * the dcache before unmounting a file system.

   */

  void shrink_dcache_sb(struct super_block *sb)

  {

  	LIST_HEAD(tmp);

  	spin_lock(&dcache_lru_lock);

  	while (!list_empty(&sb->s_dentry_lru)) {
  		list_splice_init(&sb->s_dentry_lru, &tmp);

  		spin_unlock(&dcache_lru_lock);

  		shrink_dentry_list(&tmp);

  		spin_lock(&dcache_lru_lock);

  	}

  	spin_unlock(&dcache_lru_lock);

  }

  EXPORT_SYMBOL(shrink_dcache_sb);

  
  /*

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

  	for (;;) {
  		/* descend to the first leaf in the current subtree */

  		while (!list_empty(&dentry->d_subdirs))

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

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

  			/* detach from the system */
  			dentry_lru_del(dentry);
  			__d_shrink(dentry);

  			if (dentry->d_count != 0) {

  				printk(KERN_ERR
  				       "BUG: Dentry %p{i=%lx,n=%s}"
  				       " still in use (%d)"
  				       " [unmount of %s %s]
  ",
  				       dentry,
  				       dentry->d_inode ?
  				       dentry->d_inode->i_ino : 0UL,
  				       dentry->d_name.name,

  				       dentry->d_count,

  				       dentry->d_sb->s_type->name,
  				       dentry->d_sb->s_id);
  				BUG();
  			}

  			if (IS_ROOT(dentry)) {

  				parent = NULL;

  				list_del(&dentry->d_u.d_child);
  			} else {

  				parent = dentry->d_parent;

  				parent->d_count--;

  				list_del(&dentry->d_u.d_child);

  			}

  			inode = dentry->d_inode;
  			if (inode) {
  				dentry->d_inode = NULL;
  				list_del_init(&dentry->d_alias);
  				if (dentry->d_op && dentry->d_op->d_iput)
  					dentry->d_op->d_iput(dentry, inode);
  				else
  					iput(inode);
  			}
  
  			d_free(dentry);
  
  			/* finished when we fall off the top of the tree,
  			 * otherwise we ascend to the parent and move to the
  			 * next sibling if there is one */
  			if (!parent)

  				return;

  			dentry = parent;

  		} while (list_empty(&dentry->d_subdirs));
  
  		dentry = list_entry(dentry->d_subdirs.next,
  				    struct dentry, d_u.d_child);
  	}
  }
  
  /*
   * destroy the dentries attached to a superblock on unmounting

   * - we don't need to use dentry->d_lock because:

   *   - the superblock is detached from all mountings and open files, so the
   *     dentry trees will not be rearranged by the VFS
   *   - s_umount is write-locked, so the memory pressure shrinker will ignore
   *     any dentries belonging to this superblock that it comes across
   *   - the filesystem itself is no longer permitted to rearrange the dentries
   *     in this superblock
   */
  void shrink_dcache_for_umount(struct super_block *sb)
  {
  	struct dentry *dentry;
  
  	if (down_read_trylock(&sb->s_umount))
  		BUG();
  
  	dentry = sb->s_root;
  	sb->s_root = NULL;

  	dentry->d_count--;

  	shrink_dcache_for_umount_subtree(dentry);

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

  		shrink_dcache_for_umount_subtree(dentry);
  	}
  }
  
  /*

   * This tries to ascend one level of parenthood, but
   * we can race with renaming, so we need to re-check
   * the parenthood after dropping the lock and check
   * that the sequence number still matches.
   */
  static struct dentry *try_to_ascend(struct dentry *old, int locked, unsigned seq)
  {
  	struct dentry *new = old->d_parent;
  
  	rcu_read_lock();
  	spin_unlock(&old->d_lock);
  	spin_lock(&new->d_lock);
  
  	/*
  	 * might go back up the wrong parent if we have had a rename
  	 * or deletion
  	 */
  	if (new != old->d_parent ||

  		 (old->d_flags & DCACHE_DISCONNECTED) ||

  		 (!locked && read_seqretry(&rename_lock, seq))) {
  		spin_unlock(&new->d_lock);
  		new = NULL;
  	}
  	rcu_read_unlock();
  	return new;
  }
  
  
  /*

   * 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;

  	struct list_head *next;

  	unsigned seq;

  	int locked = 0;

  	seq = read_seqbegin(&rename_lock);

  again:
  	this_parent = parent;

  	if (d_mountpoint(parent))
  		goto positive;

  	spin_lock(&this_parent->d_lock);

  repeat:
  	next = this_parent->d_subdirs.next;
  resume:
  	while (next != &this_parent->d_subdirs) {
  		struct list_head *tmp = next;

  		struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);

  		next = tmp->next;

  
  		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);

  		/* Have we found a mount point ? */

  		if (d_mountpoint(dentry)) {
  			spin_unlock(&dentry->d_lock);
  			spin_unlock(&this_parent->d_lock);

  			goto positive;

  		}

  		if (!list_empty(&dentry->d_subdirs)) {

  			spin_unlock(&this_parent->d_lock);
  			spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);

  			this_parent = dentry;

  			spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);

  			goto repeat;
  		}

  		spin_unlock(&dentry->d_lock);

  	}
  	/*
  	 * All done at this level ... ascend and resume the search.
  	 */
  	if (this_parent != parent) {

  		struct dentry *child = this_parent;
  		this_parent = try_to_ascend(this_parent, locked, seq);
  		if (!this_parent)

  			goto rename_retry;

  		next = child->d_u.d_child.next;

  		goto resume;
  	}

  	spin_unlock(&this_parent->d_lock);

  	if (!locked && read_seqretry(&rename_lock, seq))

  		goto rename_retry;

  	if (locked)
  		write_sequnlock(&rename_lock);

  	return 0; /* No mount points found in tree */
  positive:

  	if (!locked && read_seqretry(&rename_lock, seq))

  		goto rename_retry;

  	if (locked)
  		write_sequnlock(&rename_lock);

  	return 1;

  
  rename_retry:
  	locked = 1;
  	write_seqlock(&rename_lock);
  	goto again;

  }

  EXPORT_SYMBOL(have_submounts);

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

  	struct dentry *this_parent;

  	struct list_head *next;

  	unsigned seq;

  	int found = 0;

  	int locked = 0;

  	seq = read_seqbegin(&rename_lock);

  again:
  	this_parent = parent;

  	spin_lock(&this_parent->d_lock);

  repeat:
  	next = this_parent->d_subdirs.next;
  resume:
  	while (next != &this_parent->d_subdirs) {
  		struct list_head *tmp = next;

  		struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);

  		next = tmp->next;

  		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);

  		/* 
  		 * move only zero ref count dentries to the end 
  		 * of the unused list for prune_dcache
  		 */

  		if (!dentry->d_count) {

  			dentry_lru_move_tail(dentry);

  			found++;

  		} else {
  			dentry_lru_del(dentry);

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

  		if (found && need_resched()) {
  			spin_unlock(&dentry->d_lock);

  			goto out;

  		}

  
  		/*
  		 * Descend a level if the d_subdirs list is non-empty.
  		 */
  		if (!list_empty(&dentry->d_subdirs)) {

  			spin_unlock(&this_parent->d_lock);
  			spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);

  			this_parent = dentry;

  			spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);

  			goto repeat;
  		}

  
  		spin_unlock(&dentry->d_lock);

  	}
  	/*
  	 * All done at this level ... ascend and resume the search.
  	 */
  	if (this_parent != parent) {

  		struct dentry *child = this_parent;
  		this_parent = try_to_ascend(this_parent, locked, seq);
  		if (!this_parent)

  			goto rename_retry;

  		next = child->d_u.d_child.next;

  		goto resume;
  	}
  out:

  	spin_unlock(&this_parent->d_lock);

  	if (!locked && read_seqretry(&rename_lock, seq))

  		goto rename_retry;

  	if (locked)
  		write_sequnlock(&rename_lock);

  	return found;

  
  rename_retry:
  	if (found)
  		return found;
  	locked = 1;
  	write_seqlock(&rename_lock);
  	goto again;

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

  	struct super_block *sb = parent->d_sb;

  	int found;
  
  	while ((found = select_parent(parent)) != 0)

  		__shrink_dcache_sb(sb, found, 0);

  }

  EXPORT_SYMBOL(shrink_dcache_parent);

  /**

   * __d_alloc	-	allocate a dcache entry
   * @sb: filesystem it will belong to

   * @name: qstr of the name
   *
   * Allocates a dentry. It returns %NULL if there is insufficient memory
   * available. On a success the dentry is returned. The name passed in is
   * copied and the copy passed in may be reused after this call.
   */
   

  struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)

  {
  	struct dentry *dentry;
  	char *dname;

  	dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);

  	if (!dentry)
  		return NULL;
  
  	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;

  	dentry->d_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_LIST_HEAD(&dentry->d_alias);

  	INIT_LIST_HEAD(&dentry->d_u.d_child);

  	d_set_d_op(dentry, dentry->d_sb->s_d_op);

  	this_cpu_inc(nr_dentry);

  	return dentry;
  }

  
  /**
   * d_alloc	-	allocate a dcache entry
   * @parent: parent of entry to allocate
   * @name: qstr of the name
   *
   * Allocates a dentry. It returns %NULL if there is insufficient memory
   * available. On a success the dentry is returned. The name passed in is
   * copied and the copy passed in may be reused after this call.
   */
  struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
  {
  	struct dentry *dentry = __d_alloc(parent->d_sb, name);
  	if (!dentry)
  		return NULL;
  
  	spin_lock(&parent->d_lock);
  	/*
  	 * don't need child lock because it is not subject
  	 * to concurrency here
  	 */
  	__dget_dlock(parent);
  	dentry->d_parent = parent;
  	list_add(&dentry->d_u.d_child, &parent->d_subdirs);
  	spin_unlock(&parent->d_lock);
  
  	return dentry;
  }

  EXPORT_SYMBOL(d_alloc);

  struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
  {

  	struct dentry *dentry = __d_alloc(sb, name);
  	if (dentry)

  		dentry->d_flags |= DCACHE_DISCONNECTED;

  	return dentry;
  }
  EXPORT_SYMBOL(d_alloc_pseudo);

  struct dentry *d_alloc_name(struct dentry *parent, const char *name)
  {
  	struct qstr q;
  
  	q.name = name;
  	q.len = strlen(name);
  	q.hash = full_name_hash(q.name, q.len);
  	return d_alloc(parent, &q);
  }

  EXPORT_SYMBOL(d_alloc_name);

  void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
  {

  	WARN_ON_ONCE(dentry->d_op);
  	WARN_ON_ONCE(dentry->d_flags & (DCACHE_OP_HASH	|

  				DCACHE_OP_COMPARE	|
  				DCACHE_OP_REVALIDATE	|
  				DCACHE_OP_DELETE ));
  	dentry->d_op = op;
  	if (!op)
  		return;
  	if (op->d_hash)
  		dentry->d_flags |= DCACHE_OP_HASH;
  	if (op->d_compare)
  		dentry->d_flags |= DCACHE_OP_COMPARE;
  	if (op->d_revalidate)
  		dentry->d_flags |= DCACHE_OP_REVALIDATE;
  	if (op->d_delete)
  		dentry->d_flags |= DCACHE_OP_DELETE;
  
  }
  EXPORT_SYMBOL(d_set_d_op);

  static void __d_instantiate(struct dentry *dentry, struct inode *inode)
  {

  	spin_lock(&dentry->d_lock);

  	if (inode) {
  		if (unlikely(IS_AUTOMOUNT(inode)))
  			dentry->d_flags |= DCACHE_NEED_AUTOMOUNT;

  		list_add(&dentry->d_alias, &inode->i_dentry);

  	}

  	dentry->d_inode = inode;

  	dentry_rcuwalk_barrier(dentry);

  	spin_unlock(&dentry->d_lock);

  	fsnotify_d_instantiate(dentry, inode);
  }

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

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

  	if (inode)
  		spin_lock(&inode->i_lock);

  	__d_instantiate(entry, inode);

  	if (inode)
  		spin_unlock(&inode->i_lock);

  	security_d_instantiate(entry, inode);
  }

  EXPORT_SYMBOL(d_instantiate);

  
  /**
   * d_instantiate_unique - instantiate a non-aliased dentry
   * @entry: dentry to instantiate
   * @inode: inode to attach to this dentry
   *
   * Fill in inode information in the entry. On success, it returns NULL.
   * If an unhashed alias of "entry" already exists, then we return the

   * aliased dentry instead and drop one reference to inode.

   *
   * Note that in order to avoid conflicts with rename() etc, the caller
   * had better be holding the parent directory semaphore.

   *
   * This also assumes that the inode count has been incremented
   * (or otherwise set) by the caller to indicate that it is now
   * in use by the dcache.

   */

  static struct dentry *__d_instantiate_unique(struct dentry *entry,
  					     struct inode *inode)

  {
  	struct dentry *alias;
  	int len = entry->d_name.len;
  	const char *name = entry->d_name.name;
  	unsigned int hash = entry->d_name.hash;

  	if (!inode) {

  		__d_instantiate(entry, NULL);

  		return NULL;
  	}

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

  		/*
  		 * 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 (qstr->hash != hash)
  			continue;
  		if (alias->d_parent != entry->d_parent)
  			continue;

  		if (dentry_cmp(qstr->name, qstr->len, name, len))

  			continue;

  		__dget(alias);

  		return alias;
  	}

  	__d_instantiate(entry, inode);

  	return NULL;
  }

  
  struct dentry *d_instantiate_unique(struct dentry *entry, struct inode *inode)
  {
  	struct dentry *result;
  
  	BUG_ON(!list_empty(&entry->d_alias));

  	if (inode)
  		spin_lock(&inode->i_lock);

  	result = __d_instantiate_unique(entry, inode);

  	if (inode)
  		spin_unlock(&inode->i_lock);

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

  EXPORT_SYMBOL(d_instantiate_unique);
  
  /**
   * 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(root_inode->i_sb, &name);
  		if (res)

  			d_instantiate(res, root_inode);

  	}
  	return res;
  }

  EXPORT_SYMBOL(d_alloc_root);

  static struct dentry * __d_find_any_alias(struct inode *inode)
  {
  	struct dentry *alias;
  
  	if (list_empty(&inode->i_dentry))
  		return NULL;
  	alias = list_first_entry(&inode->i_dentry, struct dentry, d_alias);
  	__dget(alias);
  	return alias;
  }
  
  static 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;
  }

  /**

   * d_obtain_alias - find or allocate a dentry for a given inode
   * @inode: inode to allocate the dentry for
   *
   * Obtain a dentry for an inode resulting from NFS filehandle conversion or
   * similar open by handle operations.  The returned dentry may be anonymous,
   * or may have a full name (if the inode was already in the cache).
   *
   * When called on a directory inode, we must ensure that the inode only ever
   * has one dentry.  If a dentry is found, that is returned instead of
   * allocating a new one.
   *
   * On successful return, the reference to the inode has been transferred

   * to the dentry.  In case of an error the reference on the inode is released.
   * To make it easier to use in export operations a %NULL or IS_ERR inode may
   * be passed in and will be the error will be propagate to the return value,
   * with a %NULL @inode replaced by ERR_PTR(-ESTALE).

   */
  struct dentry *d_obtain_alias(struct inode *inode)
  {

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

  
  	if (!inode)

  		return ERR_PTR(-ESTALE);

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

  	res = d_find_any_alias(inode);

  	if (res)
  		goto out_iput;

  	tmp = __d_alloc(inode->i_sb, &anonstring);

  	if (!tmp) {
  		res = ERR_PTR(-ENOMEM);
  		goto out_iput;

  	}

  	spin_lock(&inode->i_lock);

  	res = __d_find_any_alias(inode);

  	if (res) {

  		spin_unlock(&inode->i_lock);

  		dput(tmp);
  		goto out_iput;
  	}
  
  	/* attach a disconnected dentry */
  	spin_lock(&tmp->d_lock);

  	tmp->d_inode = inode;
  	tmp->d_flags |= DCACHE_DISCONNECTED;

  	list_add(&tmp->d_alias, &inode->i_dentry);

  	hlist_bl_lock(&tmp->d_sb->s_anon);

  	hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon);

  	hlist_bl_unlock(&tmp->d_sb->s_anon);

  	spin_unlock(&tmp->d_lock);

  	spin_unlock(&inode->i_lock);

  	security_d_instantiate(tmp, inode);

  	return tmp;
  
   out_iput:

  	if (res && !IS_ERR(res))
  		security_d_instantiate(res, inode);

  	iput(inode);
  	return res;

  }

  EXPORT_SYMBOL(d_obtain_alias);

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

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

  	if (inode && S_ISDIR(inode->i_mode)) {

  		spin_lock(&inode->i_lock);

  		new = __d_find_alias(inode, 1);
  		if (new) {
  			BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));

  			spin_unlock(&inode->i_lock);

  			security_d_instantiate(new, inode);

  			d_move(new, dentry);
  			iput(inode);
  		} else {

  			/* already taking inode->i_lock, so d_add() by hand */

  			__d_instantiate(dentry, inode);

  			spin_unlock(&inode->i_lock);

  			security_d_instantiate(dentry, inode);
  			d_rehash(dentry);
  		}
  	} else
  		d_add(dentry, inode);
  	return new;
  }

  EXPORT_SYMBOL(d_splice_alias);

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

  struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,

  			struct qstr *name)
  {
  	int error;
  	struct dentry *found;
  	struct dentry *new;

  	/*
  	 * First check if a dentry matching the name already exists,
  	 * if not go ahead and create it now.
  	 */

  	found = d_hash_and_lookup(dentry->d_parent, name);

  	if (!found) {
  		new = d_alloc(dentry->d_parent, name);
  		if (!new) {
  			error = -ENOMEM;
  			goto err_out;
  		}

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

  
  	/*
  	 * If a matching dentry exists, and it's not negative use it.
  	 *
  	 * Decrement the reference count to balance the iget() done
  	 * earlier on.
  	 */

  	if (found->d_inode) {
  		if (unlikely(found->d_inode != inode)) {
  			/* This can't happen because bad inodes are unhashed. */
  			BUG_ON(!is_bad_inode(inode));
  			BUG_ON(!is_bad_inode(found->d_inode));
  		}

  		iput(inode);
  		return found;
  	}

  	/*

  	 * We are going to instantiate this dentry, unhash it and clear the
  	 * lookup flag so we can do that.

  	 */

  	if (unlikely(d_need_lookup(found)))
  		d_clear_need_lookup(found);

  	/*

  	 * Negative dentry: instantiate it unless the inode is a directory and

  	 * already has a dentry.

  	 */

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

  	}

  	return found;

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

  EXPORT_SYMBOL(d_add_ci);

  
  /**

   * __d_lookup_rcu - search for a dentry (racy, store-free)
   * @parent: parent dentry
   * @name: qstr of name we wish to find
   * @seq: returns d_seq value at the point where the dentry was found
   * @inode: returns dentry->d_inode when the inode was found valid.
   * 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.
   */
  struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
  				unsigned *seq, struct inode **inode)
  {
  	unsigned int len = name->len;
  	unsigned int hash = name->hash;
  	const unsigned char *str = name->name;

  	struct hlist_bl_head *b = d_hash(parent, hash);

  	struct hlist_bl_node *node;

  	struct dentry *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) {

  		struct inode *i;
  		const char *tname;
  		int tlen;
  
  		if (dentry->d_name.hash != hash)
  			continue;
  
  seqretry:
  		*seq = read_seqcount_begin(&dentry->d_seq);
  		if (dentry->d_parent != parent)
  			continue;
  		if (d_unhashed(dentry))
  			continue;
  		tlen = dentry->d_name.len;
  		tname = dentry->d_name.name;
  		i = dentry->d_inode;

  		prefetch(tname);

  		/*
  		 * This seqcount check is required to ensure name and
  		 * len are loaded atomically, so as not to walk off the
  		 * edge of memory when walking. If we could load this
  		 * atomically some other way, we could drop this check.
  		 */
  		if (read_seqcount_retry(&dentry->d_seq, *seq))
  			goto seqretry;

  		if (unlikely(parent->d_flags & DCACHE_OP_COMPARE)) {

  			if (parent->d_op->d_compare(parent, *inode,
  						dentry, i,
  						tlen, tname, name))
  				continue;
  		} else {

  			if (dentry_cmp(tname, tlen, str, len))

  				continue;
  		}
  		/*
  		 * No extra seqcount check is required after the name
  		 * compare. The caller must perform a seqcount check in
  		 * order to do anything useful with the returned dentry
  		 * anyway.
  		 */
  		*inode = i;
  		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(struct dentry *parent, 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(struct dentry *parent, struct qstr *name)

  {
  	unsigned int len = name->len;
  	unsigned int hash = name->hash;
  	const unsigned char *str = name->name;

  	struct hlist_bl_head *b = d_hash(parent, hash);

  	struct hlist_bl_node *node;

  	struct dentry *found = NULL;

  	struct dentry *dentry;

  	/*

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

  	 * The hash list is protected using RCU.
  	 *
  	 * Take d_lock when comparing a candidate dentry, to avoid races
  	 * with d_move().
  	 *
  	 * It is possible that concurrent renames can mess up our list
  	 * walk here and result in missing our dentry, resulting in the
  	 * false-negative result. d_lookup() protects against concurrent
  	 * renames using rename_lock seqlock.
  	 *

  	 * See Documentation/filesystems/path-lookup.txt for more details.

  	 */