/*
   * linux/fs/mbcache.c
   * (C) 2001-2002 Andreas Gruenbacher, <a.gruenbacher@computer.org>
   */
  
  /*
   * Filesystem Meta Information Block Cache (mbcache)
   *
   * The mbcache caches blocks of block devices that need to be located
   * by their device/block number, as well as by other criteria (such
   * as the block's contents).
   *
   * There can only be one cache entry in a cache per device and block number.
   * Additional indexes need not be unique in this sense. The number of
   * additional indexes (=other criteria) can be hardwired at compile time
   * or specified at cache create time.
   *
   * Each cache entry is of fixed size. An entry may be `valid' or `invalid'
   * in the cache. A valid entry is in the main hash tables of the cache,
   * and may also be in the lru list. An invalid entry is not in any hashes
   * or lists.
   *
   * A valid cache entry is only in the lru list if no handles refer to it.
   * Invalid cache entries will be freed when the last handle to the cache
   * entry is released. Entries that cannot be freed immediately are put
   * back on the lru list.
   */
  
  /*
   * Lock descriptions and usage:
   *
   * Each hash chain of both the block and index hash tables now contains
   * a built-in lock used to serialize accesses to the hash chain.
   *
   * Accesses to global data structures mb_cache_list and mb_cache_lru_list
   * are serialized via the global spinlock mb_cache_spinlock.
   *
   * Each mb_cache_entry contains a spinlock, e_entry_lock, to serialize
   * accesses to its local data, such as e_used and e_queued.
   *
   * Lock ordering:
   *
   * Each block hash chain's lock has the highest lock order, followed by an
   * index hash chain's lock, mb_cache_bg_lock (used to implement mb_cache_entry's
   * lock), and mb_cach_spinlock, with the lowest order.  While holding
   * either a block or index hash chain lock, a thread can acquire an
   * mc_cache_bg_lock, which in turn can also acquire mb_cache_spinlock.
   *
   * Synchronization:
   *
   * Since both mb_cache_entry_get and mb_cache_entry_find scan the block and
   * index hash chian, it needs to lock the corresponding hash chain.  For each
   * mb_cache_entry within the chain, it needs to lock the mb_cache_entry to
   * prevent either any simultaneous release or free on the entry and also
   * to serialize accesses to either the e_used or e_queued member of the entry.
   *
   * To avoid having a dangling reference to an already freed
   * mb_cache_entry, an mb_cache_entry is only freed when it is not on a
   * block hash chain and also no longer being referenced, both e_used,
   * and e_queued are 0's.  When an mb_cache_entry is explicitly freed it is
   * first removed from a block hash chain.
   */
  
  #include <linux/kernel.h>
  #include <linux/module.h>
  
  #include <linux/hash.h>
  #include <linux/fs.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/sched.h>
  #include <linux/list_bl.h>
  #include <linux/mbcache.h>
  #include <linux/init.h>
  #include <linux/blockgroup_lock.h>
  #include <linux/log2.h>
  
  #ifdef MB_CACHE_DEBUG
  # define mb_debug(f...) do { \
  		printk(KERN_DEBUG f); \
  		printk("
  "); \
  	} while (0)
  #define mb_assert(c) do { if (!(c)) \
  		printk(KERN_ERR "assertion " #c " failed
  "); \
  	} while(0)
  #else
  # define mb_debug(f...) do { } while(0)
  # define mb_assert(c) do { } while(0)
  #endif
  #define mb_error(f...) do { \
  		printk(KERN_ERR f); \
  		printk("
  "); \
  	} while(0)
  
  #define MB_CACHE_WRITER ((unsigned short)~0U >> 1)
  
  #define MB_CACHE_ENTRY_LOCK_BITS	ilog2(NR_BG_LOCKS)
  #define	MB_CACHE_ENTRY_LOCK_INDEX(ce)			\
  	(hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))
  
  static DECLARE_WAIT_QUEUE_HEAD(mb_cache_queue);
  static struct blockgroup_lock *mb_cache_bg_lock;
  static struct kmem_cache *mb_cache_kmem_cache;
  
  MODULE_AUTHOR("Andreas Gruenbacher <a.gruenbacher@computer.org>");
  MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
  MODULE_LICENSE("GPL");
  
  EXPORT_SYMBOL(mb_cache_create);
  EXPORT_SYMBOL(mb_cache_shrink);
  EXPORT_SYMBOL(mb_cache_destroy);
  EXPORT_SYMBOL(mb_cache_entry_alloc);
  EXPORT_SYMBOL(mb_cache_entry_insert);
  EXPORT_SYMBOL(mb_cache_entry_release);
  EXPORT_SYMBOL(mb_cache_entry_free);
  EXPORT_SYMBOL(mb_cache_entry_get);
  #if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
  EXPORT_SYMBOL(mb_cache_entry_find_first);
  EXPORT_SYMBOL(mb_cache_entry_find_next);
  #endif
  
  /*
   * Global data: list of all mbcache's, lru list, and a spinlock for
   * accessing cache data structures on SMP machines. The lru list is
   * global across all mbcaches.
   */
  
  static LIST_HEAD(mb_cache_list);
  static LIST_HEAD(mb_cache_lru_list);
  static DEFINE_SPINLOCK(mb_cache_spinlock);
  
  static inline void
  __spin_lock_mb_cache_entry(struct mb_cache_entry *ce)
  {
  	spin_lock(bgl_lock_ptr(mb_cache_bg_lock,
  		MB_CACHE_ENTRY_LOCK_INDEX(ce)));
  }
  
  static inline void
  __spin_unlock_mb_cache_entry(struct mb_cache_entry *ce)
  {
  	spin_unlock(bgl_lock_ptr(mb_cache_bg_lock,
  		MB_CACHE_ENTRY_LOCK_INDEX(ce)));
  }
  
  static inline int
  __mb_cache_entry_is_block_hashed(struct mb_cache_entry *ce)
  {
  	return !hlist_bl_unhashed(&ce->e_block_list);
  }
  
  
  static inline void
  __mb_cache_entry_unhash_block(struct mb_cache_entry *ce)
  {
  	if (__mb_cache_entry_is_block_hashed(ce))
  		hlist_bl_del_init(&ce->e_block_list);
  }
  
  static inline int
  __mb_cache_entry_is_index_hashed(struct mb_cache_entry *ce)
  {
  	return !hlist_bl_unhashed(&ce->e_index.o_list);
  }
  
  static inline void
  __mb_cache_entry_unhash_index(struct mb_cache_entry *ce)
  {
  	if (__mb_cache_entry_is_index_hashed(ce))
  		hlist_bl_del_init(&ce->e_index.o_list);
  }
  
  /*
   * __mb_cache_entry_unhash_unlock()
   *
   * This function is called to unhash both the block and index hash
   * chain.
   * It assumes both the block and index hash chain is locked upon entry.
   * It also unlock both hash chains both exit
   */
  static inline void
  __mb_cache_entry_unhash_unlock(struct mb_cache_entry *ce)
  {
  	__mb_cache_entry_unhash_index(ce);
  	hlist_bl_unlock(ce->e_index_hash_p);
  	__mb_cache_entry_unhash_block(ce);
  	hlist_bl_unlock(ce->e_block_hash_p);
  }
  
  static void
  __mb_cache_entry_forget(struct mb_cache_entry *ce, gfp_t gfp_mask)
  {
  	struct mb_cache *cache = ce->e_cache;
  
  	mb_assert(!(ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt)));
  	kmem_cache_free(cache->c_entry_cache, ce);
  	atomic_dec(&cache->c_entry_count);
  }
  
  static void
  __mb_cache_entry_release(struct mb_cache_entry *ce)
  {
  	/* First lock the entry to serialize access to its local data. */
  	__spin_lock_mb_cache_entry(ce);
  	/* Wake up all processes queuing for this cache entry. */
  	if (ce->e_queued)
  		wake_up_all(&mb_cache_queue);
  	if (ce->e_used >= MB_CACHE_WRITER)
  		ce->e_used -= MB_CACHE_WRITER;
  	/*
  	 * Make sure that all cache entries on lru_list have
  	 * both e_used and e_qued of 0s.
  	 */
  	ce->e_used--;
  	if (!(ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt))) {
  		if (!__mb_cache_entry_is_block_hashed(ce)) {
  			__spin_unlock_mb_cache_entry(ce);
  			goto forget;
  		}
  		/*
  		 * Need access to lru list, first drop entry lock,
  		 * then reacquire the lock in the proper order.
  		 */
  		spin_lock(&mb_cache_spinlock);
  		if (list_empty(&ce->e_lru_list))
  			list_add_tail(&ce->e_lru_list, &mb_cache_lru_list);
  		spin_unlock(&mb_cache_spinlock);
  	}
  	__spin_unlock_mb_cache_entry(ce);
  	return;
  forget:
  	mb_assert(list_empty(&ce->e_lru_list));
  	__mb_cache_entry_forget(ce, GFP_KERNEL);
  }
  
  /*
   * mb_cache_shrink_scan()  memory pressure callback
   *
   * This function is called by the kernel memory management when memory
   * gets low.
   *
   * @shrink: (ignored)
   * @sc: shrink_control passed from reclaim
   *
   * Returns the number of objects freed.
   */
  static unsigned long
  mb_cache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
  {
  	LIST_HEAD(free_list);
  	struct mb_cache_entry *entry, *tmp;
  	int nr_to_scan = sc->nr_to_scan;
  	gfp_t gfp_mask = sc->gfp_mask;
  	unsigned long freed = 0;
  
  	mb_debug("trying to free %d entries", nr_to_scan);
  	spin_lock(&mb_cache_spinlock);
  	while ((nr_to_scan-- > 0) && !list_empty(&mb_cache_lru_list)) {
  		struct mb_cache_entry *ce =
  			list_entry(mb_cache_lru_list.next,
  				struct mb_cache_entry, e_lru_list);
  		list_del_init(&ce->e_lru_list);
  		if (ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt))
  			continue;
  		spin_unlock(&mb_cache_spinlock);
  		/* Prevent any find or get operation on the entry */
  		hlist_bl_lock(ce->e_block_hash_p);
  		hlist_bl_lock(ce->e_index_hash_p);
  		/* Ignore if it is touched by a find/get */
  		if (ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt) ||
  			!list_empty(&ce->e_lru_list)) {
  			hlist_bl_unlock(ce->e_index_hash_p);
  			hlist_bl_unlock(ce->e_block_hash_p);
  			spin_lock(&mb_cache_spinlock);
  			continue;
  		}
  		__mb_cache_entry_unhash_unlock(ce);
  		list_add_tail(&ce->e_lru_list, &free_list);
  		spin_lock(&mb_cache_spinlock);
  	}
  	spin_unlock(&mb_cache_spinlock);
  
  	list_for_each_entry_safe(entry, tmp, &free_list, e_lru_list) {
  		__mb_cache_entry_forget(entry, gfp_mask);
  		freed++;
  	}
  	return freed;
  }
  
  static unsigned long
  mb_cache_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
  {
  	struct mb_cache *cache;
  	unsigned long count = 0;
  
  	spin_lock(&mb_cache_spinlock);
  	list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
  		mb_debug("cache %s (%d)", cache->c_name,
  			  atomic_read(&cache->c_entry_count));
  		count += atomic_read(&cache->c_entry_count);
  	}
  	spin_unlock(&mb_cache_spinlock);
  
  	return vfs_pressure_ratio(count);
  }
  
  static struct shrinker mb_cache_shrinker = {
  	.count_objects = mb_cache_shrink_count,
  	.scan_objects = mb_cache_shrink_scan,
  	.seeks = DEFAULT_SEEKS,
  };
  
  /*
   * mb_cache_create()  create a new cache
   *
   * All entries in one cache are equal size. Cache entries may be from
   * multiple devices. If this is the first mbcache created, registers
   * the cache with kernel memory management. Returns NULL if no more
   * memory was available.
   *
   * @name: name of the cache (informal)
   * @bucket_bits: log2(number of hash buckets)
   */
  struct mb_cache *
  mb_cache_create(const char *name, int bucket_bits)
  {
  	int n, bucket_count = 1 << bucket_bits;
  	struct mb_cache *cache = NULL;
  
  	if (!mb_cache_bg_lock) {
  		mb_cache_bg_lock = kmalloc(sizeof(struct blockgroup_lock),
  			GFP_KERNEL);
  		if (!mb_cache_bg_lock)
  			return NULL;
  		bgl_lock_init(mb_cache_bg_lock);
  	}
  
  	cache = kmalloc(sizeof(struct mb_cache), GFP_KERNEL);
  	if (!cache)
  		return NULL;
  	cache->c_name = name;
  	atomic_set(&cache->c_entry_count, 0);
  	cache->c_bucket_bits = bucket_bits;
  	cache->c_block_hash = kmalloc(bucket_count *
  		sizeof(struct hlist_bl_head), GFP_KERNEL);
  	if (!cache->c_block_hash)
  		goto fail;
  	for (n=0; n<bucket_count; n++)
  		INIT_HLIST_BL_HEAD(&cache->c_block_hash[n]);
  	cache->c_index_hash = kmalloc(bucket_count *
  		sizeof(struct hlist_bl_head), GFP_KERNEL);
  	if (!cache->c_index_hash)
  		goto fail;
  	for (n=0; n<bucket_count; n++)
  		INIT_HLIST_BL_HEAD(&cache->c_index_hash[n]);
  	if (!mb_cache_kmem_cache) {
  		mb_cache_kmem_cache = kmem_cache_create(name,
  			sizeof(struct mb_cache_entry), 0,
  			SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
  		if (!mb_cache_kmem_cache)
  			goto fail2;
  	}
  	cache->c_entry_cache = mb_cache_kmem_cache;
  
  	/*
  	 * Set an upper limit on the number of cache entries so that the hash
  	 * chains won't grow too long.
  	 */
  	cache->c_max_entries = bucket_count << 4;
  
  	spin_lock(&mb_cache_spinlock);
  	list_add(&cache->c_cache_list, &mb_cache_list);
  	spin_unlock(&mb_cache_spinlock);
  	return cache;
  
  fail2:
  	kfree(cache->c_index_hash);
  
  fail:
  	kfree(cache->c_block_hash);
  	kfree(cache);
  	return NULL;
  }
  
  
  /*
   * mb_cache_shrink()
   *
   * Removes all cache entries of a device from the cache. All cache entries
   * currently in use cannot be freed, and thus remain in the cache. All others
   * are freed.
   *
   * @bdev: which device's cache entries to shrink
   */
  void
  mb_cache_shrink(struct block_device *bdev)
  {
  	LIST_HEAD(free_list);
  	struct list_head *l;
  	struct mb_cache_entry *ce, *tmp;
  
  	l = &mb_cache_lru_list;
  	spin_lock(&mb_cache_spinlock);
  	while (!list_is_last(l, &mb_cache_lru_list)) {
  		l = l->next;
  		ce = list_entry(l, struct mb_cache_entry, e_lru_list);
  		if (ce->e_bdev == bdev) {
  			list_del_init(&ce->e_lru_list);
  			if (ce->e_used || ce->e_queued ||
  				atomic_read(&ce->e_refcnt))
  				continue;
  			spin_unlock(&mb_cache_spinlock);
  			/*
  			 * Prevent any find or get operation on the entry.
  			 */
  			hlist_bl_lock(ce->e_block_hash_p);
  			hlist_bl_lock(ce->e_index_hash_p);
  			/* Ignore if it is touched by a find/get */
  			if (ce->e_used || ce->e_queued ||
  				atomic_read(&ce->e_refcnt) ||
  				!list_empty(&ce->e_lru_list)) {
  				hlist_bl_unlock(ce->e_index_hash_p);
  				hlist_bl_unlock(ce->e_block_hash_p);
  				l = &mb_cache_lru_list;
  				spin_lock(&mb_cache_spinlock);
  				continue;
  			}
  			__mb_cache_entry_unhash_unlock(ce);
  			mb_assert(!(ce->e_used || ce->e_queued ||
  				atomic_read(&ce->e_refcnt)));
  			list_add_tail(&ce->e_lru_list, &free_list);
  			l = &mb_cache_lru_list;
  			spin_lock(&mb_cache_spinlock);
  		}
  	}
  	spin_unlock(&mb_cache_spinlock);
  
  	list_for_each_entry_safe(ce, tmp, &free_list, e_lru_list) {
  		__mb_cache_entry_forget(ce, GFP_KERNEL);
  	}
  }
  
  
  /*
   * mb_cache_destroy()
   *
   * Shrinks the cache to its minimum possible size (hopefully 0 entries),
   * and then destroys it. If this was the last mbcache, un-registers the
   * mbcache from kernel memory management.
   */
  void
  mb_cache_destroy(struct mb_cache *cache)
  {
  	LIST_HEAD(free_list);
  	struct mb_cache_entry *ce, *tmp;
  
  	spin_lock(&mb_cache_spinlock);
  	list_for_each_entry_safe(ce, tmp, &mb_cache_lru_list, e_lru_list) {
  		if (ce->e_cache == cache)
  			list_move_tail(&ce->e_lru_list, &free_list);
  	}
  	list_del(&cache->c_cache_list);
  	spin_unlock(&mb_cache_spinlock);
  
  	list_for_each_entry_safe(ce, tmp, &free_list, e_lru_list) {
  		list_del_init(&ce->e_lru_list);
  		/*
  		 * Prevent any find or get operation on the entry.
  		 */
  		hlist_bl_lock(ce->e_block_hash_p);
  		hlist_bl_lock(ce->e_index_hash_p);
  		mb_assert(!(ce->e_used || ce->e_queued ||
  			atomic_read(&ce->e_refcnt)));
  		__mb_cache_entry_unhash_unlock(ce);
  		__mb_cache_entry_forget(ce, GFP_KERNEL);
  	}
  
  	if (atomic_read(&cache->c_entry_count) > 0) {
  		mb_error("cache %s: %d orphaned entries",
  			  cache->c_name,
  			  atomic_read(&cache->c_entry_count));
  	}
  
  	if (list_empty(&mb_cache_list)) {
  		kmem_cache_destroy(mb_cache_kmem_cache);
  		mb_cache_kmem_cache = NULL;
  	}
  	kfree(cache->c_index_hash);
  	kfree(cache->c_block_hash);
  	kfree(cache);
  }
  
  /*
   * mb_cache_entry_alloc()
   *
   * Allocates a new cache entry. The new entry will not be valid initially,
   * and thus cannot be looked up yet. It should be filled with data, and
   * then inserted into the cache using mb_cache_entry_insert(). Returns NULL
   * if no more memory was available.
   */
  struct mb_cache_entry *
  mb_cache_entry_alloc(struct mb_cache *cache, gfp_t gfp_flags)
  {
  	struct mb_cache_entry *ce;
  
  	if (atomic_read(&cache->c_entry_count) >= cache->c_max_entries) {
  		struct list_head *l;
  
  		l = &mb_cache_lru_list;
  		spin_lock(&mb_cache_spinlock);
  		while (!list_is_last(l, &mb_cache_lru_list)) {
  			l = l->next;
  			ce = list_entry(l, struct mb_cache_entry, e_lru_list);
  			if (ce->e_cache == cache) {
  				list_del_init(&ce->e_lru_list);
  				if (ce->e_used || ce->e_queued ||
  					atomic_read(&ce->e_refcnt))
  					continue;
  				spin_unlock(&mb_cache_spinlock);
  				/*
  				 * Prevent any find or get operation on the
  				 * entry.
  				 */
  				hlist_bl_lock(ce->e_block_hash_p);
  				hlist_bl_lock(ce->e_index_hash_p);
  				/* Ignore if it is touched by a find/get */
  				if (ce->e_used || ce->e_queued ||
  					atomic_read(&ce->e_refcnt) ||
  					!list_empty(&ce->e_lru_list)) {
  					hlist_bl_unlock(ce->e_index_hash_p);
  					hlist_bl_unlock(ce->e_block_hash_p);
  					l = &mb_cache_lru_list;
  					spin_lock(&mb_cache_spinlock);
  					continue;
  				}
  				mb_assert(list_empty(&ce->e_lru_list));
  				mb_assert(!(ce->e_used || ce->e_queued ||
  					atomic_read(&ce->e_refcnt)));
  				__mb_cache_entry_unhash_unlock(ce);
  				goto found;
  			}
  		}
  		spin_unlock(&mb_cache_spinlock);
  	}
  
  	ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
  	if (!ce)
  		return NULL;
  	atomic_inc(&cache->c_entry_count);
  	INIT_LIST_HEAD(&ce->e_lru_list);
  	INIT_HLIST_BL_NODE(&ce->e_block_list);
  	INIT_HLIST_BL_NODE(&ce->e_index.o_list);
  	ce->e_cache = cache;
  	ce->e_queued = 0;
  	atomic_set(&ce->e_refcnt, 0);
  found:
  	ce->e_block_hash_p = &cache->c_block_hash[0];
  	ce->e_index_hash_p = &cache->c_index_hash[0];
  	ce->e_used = 1 + MB_CACHE_WRITER;
  	return ce;
  }
  
  
  /*
   * mb_cache_entry_insert()
   *
   * Inserts an entry that was allocated using mb_cache_entry_alloc() into
   * the cache. After this, the cache entry can be looked up, but is not yet
   * in the lru list as the caller still holds a handle to it. Returns 0 on
   * success, or -EBUSY if a cache entry for that device + inode exists
   * already (this may happen after a failed lookup, but when another process
   * has inserted the same cache entry in the meantime).
   *
   * @bdev: device the cache entry belongs to
   * @block: block number
   * @key: lookup key
   */
  int
  mb_cache_entry_insert(struct mb_cache_entry *ce, struct block_device *bdev,
  		      sector_t block, unsigned int key)
  {
  	struct mb_cache *cache = ce->e_cache;
  	unsigned int bucket;
  	struct hlist_bl_node *l;
  	struct hlist_bl_head *block_hash_p;
  	struct hlist_bl_head *index_hash_p;
  	struct mb_cache_entry *lce;
  
  	mb_assert(ce);
  	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff), 
  			   cache->c_bucket_bits);
  	block_hash_p = &cache->c_block_hash[bucket];
  	hlist_bl_lock(block_hash_p);
  	hlist_bl_for_each_entry(lce, l, block_hash_p, e_block_list) {
  		if (lce->e_bdev == bdev && lce->e_block == block) {
  			hlist_bl_unlock(block_hash_p);
  			return -EBUSY;
  		}
  	}
  	mb_assert(!__mb_cache_entry_is_block_hashed(ce));
  	__mb_cache_entry_unhash_block(ce);
  	__mb_cache_entry_unhash_index(ce);
  	ce->e_bdev = bdev;
  	ce->e_block = block;
  	ce->e_block_hash_p = block_hash_p;
  	ce->e_index.o_key = key;
  	hlist_bl_add_head(&ce->e_block_list, block_hash_p);
  	hlist_bl_unlock(block_hash_p);
  	bucket = hash_long(key, cache->c_bucket_bits);
  	index_hash_p = &cache->c_index_hash[bucket];
  	hlist_bl_lock(index_hash_p);
  	ce->e_index_hash_p = index_hash_p;
  	hlist_bl_add_head(&ce->e_index.o_list, index_hash_p);
  	hlist_bl_unlock(index_hash_p);
  	return 0;
  }
  
  
  /*
   * mb_cache_entry_release()
   *
   * Release a handle to a cache entry. When the last handle to a cache entry
   * is released it is either freed (if it is invalid) or otherwise inserted
   * in to the lru list.
   */
  void
  mb_cache_entry_release(struct mb_cache_entry *ce)
  {
  	__mb_cache_entry_release(ce);
  }
  
  
  /*
   * mb_cache_entry_free()
   *
   */
  void
  mb_cache_entry_free(struct mb_cache_entry *ce)
  {
  	mb_assert(ce);
  	mb_assert(list_empty(&ce->e_lru_list));
  	hlist_bl_lock(ce->e_index_hash_p);
  	__mb_cache_entry_unhash_index(ce);
  	hlist_bl_unlock(ce->e_index_hash_p);
  	hlist_bl_lock(ce->e_block_hash_p);
  	__mb_cache_entry_unhash_block(ce);
  	hlist_bl_unlock(ce->e_block_hash_p);
  	__mb_cache_entry_release(ce);
  }
  
  
  /*
   * mb_cache_entry_get()
   *
   * Get a cache entry  by device / block number. (There can only be one entry
   * in the cache per device and block.) Returns NULL if no such cache entry
   * exists. The returned cache entry is locked for exclusive access ("single
   * writer").
   */
  struct mb_cache_entry *
  mb_cache_entry_get(struct mb_cache *cache, struct block_device *bdev,
  		   sector_t block)
  {
  	unsigned int bucket;
  	struct hlist_bl_node *l;
  	struct mb_cache_entry *ce;
  	struct hlist_bl_head *block_hash_p;
  
  	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff),
  			   cache->c_bucket_bits);
  	block_hash_p = &cache->c_block_hash[bucket];
  	/* First serialize access to the block corresponding hash chain. */
  	hlist_bl_lock(block_hash_p);
  	hlist_bl_for_each_entry(ce, l, block_hash_p, e_block_list) {
  		mb_assert(ce->e_block_hash_p == block_hash_p);
  		if (ce->e_bdev == bdev && ce->e_block == block) {
  			/*
  			 * Prevent a free from removing the entry.
  			 */
  			atomic_inc(&ce->e_refcnt);
  			hlist_bl_unlock(block_hash_p);
  			__spin_lock_mb_cache_entry(ce);
  			atomic_dec(&ce->e_refcnt);
  			if (ce->e_used > 0) {
  				DEFINE_WAIT(wait);
  				while (ce->e_used > 0) {
  					ce->e_queued++;
  					prepare_to_wait(&mb_cache_queue, &wait,
  							TASK_UNINTERRUPTIBLE);
  					__spin_unlock_mb_cache_entry(ce);
  					schedule();
  					__spin_lock_mb_cache_entry(ce);
  					ce->e_queued--;
  				}
  				finish_wait(&mb_cache_queue, &wait);
  			}
  			ce->e_used += 1 + MB_CACHE_WRITER;
  			__spin_unlock_mb_cache_entry(ce);
  
  			if (!list_empty(&ce->e_lru_list)) {
  				spin_lock(&mb_cache_spinlock);
  				list_del_init(&ce->e_lru_list);
  				spin_unlock(&mb_cache_spinlock);
  			}
  			if (!__mb_cache_entry_is_block_hashed(ce)) {
  				__mb_cache_entry_release(ce);
  				return NULL;
  			}
  			return ce;
  		}
  	}
  	hlist_bl_unlock(block_hash_p);
  	return NULL;
  }
  
  #if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
  
  static struct mb_cache_entry *
  __mb_cache_entry_find(struct hlist_bl_node *l, struct hlist_bl_head *head,
  		      struct block_device *bdev, unsigned int key)
  {
  
  	/* The index hash chain is alredy acquire by caller. */
  	while (l != NULL) {
  		struct mb_cache_entry *ce =
  			hlist_bl_entry(l, struct mb_cache_entry,
  				e_index.o_list);
  		mb_assert(ce->e_index_hash_p == head);
  		if (ce->e_bdev == bdev && ce->e_index.o_key == key) {
  			/*
  			 * Prevent a free from removing the entry.
  			 */
  			atomic_inc(&ce->e_refcnt);
  			hlist_bl_unlock(head);
  			__spin_lock_mb_cache_entry(ce);
  			atomic_dec(&ce->e_refcnt);
  			ce->e_used++;
  			/* Incrementing before holding the lock gives readers
  			   priority over writers. */
  			if (ce->e_used >= MB_CACHE_WRITER) {
  				DEFINE_WAIT(wait);
  
  				while (ce->e_used >= MB_CACHE_WRITER) {
  					ce->e_queued++;
  					prepare_to_wait(&mb_cache_queue, &wait,
  							TASK_UNINTERRUPTIBLE);
  					__spin_unlock_mb_cache_entry(ce);
  					schedule();
  					__spin_lock_mb_cache_entry(ce);
  					ce->e_queued--;
  				}
  				finish_wait(&mb_cache_queue, &wait);
  			}
  			__spin_unlock_mb_cache_entry(ce);
  			if (!list_empty(&ce->e_lru_list)) {
  				spin_lock(&mb_cache_spinlock);
  				list_del_init(&ce->e_lru_list);
  				spin_unlock(&mb_cache_spinlock);
  			}
  			if (!__mb_cache_entry_is_block_hashed(ce)) {
  				__mb_cache_entry_release(ce);
  				return ERR_PTR(-EAGAIN);
  			}
  			return ce;
  		}
  		l = l->next;
  	}
  	hlist_bl_unlock(head);
  	return NULL;
  }
  
  
  /*
   * mb_cache_entry_find_first()
   *
   * Find the first cache entry on a given device with a certain key in
   * an additional index. Additional matches can be found with
   * mb_cache_entry_find_next(). Returns NULL if no match was found. The
   * returned cache entry is locked for shared access ("multiple readers").
   *
   * @cache: the cache to search
   * @bdev: the device the cache entry should belong to
   * @key: the key in the index
   */
  struct mb_cache_entry *
  mb_cache_entry_find_first(struct mb_cache *cache, struct block_device *bdev,
  			  unsigned int key)
  {
  	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
  	struct hlist_bl_node *l;
  	struct mb_cache_entry *ce = NULL;
  	struct hlist_bl_head *index_hash_p;
  
  	index_hash_p = &cache->c_index_hash[bucket];
  	hlist_bl_lock(index_hash_p);
  	if (!hlist_bl_empty(index_hash_p)) {
  		l = hlist_bl_first(index_hash_p);
  		ce = __mb_cache_entry_find(l, index_hash_p, bdev, key);
  	} else
  		hlist_bl_unlock(index_hash_p);
  	return ce;
  }
  
  
  /*
   * mb_cache_entry_find_next()
   *
   * Find the next cache entry on a given device with a certain key in an
   * additional index. Returns NULL if no match could be found. The previous
   * entry is atomatically released, so that mb_cache_entry_find_next() can
   * be called like this:
   *
   * entry = mb_cache_entry_find_first();
   * while (entry) {
   * 	...
   *	entry = mb_cache_entry_find_next(entry, ...);
   * }
   *
   * @prev: The previous match
   * @bdev: the device the cache entry should belong to
   * @key: the key in the index
   */
  struct mb_cache_entry *
  mb_cache_entry_find_next(struct mb_cache_entry *prev,
  			 struct block_device *bdev, unsigned int key)
  {
  	struct mb_cache *cache = prev->e_cache;
  	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
  	struct hlist_bl_node *l;
  	struct mb_cache_entry *ce;
  	struct hlist_bl_head *index_hash_p;
  
  	index_hash_p = &cache->c_index_hash[bucket];
  	mb_assert(prev->e_index_hash_p == index_hash_p);
  	hlist_bl_lock(index_hash_p);
  	mb_assert(!hlist_bl_empty(index_hash_p));
  	l = prev->e_index.o_list.next;
  	ce = __mb_cache_entry_find(l, index_hash_p, bdev, key);
  	__mb_cache_entry_release(prev);
  	return ce;
  }
  
  #endif  /* !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0) */
  
  static int __init init_mbcache(void)
  {
  	register_shrinker(&mb_cache_shrinker);
  	return 0;
  }
  
  static void __exit exit_mbcache(void)
  {
  	unregister_shrinker(&mb_cache_shrinker);
  }
  
  module_init(init_mbcache)
  module_exit(exit_mbcache)