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/*
 * 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