[PATCH] lockdep: add more rwsem.h documentation

#ifdef CONFIG_DEBUG_LOCK_ALLOC

/*

 * nested locking:

 * nested locking. NOTE: rwsems are not allowed to recurse

 * (which occurs if the same task tries to acquire the same

 * lock instance multiple times), but multiple locks of the

 * same lock class might be taken, if the order of the locks

 * is always the same. This ordering rule can be expressed

 * to lockdep via the _nested() APIs, but enumerating the

 * subclasses that are used. (If the nesting relationship is

 * static then another method for expressing nested locking is

 * the explicit definition of lock class keys and the use of

 * lockdep_set_class() at lock initialization time.

 * See Documentation/lockdep-design.txt for more details.)

 */

extern void down_read_nested(struct rw_semaphore *sem, int subclass);

extern void down_write_nested(struct rw_semaphore *sem, int subclass);

/*

 * Take/release a lock when not the owner will release it:

 * Take/release a lock when not the owner will release it.

 *

 * [ This API should be avoided as much as possible - the

 *   proper abstraction for this case is completions. ]

 */

extern void down_read_non_owner(struct rw_semaphore *sem);

extern void up_read_non_owner(struct rw_semaphore *sem);