/* rwsem.h: R/W semaphores, public interface
   *
   * Written by David Howells (dhowells@redhat.com).
   * Derived from asm-i386/semaphore.h
   */
  
  #ifndef _LINUX_RWSEM_H
  #define _LINUX_RWSEM_H
  
  #include <linux/linkage.h>

  #ifdef __KERNEL__

  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <asm/system.h>
  #include <asm/atomic.h>
  
  struct rw_semaphore;
  
  #ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
  #include <linux/rwsem-spinlock.h> /* use a generic implementation */
  #else
  #include <asm/rwsem.h> /* use an arch-specific implementation */
  #endif

  /*
   * lock for reading
   */

  extern void down_read(struct rw_semaphore *sem);

  
  /*
   * trylock for reading -- returns 1 if successful, 0 if contention
   */

  extern int down_read_trylock(struct rw_semaphore *sem);

  
  /*
   * lock for writing
   */

  extern void down_write(struct rw_semaphore *sem);

  
  /*
   * trylock for writing -- returns 1 if successful, 0 if contention
   */

  extern int down_write_trylock(struct rw_semaphore *sem);

  
  /*
   * release a read lock
   */

  extern void up_read(struct rw_semaphore *sem);

  
  /*
   * release a write lock
   */

  extern void up_write(struct rw_semaphore *sem);

  
  /*
   * downgrade write lock to read lock
   */

  extern void downgrade_write(struct rw_semaphore *sem);
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  /*

   * 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.
   *
   * [ 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);
  #else
  # define down_read_nested(sem, subclass)		down_read(sem)
  # define down_write_nested(sem, subclass)	down_write(sem)
  # define down_read_non_owner(sem)		down_read(sem)
  # define up_read_non_owner(sem)			up_read(sem)
  #endif

  
  #endif /* __KERNEL__ */
  #endif /* _LINUX_RWSEM_H */