text: /* 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>
  
  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <linux/list.h>
  #include <linux/spinlock.h>
  
  #include <linux/atomic.h>
  
  struct rw_semaphore;
  
  #ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
  #include <linux/rwsem-spinlock.h> /* use a generic implementation */
  #else
  /* All arch specific implementations share the same struct */
  struct rw_semaphore {
  	long			count;
  	raw_spinlock_t		wait_lock;
  	struct list_head	wait_list;
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	struct lockdep_map	dep_map;
  #endif
  };
  
  extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
  extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
  extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
  extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
  
  /* Include the arch specific part */
  #include <asm/rwsem.h>
  
  /* In all implementations count != 0 means locked */
  static inline int rwsem_is_locked(struct rw_semaphore *sem)
  {
  	return sem->count != 0;
  }
  
  #endif
  
  /* Common initializer macros and functions */
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  #else
  # define __RWSEM_DEP_MAP_INIT(lockname)
  #endif
  
  #define __RWSEM_INITIALIZER(name)			\
  	{ RWSEM_UNLOCKED_VALUE,				\
  	  __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),	\
  	  LIST_HEAD_INIT((name).wait_list)		\
  	  __RWSEM_DEP_MAP_INIT(name) }
  
  #define DECLARE_RWSEM(name) \
  	struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  
  extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
  			 struct lock_class_key *key);
  
  #define init_rwsem(sem)						\
  do {								\
  	static struct lock_class_key __key;			\
  								\
  	__init_rwsem((sem), #sem, &__key);			\
  } while (0)
  
  /*
   * 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);
  extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
  
  # define down_write_nest_lock(sem, nest_lock)			\
  do {								\
  	typecheck(struct lockdep_map *, &(nest_lock)->dep_map);	\
  	_down_write_nest_lock(sem, &(nest_lock)->dep_map);	\
  } while (0);
  
  /*
   * 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_nest_lock(sem, nest_lock)	down_write(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 /* _LINUX_RWSEM_H */
  