/* SPDX-License-Identifier: GPL-2.0 */

  /* 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>

  #include <linux/err.h>

  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER

  #include <linux/osq_lock.h>

  #endif

  /*
   * For an uncontended rwsem, count and owner are the only fields a task
   * needs to touch when acquiring the rwsem. So they are put next to each
   * other to increase the chance that they will share the same cacheline.
   *
   * In a contended rwsem, the owner is likely the most frequently accessed
   * field in the structure as the optimistic waiter that holds the osq lock
   * will spin on owner. For an embedded rwsem, other hot fields in the
   * containing structure should be moved further away from the rwsem to
   * reduce the chance that they will share the same cacheline causing
   * cacheline bouncing problem.
   */

  struct rw_semaphore {

  	atomic_long_t count;

  	/*

  	 * Write owner or one of the read owners as well flags regarding
  	 * the current state of the rwsem. Can be used as a speculative
  	 * check to see if the write owner is running on the cpu.

  	 */

  	atomic_long_t owner;

  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER

  	struct optimistic_spin_queue osq; /* spinner MCS lock */

  #endif

  	raw_spinlock_t wait_lock;
  	struct list_head wait_list;

  #ifdef CONFIG_DEBUG_RWSEMS
  	void *magic;
  #endif

  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	struct lockdep_map	dep_map;
  #endif
  };

  /*

   * Setting all bits of the owner field except bit 0 will indicate

   * that the rwsem is writer-owned with an unknown owner.
   */

  #define RWSEM_OWNER_UNKNOWN	(-2L)

  /* In all implementations count != 0 means locked */
  static inline int rwsem_is_locked(struct rw_semaphore *sem)
  {

  	return atomic_long_read(&sem->count) != 0;

  }

  #define RWSEM_UNLOCKED_VALUE		0L

  #define __RWSEM_INIT_COUNT(name)	.count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)

  /* 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

  #ifdef CONFIG_DEBUG_RWSEMS
  # define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname
  #else
  # define __DEBUG_RWSEM_INITIALIZER(lockname)
  #endif

  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER

  #define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED

  #else

  #define __RWSEM_OPT_INIT(lockname)

  #endif

  #define __RWSEM_INITIALIZER(name)				\

  	{ __RWSEM_INIT_COUNT(name),				\

  	  .owner = ATOMIC_LONG_INIT(0),				\

  	  .wait_list = LIST_HEAD_INIT((name).wait_list),	\
  	  .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock)	\
  	  __RWSEM_OPT_INIT(name)				\

  	  __DEBUG_RWSEM_INITIALIZER(name)			\

  	  __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)

  /*

   * This is the same regardless of which rwsem implementation that is being used.
   * It is just a heuristic meant to be called by somebody alreadying holding the
   * rwsem to see if somebody from an incompatible type is wanting access to the
   * lock.
   */
  static inline int rwsem_is_contended(struct rw_semaphore *sem)
  {
  	return !list_empty(&sem->wait_list);
  }
  
  /*

   * lock for reading
   */

  extern void down_read(struct rw_semaphore *sem);

  extern int __must_check down_read_killable(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);

  extern int __must_check down_write_killable(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/locking/lockdep-design.rst 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 int down_write_killable_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_write_killable_nested(sem, subclass)	down_write_killable(sem)

  # define down_read_non_owner(sem)		down_read(sem)
  # define up_read_non_owner(sem)			up_read(sem)

  #endif

  #endif /* _LINUX_RWSEM_H */