/* * The owner field of the rw_semaphore structure will be set to * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear * the owner field when it unlocks. A reader, on the other hand, will * not touch the owner field when it unlocks. * * In essence, the owner field now has the following 3 states: *  1) 0 *     - lock is free or the owner hasn't set the field yet *  2) RWSEM_READER_OWNED *     - lock is currently or previously owned by readers (lock is free *       or not set by owner yet) *  3) Other non-zero value *     - a writer owns the lock */#define RWSEM_READER_OWNED	((struct task_struct *)1UL) #ifdef CONFIG_RWSEM_SPIN_ON_OWNER/* * All writes to owner are protected by WRITE_ONCE() to make sure that * store tearing can't happen as optimistic spinners may read and use * the owner value concurrently without lock. Read from owner, however, * may not need READ_ONCE() as long as the pointer value is only used * for comparison and isn't being dereferenced. */static inline void rwsem_set_owner(struct rw_semaphore *sem){	WRITE_ONCE(sem->owner, current);} static inline void rwsem_clear_owner(struct rw_semaphore *sem){	WRITE_ONCE(sem->owner, NULL);} static inline void rwsem_set_reader_owned(struct rw_semaphore *sem){	/*	 * We check the owner value first to make sure that we will only	 * do a write to the rwsem cacheline when it is really necessary	 * to minimize cacheline contention.	 */	if (sem->owner != RWSEM_READER_OWNED)		WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);} static inline bool rwsem_owner_is_writer(struct task_struct *owner){	return owner && owner != RWSEM_READER_OWNED;} static inline bool rwsem_owner_is_reader(struct task_struct *owner){	return owner == RWSEM_READER_OWNED;}#elsestatic inline void rwsem_set_owner(struct rw_semaphore *sem){} static inline void rwsem_clear_owner(struct rw_semaphore *sem){} static inline void rwsem_set_reader_owned(struct rw_semaphore *sem){}#endif