1 /* 2 * The owner field of the rw_semaphore structure will be set to 3 * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear 4 * the owner field when it unlocks. A reader, on the other hand, will 5 * not touch the owner field when it unlocks. 6 * 7 * In essence, the owner field now has the following 3 states: 8 * 1) 0 9 * - lock is free or the owner hasn't set the field yet 10 * 2) RWSEM_READER_OWNED 11 * - lock is currently or previously owned by readers (lock is free 12 * or not set by owner yet) 13 * 3) Other non-zero value 14 * - a writer owns the lock 15 */ 16 #define RWSEM_READER_OWNED ((struct task_struct *)1UL) 17 18 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER 19 /* 20 * All writes to owner are protected by WRITE_ONCE() to make sure that 21 * store tearing can't happen as optimistic spinners may read and use 22 * the owner value concurrently without lock. Read from owner, however, 23 * may not need READ_ONCE() as long as the pointer value is only used 24 * for comparison and isn't being dereferenced. 25 */ rwsem_set_owner(struct rw_semaphore * sem)26static inline void rwsem_set_owner(struct rw_semaphore *sem) 27 { 28 WRITE_ONCE(sem->owner, current); 29 } 30 rwsem_clear_owner(struct rw_semaphore * sem)31static inline void rwsem_clear_owner(struct rw_semaphore *sem) 32 { 33 WRITE_ONCE(sem->owner, NULL); 34 } 35 rwsem_set_reader_owned(struct rw_semaphore * sem)36static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) 37 { 38 /* 39 * We check the owner value first to make sure that we will only 40 * do a write to the rwsem cacheline when it is really necessary 41 * to minimize cacheline contention. 42 */ 43 if (sem->owner != RWSEM_READER_OWNED) 44 WRITE_ONCE(sem->owner, RWSEM_READER_OWNED); 45 } 46 rwsem_owner_is_writer(struct task_struct * owner)47static inline bool rwsem_owner_is_writer(struct task_struct *owner) 48 { 49 return owner && owner != RWSEM_READER_OWNED; 50 } 51 rwsem_owner_is_reader(struct task_struct * owner)52static inline bool rwsem_owner_is_reader(struct task_struct *owner) 53 { 54 return owner == RWSEM_READER_OWNED; 55 } 56 #else rwsem_set_owner(struct rw_semaphore * sem)57static inline void rwsem_set_owner(struct rw_semaphore *sem) 58 { 59 } 60 rwsem_clear_owner(struct rw_semaphore * sem)61static inline void rwsem_clear_owner(struct rw_semaphore *sem) 62 { 63 } 64 rwsem_set_reader_owned(struct rw_semaphore * sem)65static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) 66 { 67 } 68 #endif 69