The setting of owner field is specific to rwsem-xadd, it is not needed for rwsem-spinlock. This patch moves all the owner setting code to the fast paths directly within rwsem-add.h file.
The rwsem_set_reader_owned() is now only called by the first reader. So there is no need to do a read to check if the owner is properly set first. Signed-off-by: Waiman Long <[email protected]> --- kernel/locking/rwsem-xadd.c | 7 +++---- kernel/locking/rwsem-xadd.h | 19 ++++++++++++++++--- kernel/locking/rwsem.c | 17 ++--------------- kernel/locking/rwsem.h | 11 ++++------- 4 files changed, 25 insertions(+), 29 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 39dc5be..30bc163 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -132,11 +132,10 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, return; } /* - * It is not really necessary to set it to reader-owned here, - * but it gives the spinners an early indication that the - * readers now have the lock. + * Set it to reader-owned for first reader. */ - rwsem_set_reader_owned(sem); + if (!(oldcount >> RWSEM_READER_SHIFT)) + rwsem_set_reader_owned(sem); } /* diff --git a/kernel/locking/rwsem-xadd.h b/kernel/locking/rwsem-xadd.h index 3820719..abcb484 100644 --- a/kernel/locking/rwsem-xadd.h +++ b/kernel/locking/rwsem-xadd.h @@ -29,9 +29,12 @@ */ static inline void __down_read(struct rw_semaphore *sem) { - if (unlikely(atomic_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count) - & RWSEM_READ_FAILED_MASK)) + int count = atomic_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count); + + if (unlikely(count & RWSEM_READ_FAILED_MASK)) rwsem_down_read_failed(sem); + else if ((count >> RWSEM_READER_SHIFT) == 1) + rwsem_set_reader_owned(sem); } static inline int __down_read_trylock(struct rw_semaphore *sem) @@ -41,6 +44,8 @@ static inline int __down_read_trylock(struct rw_semaphore *sem) while (!((tmp = atomic_read(&sem->count)) & RWSEM_READ_FAILED_MASK)) { if (tmp == atomic_cmpxchg_acquire(&sem->count, tmp, tmp + RWSEM_READER_BIAS)) { + if (!(tmp >> RWSEM_READER_SHIFT)) + rwsem_set_reader_owned(sem); return 1; } } @@ -55,6 +60,7 @@ static inline void __down_write(struct rw_semaphore *sem) if (unlikely(atomic_cmpxchg_acquire(&sem->count, 0, RWSEM_WRITER_LOCKED))) rwsem_down_write_failed(sem); + rwsem_set_owner(sem); } static inline int __down_write_killable(struct rw_semaphore *sem) @@ -63,12 +69,17 @@ static inline int __down_write_killable(struct rw_semaphore *sem) RWSEM_WRITER_LOCKED))) if (IS_ERR(rwsem_down_write_failed_killable(sem))) return -EINTR; + rwsem_set_owner(sem); return 0; } static inline int __down_write_trylock(struct rw_semaphore *sem) { - return !atomic_cmpxchg_acquire(&sem->count, 0, RWSEM_WRITER_LOCKED); + bool taken = !atomic_cmpxchg_acquire(&sem->count, 0, + RWSEM_WRITER_LOCKED); + if (taken) + rwsem_set_owner(sem); + return taken; } /* @@ -89,6 +100,7 @@ static inline void __up_read(struct rw_semaphore *sem) */ static inline void __up_write(struct rw_semaphore *sem) { + rwsem_clear_owner(sem); if (unlikely(atomic_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count) & RWSEM_FLAG_WAITERS)) rwsem_wake(sem); @@ -110,6 +122,7 @@ static inline void __downgrade_write(struct rw_semaphore *sem) */ tmp = atomic_fetch_add_release(-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count); + rwsem_set_reader_owned(sem); if (tmp & RWSEM_FLAG_WAITERS) rwsem_downgrade_wake(sem); } diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 4d48b1c..0a32725 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -23,7 +23,6 @@ void __sched down_read(struct rw_semaphore *sem) rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_); LOCK_CONTENDED(sem, __down_read_trylock, __down_read); - rwsem_set_reader_owned(sem); } EXPORT_SYMBOL(down_read); @@ -35,10 +34,8 @@ int down_read_trylock(struct rw_semaphore *sem) { int ret = __down_read_trylock(sem); - if (ret == 1) { + if (ret == 1) rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_); - rwsem_set_reader_owned(sem); - } return ret; } @@ -53,7 +50,6 @@ void __sched down_write(struct rw_semaphore *sem) rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); LOCK_CONTENDED(sem, __down_write_trylock, __down_write); - rwsem_set_owner(sem); } EXPORT_SYMBOL(down_write); @@ -71,7 +67,6 @@ int __sched down_write_killable(struct rw_semaphore *sem) return -EINTR; } - rwsem_set_owner(sem); return 0; } @@ -84,10 +79,8 @@ int down_write_trylock(struct rw_semaphore *sem) { int ret = __down_write_trylock(sem); - if (ret == 1) { + if (ret == 1) rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_); - rwsem_set_owner(sem); - } return ret; } @@ -113,7 +106,6 @@ void up_write(struct rw_semaphore *sem) { rwsem_release(&sem->dep_map, 1, _RET_IP_); - rwsem_clear_owner(sem); __up_write(sem); } @@ -126,7 +118,6 @@ void downgrade_write(struct rw_semaphore *sem) { lock_downgrade(&sem->dep_map, _RET_IP_); - rwsem_set_reader_owned(sem); __downgrade_write(sem); } @@ -140,7 +131,6 @@ void down_read_nested(struct rw_semaphore *sem, int subclass) rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_); LOCK_CONTENDED(sem, __down_read_trylock, __down_read); - rwsem_set_reader_owned(sem); } EXPORT_SYMBOL(down_read_nested); @@ -151,7 +141,6 @@ void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest) rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_); LOCK_CONTENDED(sem, __down_write_trylock, __down_write); - rwsem_set_owner(sem); } EXPORT_SYMBOL(_down_write_nest_lock); @@ -171,7 +160,6 @@ void down_write_nested(struct rw_semaphore *sem, int subclass) rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_); LOCK_CONTENDED(sem, __down_write_trylock, __down_write); - rwsem_set_owner(sem); } EXPORT_SYMBOL(down_write_nested); @@ -186,7 +174,6 @@ int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass) return -EINTR; } - rwsem_set_owner(sem); return 0; } diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h index adcc5af..612109f 100644 --- a/kernel/locking/rwsem.h +++ b/kernel/locking/rwsem.h @@ -33,15 +33,12 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem) WRITE_ONCE(sem->owner, NULL); } +/* + * This should only be called by the first reader that acquires the read lock. + */ 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); + WRITE_ONCE(sem->owner, RWSEM_READER_OWNED); } static inline bool rwsem_owner_is_writer(struct task_struct *owner) -- 1.8.3.1
