Commit-ID: c0fcb6c2d332041256dc55d8a1ec3c0a2d0befb8 Gitweb: http://git.kernel.org/tip/c0fcb6c2d332041256dc55d8a1ec3c0a2d0befb8 Author: Jason Low <[email protected]> AuthorDate: Mon, 16 May 2016 17:38:00 -0700 Committer: Ingo Molnar <[email protected]> CommitDate: Fri, 3 Jun 2016 09:47:13 +0200
locking/rwsem: Optimize write lock by reducing operations in slowpath When acquiring the rwsem write lock in the slowpath, we first try to set count to RWSEM_WAITING_BIAS. When that is successful, we then atomically add the RWSEM_WAITING_BIAS in cases where there are other tasks on the wait list. This causes write lock operations to often issue multiple atomic operations. We can instead make the list_is_singular() check first, and then set the count accordingly, so that we issue at most 1 atomic operation when acquiring the write lock and reduce unnecessary cacheline contention. Signed-off-by: Jason Low <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Acked-by: Waiman Long<[email protected]> Acked-by: Davidlohr Bueso <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Arnd Bergmann <[email protected]> Cc: Christoph Lameter <[email protected]> Cc: Fenghua Yu <[email protected]> Cc: Heiko Carstens <[email protected]> Cc: Ivan Kokshaysky <[email protected]> Cc: Jason Low <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Martin Schwidefsky <[email protected]> Cc: Matt Turner <[email protected]> Cc: Paul E. McKenney <[email protected]> Cc: Peter Hurley <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Richard Henderson <[email protected]> Cc: Terry Rudd <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Tim Chen <[email protected]> Cc: Tony Luck <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> --- kernel/locking/rwsem-xadd.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index fcbf75a..b957da7 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -261,17 +261,28 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) } EXPORT_SYMBOL(rwsem_down_read_failed); +/* + * This function must be called with the sem->wait_lock held to prevent + * race conditions between checking the rwsem wait list and setting the + * sem->count accordingly. + */ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) { /* - * Try acquiring the write lock. Check count first in order - * to reduce unnecessary expensive cmpxchg() operations. + * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS. */ - if (count == RWSEM_WAITING_BIAS && - cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, - RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { - if (!list_is_singular(&sem->wait_list)) - rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); + if (count != RWSEM_WAITING_BIAS) + return false; + + /* + * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there + * are other tasks on the wait list, we need to add on WAITING_BIAS. + */ + count = list_is_singular(&sem->wait_list) ? + RWSEM_ACTIVE_WRITE_BIAS : + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS; + + if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) { rwsem_set_owner(sem); return true; }
