Messages in this thread Patch in this message | | | Subject | [PATCH v2] locking/rwsem: Optimize write lock by reducing operations in slowpath | From | Jason Low <> | Date | Mon, 16 May 2016 13:10:13 -0700 |
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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 <jason.low2@hp.com> Acked-by: Waiman Long<Waiman.Long@hpe.com> Acked-by: Davidlohr Bueso <dave@stgolabs.net> --- 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 df4dcb8..956f088 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -255,17 +255,28 @@ 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; } -- 2.1.4
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