[PATCH v2] locking/rwsem: Optimize write lock by reducing operations in slowpath

From: Jason Low
Date: Mon May 16 2016 - 16:11:00 EST


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@xxxxxx>
Acked-by: Waiman Long<Waiman.Long@xxxxxxx>
Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
---
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