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

[Jason Low]

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