[PATCH v2 4/5] rwsem/wake: check lock before do atomic update

[Tim Chen <tim.c.chen@linux.intel.com>]

Atomic update lock and roll back will cause cache bouncing in large
machine. A lock status pre-read can relieve this problem.

Suggested-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Suggested-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Alex Shi <alex.shi@intel.com>
---
 lib/rwsem.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index a8055cf..1d6e6e8 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -64,7 +64,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 	struct rwsem_waiter *waiter;
 	struct task_struct *tsk;
 	struct list_head *next;
-	long oldcount, woken, loop, adjustment;
+	long woken, loop, adjustment;
 
 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
 	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
@@ -86,6 +86,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 	if (wake_type != RWSEM_WAKE_READ_OWNED) {
 		adjustment = RWSEM_ACTIVE_READ_BIAS;
 		while (1) {
+			long oldcount;
+
+			/* A writer stole the lock. */
+			if (unlikely(sem->count < RWSEM_WAITING_BIAS))
+				return sem;
+
 			oldcount = rwsem_atomic_update(adjustment, sem)
 								- adjustment;
 			if (likely(oldcount >= RWSEM_WAITING_BIAS))
-- 
1.7.4.4