[RFC PATCH-tip/locking/core v3 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

[Waiman Long]

The osq_lock() and osq_unlock() function may not provide the necessary
acquire and release barrier in some cases. This patch makes sure
that the proper barriers are provided when osq_lock() is successful
or when osq_unlock() is called.

The change on the unlock side is more for documentation purpose than
is actually needed.

Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 kernel/locking/osq_lock.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..d957b90 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -124,6 +124,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 
 		cpu_relax_lowlatency();
 	}
+	/*
+	 * Add an acquire memory barrier for pairing with the release barrier
+	 * in unlock.
+	 */
+	smp_acquire__after_ctrl_dep();
 	return true;
 
 unqueue:
@@ -198,7 +203,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 	 * Second most likely case.
 	 */
 	node = this_cpu_ptr(&osq_node);
-	next = xchg(&node->next, NULL);
+	next = xchg_release(&node->next, NULL);
 	if (next) {
 		WRITE_ONCE(next->locked, 1);
 		return;
-- 
1.7.1