Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

[Davidlohr Bueso <dave@stgolabs.net>]

On Tue, 14 Jun 2016, Waiman Long wrote:

>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.
>
>Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
>---
> kernel/locking/osq_lock.c |    4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
>index 05a3785..7dd4ee5 100644
>--- a/kernel/locking/osq_lock.c
>+++ b/kernel/locking/osq_lock.c
>@@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> 	 * cmpxchg in an attempt to undo our queueing.
> 	 */
>
>-	while (!READ_ONCE(node->locked)) {
>+	while (!smp_load_acquire(&node->locked)) {

Hmm this being a polling path, that barrier can get pretty expensive and
last I checked it was unnecessary:

036cc30c6b6 (locking/osq: No need for load/acquire when acquire-polling)

Thanks,
Davidlohr