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

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Date:	Wed, 15 Jun 2016 09:56:59 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper
 acquire/release barrier

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@....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