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

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Message-ID: <20160615171250.GO30921@twins.programming.kicks-ass.net>
Date:	Wed, 15 Jun 2016 19:12:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Waiman Long <Waiman.Long@....com>, 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 Wed, Jun 15, 2016 at 09:56:59AM -0700, Davidlohr Bueso wrote:
> On Tue, 14 Jun 2016, Waiman Long wrote:

> >+++ 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:

I think he'll go rely on it later on.

In any case, its fairly simple to cure, just add
smp_acquire__after_ctrl_dep() at the end. If we bail because
need_resched() we don't need the acquire I think.