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 15:08:48 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Davidlohr Bueso <dave@...olabs.net>,
	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 06/15/2016 01:12 PM, Peter Zijlstra wrote:
> 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.

Yes, I only need the acquire barrier when the locking is successful. 
Thanks for the suggestion. I will make the change accordingly.

Cheers,
Longman