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

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Message-ID: <5761B4B7.3070102@hpe.com>
Date:	Wed, 15 Jun 2016 16:04:07 -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 03:08 PM, Waiman Long wrote:
> 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.
>

BTW, when will the smp_acquire__after_ctrl_dep() patch goes into the tip 
tree? My patch will have a dependency on that when I make the change.

Cheers,
Longman