linux-kernel - Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Message-ID: <20170814120121.GA24249@arm.com>
Date:   Mon, 14 Aug 2017 13:01:22 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

On Fri, Aug 11, 2017 at 11:06:01AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 10, 2017 at 02:18:30PM -0400, Waiman Long wrote:
> > On 08/10/2017 12:22 PM, Waiman Long wrote:
> > > On 08/10/2017 12:15 PM, Peter Zijlstra wrote:
> 
> > >> Might as well do an explicit:
> > >>
> > >> 	smp_mb__before_atomic()
> > >> 	cmpxchg_relaxed()
> > >> 	smp_mb__after_atomic()
> > >>
> > >> I suppose and not introduce new primitives.
> > 
> > I think we don't need smp_mb__after_atomic(). The read has to be fully
> > ordered, but the write part may not need it as the control dependency of
> > the old value should guard against incorrect action. Right?
> 
> You'd think that, but IIRC there was something funny about using the SC
> return flag for control dependencies. Will?

Yeah, that's right, you can't use the STXR status flag to create control
dependencies.

Will