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

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Message-ID: <20170822104014.GA27058@arm.com>
Date:   Tue, 22 Aug 2017 11:40:14 +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>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

On Mon, Aug 21, 2017 at 09:25:50PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 21, 2017 at 07:00:02PM +0100, Will Deacon wrote:
> > > No, I meant _from_ the LL load, not _to_ a later load.
> > 
> > Sorry, I'm still not following enough to give you a definitive answer on
> > that. Could you give an example, please? These sequences usually run in
> > a loop, so the conditional branch back (based on the status flag) is where
> > the read-after-read comes in.
> > 
> > Any control dependencies from the loaded data exist regardless of the status
> > flag.
> 
> Basically what Waiman ended up doing, something like:
> 
>         if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
>                 return;
> 
>         WRITE_ONCE(l->locked, _Q_SLOW_VAL);
> 
> Where the STORE depends on the LL value being 'complete'.

Yup, that's ordered as you would expect. Thanks for the example!

Will