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

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Date:   Mon, 14 Aug 2017 11:01:10 -0400
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 08/14/2017 08:01 AM, Will Deacon wrote:
> 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

Actually, the code sequence that I plan to use are:

        smp_mb__before_atomic();
        if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed)
            != vcpu_halted)
                return;

        WRITE_ONCE(l->locked, _Q_SLOW_VAL);
        (void)pv_hash(lock, pn);

I am planning to use the comparison of the returned value (pn->state)
again vcpu_halted as the control dependency. I don't see how the status
flag of STXR is affecting this.

Cheers,
Longman