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

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Message-ID: <64b4957e-34f3-cc54-94bd-7ab64334f590@redhat.com>
Date:   Tue, 22 Aug 2017 11:35:28 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
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>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On 08/21/2017 03:42 PM, Peter Zijlstra wrote:
> 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'.
>>

pn->state == vcpu_halted is the prerequisite of putting _Q_SLOW_VAL into
the lock. The order of writing vcpu_hashed into pn->state doesn't really
matter. The cmpxchg_relaxed() here should synchronize with the cmpxchg()
in pv_wait_node().

Cheers,
Longman