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Date: Mon, 14 Sep 2015 15:38:32 -0400
From: Waiman Long <waiman.long@hpe.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>, Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt
References: <1441996658-62854-1-git-send-email-Waiman.Long@hpe.com> <1441996658-62854-6-git-send-email-Waiman.Long@hpe.com> <20150914140051.GT18489@twins.programming.kicks-ass.net> <55F71CC8.3050306@hpe.com>
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On 09/14/2015 03:15 PM, Waiman Long wrote:
> On 09/14/2015 10:00 AM, Peter Zijlstra wrote:
>> On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
>>> This patch allows one attempt for the lock waiter to steal the lock
>>> when entering the PV slowpath.  This helps to reduce the performance
>>> penalty caused by lock waiter preemption while not having much of
>>> the downsides of a real unfair lock.
>>> @@ -415,8 +458,12 @@ static void pv_wait_head(struct qspinlock 
>>> *lock, struct mcs_spinlock *node)
>>>
>>>       for (;; waitcnt++) {
>>>           for (loop = SPIN_THRESHOLD; loop; loop--) {
>>> -            if (!READ_ONCE(l->locked))
>>> -                return;
>>> +            /*
>>> +             * Try to acquire the lock when it is free.
>>> +             */
>>> +            if (!READ_ONCE(l->locked)&&
>>> +               (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0))
>>> +                goto gotlock;
>>>               cpu_relax();
>>>           }
>>>
>> This isn't _once_, this is once per 'wakeup'. And note that interrupts
>> unrelated to the kick can equally wake the vCPU up.
>>
>
> Oh! There is a minor bug that I shouldn't need to have a second 
> READ_ONCE() call here.

Oh! I misread the diff, the code was OK.

Cheers,
Longman