linux-kernel - Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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Date:	Mon, 14 Sep 2015 15:38:32 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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
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