linux-kernel - Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

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Date:	Tue, 13 Oct 2015 16:41:41 -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 v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

On 10/13/2015 02:23 PM, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
>>   	for (;; waitcnt++) {
>> +		loop = SPIN_THRESHOLD;
>> +		while (loop) {
>> +			/*
>> +			 * Spin until the lock is free
>> +			 */
>> +			for (; loop&&  READ_ONCE(l->locked); loop--)
>> +				cpu_relax();
>> +			/*
>> +			 * Seeing the lock is free, this queue head vCPU is
>> +			 * the rightful next owner of the lock. However, the
>> +			 * lock may have just been stolen by another task which
>> +			 * has entered the slowpath. So we need to use atomic
>> +			 * operation to make sure that we really get the lock.
>> +			 * Otherwise, we have to wait again.
>> +			 */
>> +			if (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)
>> +				goto gotlock;
>>   		}
> 		for (loop = SPIN_THRESHOLD; loop; --loop) {
> 			if (!READ_ONCE(l->locked)&&
> 			cmpxchg(&l->locked, 0, _Q_LOCKED_VA) == 0)
> 				goto gotlock;
>
> 			cpu_relax();
> 		}
>

This was the code that I used in my original patch, but it seems to 
confuse you about doing too many lock stealing. So I separated it out to 
make my intention more explicit. I will change it back to the old code.

Cheers,
Longman


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