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

[Waiman Long <waiman.long@hpe.com>]

On 10/13/2015 04:44 PM, Peter Zijlstra wrote:
> On Tue, Oct 13, 2015 at 04:41:41PM -0400, Waiman Long wrote:
>> 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.
> Code should be compact; its the purpose of Changelogs and comments to
> explain it if its subtle.
>
> Here you made weird code and the comments still don't explain how its
> starvation proof and the Changelog is almost empty of useful.

You are right. The current patch can't guarantee that there will be no 
lock starvation. I will make some code changes to make sure that lock 
starvation won't happen. I will also try to clean up the code and the 
comments at the same time.

Cheers,
Longman