linux-kernel - Re: [PATCH v3 2/7] locking/pvqspinlock: Add pending bit support

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Date:	Wed, 29 Jul 2015 16:49:58 -0400
From:	Waiman Long <waiman.long@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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>
Subject: Re: [PATCH v3 2/7] locking/pvqspinlock: Add pending bit support

On 07/27/2015 03:39 PM, Davidlohr Bueso wrote:
> On Mon, 2015-07-27 at 13:30 -0400, Waiman Long wrote:
>> The pending bit acts as a 1-slot waiting queue. So if the vCPU needs to
>> fall back to regular queuing, it needs to clear the bit.
> Right, that's what I thought. So you would also need to call
> clear_pending() as soon as the trylock/pending loop hits zero. Or am I
> missing something?
>
> /*
>   * trylock || pending
>   */
> for (;;) {
> 	...
> 	if (loop--<= 0) {
> 		clear_pending(lock);
> 		goto queue;
> 	}
> }
>
> Also, no need to check for negative loop, zero should be enough to
> breakout.

The while loop below can potentially make the loop variable become negative:


         /*
          * wait for in-progress pending->locked hand-overs
          */
         if (val == _Q_PENDING_VAL) {
                 while (((val = atomic_read(&lock->val)) == 
_Q_PENDING_VAL) &&
                         loop--)
                         cpu_relax();
         }

Cheers,
Longman
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