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

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Message-ID: <1438025945.25997.46.camel@stgolabs.net>
Date:	Mon, 27 Jul 2015 12:39:05 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <waiman.long@...com>
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 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.

Thanks,
Davidlohr

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