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

From: Davidlohr Bueso <dave@stgolabs.net>
To: Waiman Long <waiman.long@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Scott J Norton <scott.norton@hp.com>,
	Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH v3 2/7] locking/pvqspinlock: Add pending bit support
Date: Mon, 27 Jul 2015 12:39:05 -0700	[thread overview]
Message-ID: <1438025945.25997.46.camel@stgolabs.net> (raw)
In-Reply-To: <55B66ACB.6010702@hp.com>

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