linux-kernel - Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive spinning

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Message-ID: <20140226092210.GH18404@twins.programming.kicks-ass.net>
Date:	Wed, 26 Feb 2014 10:22:10 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	linux-kernel@...r.kernel.org, Waiman Long <waiman.long@...com>,
	mingo@...nel.org, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	andi@...stfloor.org, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive
 spinning

On Tue, Feb 25, 2014 at 11:56:19AM -0800, Jason Low wrote:
> On Mon, 2014-02-10 at 20:58 +0100, Peter Zijlstra wrote:
> 
> > +unqueue:
> > +	/*
> > +	 * Step - A  -- stabilize @prev
> > +	 *
> > +	 * Undo our @prev->next assignment; this will make @prev's
> > +	 * unlock()/unqueue() wait for a next pointer since @lock points to us
> > +	 * (or later).
> > +	 */
> > +
> > +	for (;;) {
> > +		if (prev->next == node &&
> > +		    cmpxchg(&prev->next, node, NULL) == node)
> > +			break;
> > +
> > +		/*
> > +		 * We can only fail the cmpxchg() racing against an unlock(),
> > +		 * in which case we should observe @node->locked becomming
> > +		 * true.
> > +		 */
> > +		if (smp_load_acquire(&node->locked))
> > +			return true;

I've stuck on in right about here. So that we don't unduly delay the
cmpxchg() after the load of prev.

> > +
> > +		/*
> > +		 * Or we race against a concurrent unqueue()'s step-B, in which
> > +		 * case its step-C will write us a new @node->prev pointer.
> > +		 */
> > +		prev = ACCESS_ONCE(node->prev);
> 
> Should we also add an arch_mutex_cpu_relax() to this loop?
> 
> 
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