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

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Date:	Tue, 25 Feb 2014 11:56:19 -0800
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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;
> +
> +		/*
> +		 * 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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