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

[Jason Low <jason.low2@hp.com>]

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?