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:
> +void osq_unlock(struct optimistic_spin_queue **lock)
> +{
> +	struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
> +	struct optimistic_spin_queue *next;
> +
> +	/*
> +	 * Fast path for the uncontended case.
> +	 */
> +	if (likely(cmpxchg(lock, node, NULL) == node))
> +		return;

Can we can also add the following code here as I'm noticing next != NULL
is the much more likely scenario on my box:

        next = xchg(&node->next, NULL);
        if (next) {
                ACCESS_ONCE(next->locked) = 1;
                return;


> +	next = osq_wait_next(lock, node, NULL);
> +	if (next)
> +		ACCESS_ONCE(next->locked) = 1;
> +}