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

[Peter Zijlstra]

On Mon, Feb 10, 2014 at 01:15:59PM -0800, Jason Low wrote:
> 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;

Is adding that really much faster than the relatively straight path
oqs_wait_next() would walk to bit the same exit?

The only reason I pulled out the above cmpxchg() is because its the
uncontended fast path, which seems like a special enough case.

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