Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance to spin_on_owner if need_resched() triggered while queued

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

On Sun, 2014-02-02 at 22:12 +0100, Peter Zijlstra wrote:
> The way I wrote that same loop in step-B, is:
> 
> 
> 	for (;;) {
> 		if (*lock == node && cmpxchg(lock, node, prev) == node)
> 			return
> 
> 		next = xchg(&node->next, NULL); /* B -> A */
> 		if (next)
> 			break;
> 
> 		arch_mutex_cpu_relax();
> 	}
> 
> I suppose we can make that something like:
> 
> 
> 	if (node->next) {
> 		next = xchg(&node->next, NULL);
> 		if (next)
> 			break
> 	}
> 
> To avoid the xchg on every loop.

Ah yes, we want to use xchg() on &node->next.

Since the cmpxchg() is now in a loop in the unlock function, an
additional (*lock == node) check before the cmpxchg() would also be nice
to avoid spinning on cmpxchg() there too.