Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2009-01-09 at 10:59 -0500, Steven Rostedt wrote:

> > 
> > Adding that blocking on !owner utterly destroys everything.
> 
> I was going to warn you about that ;-)
> 
> Without the check for !owner, you are almost guaranteed to go to sleep 
> every time. Here's why:
> 
> You are spinning and thus have a hot cache on that CPU.
> 
> The owner goes to unlock but will be in a cold cache. It sets lock->owner 
> to NULL, but is still in cold cache so it is a bit slower.
> 
> Once the spinner sees the NULL, it shoots out of the spin but sees the 
> lock is still not available then goes to sleep. All before the owner could 
> release it. This could probably happen at every contention. Thus, you lose 
> the benefit of spinning. You probably make things worse because you add a 
> spin before every sleep.

Which is why I changed the inner loop to:

  l_owner = ACCESS_ONCE(lock->owner)
  if (l_owner && l_owner != owner)
    break

So that that would continue spinning.