Re: [PATCH -v9][RFC] mutex: implement adaptive spinning

Peter Zijlstra
2009-01-13T16:21:50Z

On Tue, 2009-01-13 at 08:16 -0800, Linus Torvalds wrote:
>
> On Tue, 13 Jan 2009, Peter Zijlstra wrote:
> >
> > Change mutex contention behaviour such that it will sometimes busy wait on
> > acquisition - moving its behaviour closer to that of spinlocks.
>
> Okey, dokey. Looks reasonable, but I wonder if this part came from v8 and
> wasn't intentional:
>
> > + if (atomic_xchg(&lock->count, -1) == 1) {
> > + lock_acquired(&lock->dep_map, ip);
> > + mutex_set_owner(lock);
> > + preempt_enable();
> > + return 0;
> > + }
>
> Now you're forcing the slow-path on unlock. Maybe it was intentional,
> maybe it wasn't. Did you perhaps mean
>
> if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
>
> here? I thought we agreed it was safe, if only because it should be
> equivalent to just having done "mutex_trylock()" instead of a "real" lock
> sequence.

Yes, that was an 'accident' from -v8, yes we did think the cmpxchg was
good, however I did get some spurious lockups on -v7, and I only noticed
the thing after I'd done most of the testing, so I decided to let it be
for now.

Let me put the cmpxchg back in and see if this is all still good (only
3*2*2 configs to test :-).