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

Peter Zijlstra
2009-01-13T16:40:54Z

On Tue, 2009-01-13 at 17:21 +0100, Peter Zijlstra wrote:
> 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 :-).

Ok, tested only 1, but that was the one I remember lockups from -- and
that seems to be good with the cmpxchg.

Do you fancy me sending v10 or will you make that change locally?