On Thu, Aug 09, 2012 at 07:09:02PM +0100, Nicolas Pitre wrote:
> On Thu, 9 Aug 2012, Will Deacon wrote:
> > On Thu, Aug 09, 2012 at 05:57:33PM +0100, Nicolas Pitre wrote:
> > > On Thu, 9 Aug 2012, Nicolas Pitre wrote:
> > > diff --git a/include/asm-generic/mutex-xchg.h
> > > b/include/asm-generic/mutex-xchg.h
> > > index 580a6d35c7..44a66c99c8 100644
> > > --- a/include/asm-generic/mutex-xchg.h
> > > +++ b/include/asm-generic/mutex-xchg.h
> > > @@ -25,8 +25,11 @@
> > > static inline void
> > > __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
> > > {
> > > - if (unlikely(atomic_xchg(count, 0) != 1))
> > > - fail_fn(count);
> > > + if (unlikely(atomic_xchg(count, 0) != 1)) {
> > > + /* Mark lock contention explicitly */
> > > + if (likely(atomic_xchg(count, -1) != 1))
> > > + fail_fn(count);
> > > + }
> > > }
> > >
> > > /**
> >
> > Doesn't this mean that we're no longer just swapping 0 for a 0 if the lock
> > was taken, therefore needlessly sending the current owner down the slowpath
> > on unlock?
>
> If the lock was taken, this means the count was either 0 or -1. If it
> was 1 then we just put a 0 there and we own it. But if the cound was 0
> then we should store -1 instead, which is what the inner xchg does. If
> the count was already -1 then we store -1 back. That more closely mimic
> what the atomic dec does which is what we want.
Ok, I just wasn't sure that marking the lock contended was required when it
was previously locked, given that we'll drop into spinning on the owner
anyway.
I'll add a commit message to the above and re-post if that's ok?
Will
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