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:
> > > Yes, that looks fine. I'd remove that if (prev < 0) entirely though.
> > > We'll just swap a 0 for a 0 if the count wasn't < 0, or a 0 for a 1 if
> > > the mutex just got unlocked which is also fine. This is especially
> > > beneficial when a native xchg processor instruction is used.
> >
> > In fact, this suggestion isn't entirely correct either. The inner xchg
> > in this case should be -1 and not 'count'. If 'count' is 0 and the
> > mutex becomes contended in the small window between the two xchg's then
> > the contention mark would be lost again.
> >
> > In other words, I think this should look like this:
> >
> > 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.
Nicolas
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