On Thu, 9 Aug 2012, Nicolas Pitre wrote:
> On Thu, 9 Aug 2012, Will Deacon wrote:
>
> > I think we could actually fix this entirely in mutex-xchg.h by doing
> > something in fastpath_lock similar to what we do for trylock:
> >
> >
> > diff --git a/include/asm-generic/mutex-xchg.h
> > b/include/asm-generic/mutex-xchg.h
> > index 580a6d3..c082e99 100644
> > --- a/include/asm-generic/mutex-xchg.h
> > +++ b/include/asm-generic/mutex-xchg.h
> > @@ -25,8 +25,19 @@
> > static inline void
> > __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
> > {
> > - if (unlikely(atomic_xchg(count, 0) != 1))
> > - fail_fn(count);
> > + int prev = atomic_xchg(count, 0);
> > +
> > + if (unlikely(prev != 1)) {
> > + if (prev < 0)
> > + /*
> > + * The lock was contended, so we need to restore
> > + * its original state to ensure that any waiting
> > + * tasks are woken up by the unlock slow path.
> > + */
> > + prev = atomic_xchg(count, prev);
> > + if (prev != 1)
> > + fail_fn(count);
> > + }
> > }
> >
> > What do you reckon?
>
> 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);
+ }
}
/**
Nicolas
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