RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h

[Nicolas Pitre]

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