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 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.

That's fine, and the owner will put 1 back when it unlocks it as well as 
processing the wait queue which is what we need.

> I'll add a commit message to the above and re-post if that's ok?

Sure.  Don't forget to update __mutex_fastpath_lock_retval() as well.


Nicolas