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

[Nicolas Pitre]

On Tue, 7 Aug 2012, Will Deacon wrote:

> On Tue, Aug 07, 2012 at 06:14:36PM +0100, Nicolas Pitre wrote:
> > On Tue, 7 Aug 2012, Will Deacon wrote:
> > > The symptoms are that a bunch of hackbench tasks are left waiting on an
> > > unlocked mutex and therefore never get woken up to claim it. I think this
> > > boils down to the following sequence:
> > > 
> > > 
> > >         Task A        Task B        Task C        Lock value
> > > 0                                                     1
> > > 1       lock()                                        0
> > > 2                     lock()                          0
> > > 3                     spin(A)                         0
> > > 4       unlock()                                      1
> > > 5                                   lock()            0
> > > 6                     cmpxchg(1,0)                    0
> > > 7                     contended()                    -1
> > > 8       lock()                                        0
> > > 9       spin(C)                                       0
> > > 10                                  unlock()          1
> > > 11      cmpxchg(1,0)                                  0
> > > 12      unlock()                                      1
> > > 
> > > 
> > > At this point, the lock is unlocked, but Task B is in an uninterruptible
> > > sleep with nobody to wake it up.
> > 
> > I fail to see how the lock value would go from -1 to 0 on line 8.  How 
> > does that happen?
> 
> [...]

Forget that.  I assumed cmpxchg when it is just xchg.

(And, for that matter, I'm even the original author for some of that 
 code.: http://lkml.org/lkml/2005/12/26/83).

Back to thinking.


Nicolas