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

On Tue, 7 Aug 2012, Will Deacon wrote:

On Tue, Aug 07, 2012 at 06:14:36PM +0100, Nicolas Pitre wrote:

quoted

On Tue, 7 Aug 2012, Will Deacon wrote:

quoted

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