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

[will.deacon@arm.com (Will Deacon)]

Hello,

ARM recently moved to asm-generic/mutex-xchg.h for its mutex implementation
after our previous implementation was found to be missing some crucial
memory barriers. However, I'm seeing some problems running hackbench on
SMP platforms due to the way in which the MUTEX_SPIN_ON_OWNER code operates.

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.

The following patch fixes the problem by ensuring we put the lock into
the contended state if we acquire it from the spin loop on the slowpath
but I'd like to be sure that this won't cause problems with other mutex
implementations:


diff --git a/kernel/mutex.c b/kernel/mutex.c
index a307cc9..27b7887 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -170,7 +170,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                if (owner && !mutex_spin_on_owner(lock, owner))
                        break;
 
-               if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
+               if (atomic_cmpxchg(&lock->count, 1, -1) == 1) {
                        lock_acquired(&lock->dep_map, ip);
                        mutex_set_owner(lock);
                        preempt_enable();


All comments welcome.

Cheers,

Will