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

On Mon, 2012-08-13 at 09:35 -0400, Nicolas Pitre wrote:

Date: Fri, 10 Aug 2012 15:22:09 +0100
From: Will Deacon <redacted>
Subject: [PATCH] mutex: place lock in contended state after fastpath_lock failure

ARM recently moved to asm-generic/mutex-xchg.h for its mutex
implementation after the previous implementation was found to be missing
some crucial memory barriers. However, this has revealed 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. This boils
down to the following sequence of events:

        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.

This patch fixes the problem by ensuring we put the lock into the
contended state if we fail to acquire it on the fastpath, ensuring that
any blocked waiters are woken up when the mutex is released.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <redacted>
Cc: Chris Mason <redacted>
Cc: Ingo Molnar <redacted>
Cc: <redacted>
Signed-off-by: Will Deacon <redacted>
Reviewed-by: Nicolas Pitre <redacted> 

Acked-by: Peter Zijlstra <redacted>


Will you carry this through the ARM tree or do you want me/Ingo to take
it?