Re: RFC: mutex: hung tasks on SMP platforms withasm-generic/mutex-xchg.h

[Peter Zijlstra]

On Mon, 2012-08-13 at 09:35 -0400, Nicolas Pitre wrote:
> Date: Fri, 10 Aug 2012 15:22:09 +0100
> From: Will Deacon <will.deacon@xxxxxxx>
> 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@xxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Chris Mason <chris.mason@xxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> Reviewed-by: Nicolas Pitre <nico@xxxxxxxxxx> 

Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>


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


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