ACK: [Precise][SRU][PATCH 1/1] mutex: Place lock in contended state after fastpath_lock failure

[Colin Ian King]

On 24/08/12 11:47, Luis Henriques wrote:
> From: Will Deacon <will.deacon at arm.com>
>
> BugLink: http://bugs.launchpad.net/bugs/1041114
>
> 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.
>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Chris Mason <chris.mason at fusionio.com>
> Cc: Ingo Molnar <mingo at elte.hu>
> Cc: <stable at vger.kernel.org>
> Reviewed-by: Nicolas Pitre <nico at linaro.org>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra at chello.nl>
> Link: http://lkml.kernel.org/n/tip-6e9lrw2avczr0617fzl5vqb8@git.kernel.org
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> (cherry picked from commit 0bce9c46bf3b15f485d82d7e81dabed6ebcc24b1)
>
> Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
> ---
>   include/asm-generic/mutex-xchg.h |   11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
> index 580a6d3..c04e0db 100644
> --- a/include/asm-generic/mutex-xchg.h
> +++ b/include/asm-generic/mutex-xchg.h
> @@ -26,7 +26,13 @@ static inline void
>   __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
>   {
>   	if (unlikely(atomic_xchg(count, 0) != 1))
> -		fail_fn(count);
> +		/*
> +		 * We failed to acquire the lock, so mark it contended
> +		 * to ensure that any waiting tasks are woken up by the
> +		 * unlock slow path.
> +		 */
> +		if (likely(atomic_xchg(count, -1) != 1))
> +			fail_fn(count);
>   }
>
>   /**
> @@ -43,7 +49,8 @@ static inline int
>   __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
>   {
>   	if (unlikely(atomic_xchg(count, 0) != 1))
> -		return fail_fn(count);
> +		if (likely(atomic_xchg(count, -1) != 1))
> +			return fail_fn(count);
>   	return 0;
>   }
>
>