Re: [PATCH] mutex: place lock in contended state after fastpath_lock failure

[Nicolas Pitre <nico@fluxnic.net>]

On Fri, 10 Aug 2012, Will Deacon wrote:

> 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 <tglx@linutronix.de>
> Cc: Chris Mason <chris.mason@fusionio.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Nicolas Pitre <nico@fluxnic.net>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

Reviewed-by: Nicolas Pitre <nico@linaro.org>

> ---
> 
> Nico: Can I add your S-o-B to this please? Also, preliminary benchmarks
>       are now showing a slight performance improvement on A15 if I use
>       the -dec variant rather than -xchg. I'll follow up with a patch
>       once I've got more numbers.

Good.


> 
>  include/asm-generic/mutex-xchg.h |   11 +++++++++--
>  1 files 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;
>  }
>  
> -- 
> 1.7.4.1
>