Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756072Ab2HJQKx (ORCPT ); Fri, 10 Aug 2012 12:10:53 -0400 Received: from relais.videotron.ca ([24.201.245.36]:48924 "EHLO relais.videotron.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753283Ab2HJQKu (ORCPT ); Fri, 10 Aug 2012 12:10:50 -0400 MIME-version: 1.0 Content-transfer-encoding: 7BIT Content-type: TEXT/PLAIN; CHARSET=US-ASCII Date: Fri, 10 Aug 2012 12:10:42 -0400 (EDT) From: Nicolas Pitre To: Will Deacon Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Arnd Bergmann , Thomas Gleixner , Chris Mason , Ingo Molnar , stable@vger.kernel.org Subject: Re: [PATCH] mutex: place lock in contended state after fastpath_lock failure In-reply-to: <1344608529-2377-1-git-send-email-will.deacon@arm.com> Message-id: References: <1344608529-2377-1-git-send-email-will.deacon@arm.com> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3589 Lines: 95 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 > Cc: Thomas Gleixner > Cc: Chris Mason > Cc: Ingo Molnar > Cc: Nicolas Pitre > Cc: > Signed-off-by: Will Deacon Reviewed-by: Nicolas Pitre > --- > > 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/