From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755897Ab2IIXey (ORCPT ); Sun, 9 Sep 2012 19:34:54 -0400 Received: from mail-vc0-f174.google.com ([209.85.220.174]:58873 "EHLO mail-vc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755939Ab2IIXeu (ORCPT ); Sun, 9 Sep 2012 19:34:50 -0400 Date: Sun, 9 Sep 2012 19:34:47 -0400 (EDT) From: Nicolas Pitre To: Ben Hutchings cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Will Deacon , Arnd Bergmann , Chris Mason , Ingo Molnar , Peter Zijlstra , Thomas Gleixner Subject: Re: [ 25/95] mutex: Place lock in contended state after fastpath_lock failure In-Reply-To: <20120909224154.303617964@decadent.org.uk> Message-ID: References: <20120909224154.303617964@decadent.org.uk> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 9 Sep 2012, Ben Hutchings wrote: > 3.2-stable review patch. If anyone has any objections, please let me know. Once you have this patch included, could you pick up commit a76d7bd96d as well please? Thanks. > ------------------ > > From: Will Deacon > > commit 0bce9c46bf3b15f485d82d7e81dabed6ebcc24b1 upstream. > > 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 > Cc: Arnd Bergmann > Cc: Chris Mason > Cc: Ingo Molnar > Reviewed-by: Nicolas Pitre > Signed-off-by: Peter Zijlstra > Link: http://lkml.kernel.org/n/tip-6e9lrw2avczr0617fzl5vqb8@git.kernel.org > Signed-off-by: Thomas Gleixner > Signed-off-by: Ben Hutchings > --- > 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; > } > > >