From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752718AbaATCdc (ORCPT ); Sun, 19 Jan 2014 21:33:32 -0500 Received: from e39.co.us.ibm.com ([32.97.110.160]:32983 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752383AbaATCd3 (ORCPT ); Sun, 19 Jan 2014 21:33:29 -0500 Date: Sun, 19 Jan 2014 18:33:22 -0800 From: "Paul E. McKenney" To: Tim Chen Cc: Ingo Molnar , Andrew Morton , Thomas Gleixner , Will Deacon , linux-kernel@vger.kernel.org, linux-mm , linux-arch@vger.kernel.org, Linus Torvalds , Waiman Long , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton , "Figo.zhang" Subject: Re: [PATCH v7 4/6] MCS Lock: Barrier corrections Message-ID: <20140120023322.GL10038@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1389917308.3138.14.camel@schen9-DESK> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1389917308.3138.14.camel@schen9-DESK> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14012002-9332-0000-0000-000002D161EA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 16, 2014 at 04:08:28PM -0800, Tim Chen wrote: > This patch corrects the way memory barriers are used in the MCS lock > with smp_load_acquire and smp_store_release fucnction. > It removes ones that are not needed. > > Note that using the smp_load_acquire/smp_store_release pair is not > sufficient to form a full memory barrier across > cpus for many architectures (except x86) for mcs_unlock and mcs_lock. > For applications that absolutely need a full barrier across multiple cpus > with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be > used after mcs_lock if a full memory barrier needs to be guaranteed. > > From: Waiman Long > Suggested-by: Michel Lespinasse > Signed-off-by: Waiman Long > Signed-off-by: Jason Low > Signed-off-by: Tim Chen And this fixes my gripes in the first patch in this series, good! Reviewed-by: Paul E. McKenney > --- > kernel/locking/mcs_spinlock.c | 18 +++++++++++++----- > 1 file changed, 13 insertions(+), 5 deletions(-) > > diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c > index 44fb092..6cdc730 100644 > --- a/kernel/locking/mcs_spinlock.c > +++ b/kernel/locking/mcs_spinlock.c > @@ -43,9 +43,12 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > return; > } > ACCESS_ONCE(prev->next) = node; > - smp_wmb(); > - /* Wait until the lock holder passes the lock down */ > - while (!ACCESS_ONCE(node->locked)) > + /* > + * Wait until the lock holder passes the lock down. > + * Using smp_load_acquire() provides a memory barrier that > + * ensures subsequent operations happen after the lock is acquired. > + */ > + while (!(smp_load_acquire(&node->locked))) > arch_mutex_cpu_relax(); > } > EXPORT_SYMBOL_GPL(mcs_spin_lock); > @@ -68,7 +71,12 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > while (!(next = ACCESS_ONCE(node->next))) > arch_mutex_cpu_relax(); > } > - ACCESS_ONCE(next->locked) = 1; > - smp_wmb(); > + /* > + * Pass lock to next waiter. > + * smp_store_release() provides a memory barrier to ensure > + * all operations in the critical section has been completed > + * before unlocking. > + */ > + smp_store_release(&next->locked, 1); > } > EXPORT_SYMBOL_GPL(mcs_spin_unlock); > -- > 1.7.11.7 > > >