From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qe0-f43.google.com (mail-qe0-f43.google.com [209.85.128.43]) by kanga.kvack.org (Postfix) with ESMTP id CC42C6B0036 for ; Sun, 19 Jan 2014 21:33:28 -0500 (EST) Received: by mail-qe0-f43.google.com with SMTP id nc12so6036432qeb.30 for ; Sun, 19 Jan 2014 18:33:28 -0800 (PST) Received: from e39.co.us.ibm.com (e39.co.us.ibm.com. [32.97.110.160]) by mx.google.com with ESMTPS id t101si6055966qge.151.2014.01.19.18.33.27 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Sun, 19 Jan 2014 18:33:28 -0800 (PST) Received: from /spool/local by e39.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sun, 19 Jan 2014 19:33:27 -0700 Received: from b03cxnp08028.gho.boulder.ibm.com (b03cxnp08028.gho.boulder.ibm.com [9.17.130.20]) by d03dlp03.boulder.ibm.com (Postfix) with ESMTP id 739FE19D8041 for ; Sun, 19 Jan 2014 19:33:16 -0700 (MST) Received: from d03av06.boulder.ibm.com (d03av06.boulder.ibm.com [9.17.195.245]) by b03cxnp08028.gho.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id s0K2XPoR62193906 for ; Mon, 20 Jan 2014 03:33:25 +0100 Received: from d03av06.boulder.ibm.com (loopback [127.0.0.1]) by d03av06.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id s0K2af2x010048 for ; Sun, 19 Jan 2014 19:36:43 -0700 Date: Sun, 19 Jan 2014 18:33:22 -0800 From: "Paul E. McKenney" 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> Sender: owner-linux-mm@kvack.org List-ID: 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" 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 > > > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org