From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f172.google.com (mail-pd0-f172.google.com [209.85.192.172]) by kanga.kvack.org (Postfix) with ESMTP id 671B16B007B for ; Tue, 5 Nov 2013 13:39:35 -0500 (EST) Received: by mail-pd0-f172.google.com with SMTP id w10so8993891pde.3 for ; Tue, 05 Nov 2013 10:39:35 -0800 (PST) Received: from psmtp.com ([74.125.245.175]) by mx.google.com with SMTP id z1si14368579pbw.129.2013.11.05.10.39.33 for ; Tue, 05 Nov 2013 10:39:34 -0800 (PST) Date: Tue, 5 Nov 2013 18:37:44 +0000 From: Will Deacon Subject: Re: [PATCH v2 3/4] MCS Lock: Barrier corrections Message-ID: <20131105183744.GJ26895@mudshark.cambridge.arm.com> References: <1383673356.11046.279.camel@schen9-DESK> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1383673356.11046.279.camel@schen9-DESK> Sender: owner-linux-mm@kvack.org List-ID: To: Tim Chen Cc: Ingo Molnar , Andrew Morton , Thomas Gleixner , "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 , "Paul E.McKenney" , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton On Tue, Nov 05, 2013 at 05:42:36PM +0000, Tim Chen wrote: > This patch corrects the way memory barriers are used in the MCS lock > and removes ones that are not needed. Also add comments on all barriers. Hmm, I see that you're fixing up the barriers, but I still don't completely understand how what you have is correct. Hopefully you can help me out :) > Reviewed-by: Paul E. McKenney > Reviewed-by: Tim Chen > Signed-off-by: Jason Low > --- > include/linux/mcs_spinlock.h | 13 +++++++++++-- > 1 files changed, 11 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h > index 96f14299..93d445d 100644 > --- a/include/linux/mcs_spinlock.h > +++ b/include/linux/mcs_spinlock.h > @@ -36,16 +36,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > node->locked = 0; > node->next = NULL; > > + /* xchg() provides a memory barrier */ > prev = xchg(lock, node); > if (likely(prev == NULL)) { > /* Lock acquired */ > return; > } > ACCESS_ONCE(prev->next) = node; > - smp_wmb(); > /* Wait until the lock holder passes the lock down */ > while (!ACCESS_ONCE(node->locked)) > arch_mutex_cpu_relax(); > + > + /* Make sure subsequent operations happen after the lock is acquired */ > + smp_rmb(); Ok, so this is an smp_rmb() because we assume that stores aren't speculated, right? (i.e. the control dependency above is enough for stores to be ordered with respect to taking the lock)... > } > > /* > @@ -58,6 +61,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod > > if (likely(!next)) { > /* > + * cmpxchg() provides a memory barrier. > * Release the lock by setting it to NULL > */ > if (likely(cmpxchg(lock, node, NULL) == node)) > @@ -65,9 +69,14 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod > /* Wait until the next pointer is set */ > while (!(next = ACCESS_ONCE(node->next))) > arch_mutex_cpu_relax(); > + } else { > + /* > + * Make sure all operations within the critical section > + * happen before the lock is released. > + */ > + smp_wmb(); ...but I don't see what prevents reads inside the critical section from moving across the smp_wmb() here. What am I missing? Will -- 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