Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754734Ab3I1CTz (ORCPT ); Fri, 27 Sep 2013 22:19:55 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:34991 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753946Ab3I1CTx (ORCPT ); Fri, 27 Sep 2013 22:19:53 -0400 Date: Fri, 27 Sep 2013 19:19:47 -0700 From: "Paul E. McKenney" To: Jason Low Cc: Tim Chen , Waiman Long , Ingo Molnar , Andrew Morton , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , linux-kernel@vger.kernel.org, linux-mm Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file Message-ID: <20130928021947.GF9093@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1380147049.3467.67.camel@schen9-DESK> <20130927152953.GA4464@linux.vnet.ibm.com> <1380310733.3467.118.camel@schen9-DESK> <20130927203858.GB9093@linux.vnet.ibm.com> <1380322005.3467.186.camel@schen9-DESK> <20130927230137.GE9093@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13092802-3532-0000-0000-000001C1B020 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2984 Lines: 81 On Fri, Sep 27, 2013 at 04:54:06PM -0700, Jason Low wrote: > On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney > wrote: > > Yep. The previous lock holder's smp_wmb() won't keep either the compiler > > or the CPU from reordering things for the new lock holder. They could for > > example reorder the critical section to precede the node->locked check, > > which would be very bad. > > Paul, Tim, Longman, > > How would you like the proposed changes below? Could you point me at what this applies to? I can find flaws looking at random pieces, given a little luck, but at some point I need to look at the whole thing. ;-) Thanx, Paul > --- > Subject: [PATCH] MCS: optimizations and barrier corrections > > Delete the node->locked = 1 assignment if the lock is free as it won't be used. > > Delete the smp_wmb() in mcs_spin_lock() and add a full memory barrier at the > end of the mcs_spin_lock() function. As Paul McKenney suggested, "you do need a > full memory barrier here in order to ensure that you see the effects of the > previous lock holder's critical section." And in the mcs_spin_unlock(), move the > memory barrier so that it is before the "ACCESS_ONCE(next->locked) = 1;". > > Signed-off-by: Jason Low > Signed-off-by: Paul E. McKenney > Signed-off-by: Tim Chen > --- > include/linux/mcslock.h | 7 +++---- > 1 files changed, 3 insertions(+), 4 deletions(-) > > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h > index 20fd3f0..edd57d2 100644 > --- a/include/linux/mcslock.h > +++ b/include/linux/mcslock.h > @@ -26,15 +26,14 @@ void mcs_spin_lock(struct mcs_spin_node **lock, > struct mcs_spin_node *node) > > prev = xchg(lock, node); > if (likely(prev == NULL)) { > - /* Lock acquired */ > - node->locked = 1; > + /* Lock acquired. No need to set node->locked since it > won't be used */ > 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(); > + smp_mb(); > } > > static void mcs_spin_unlock(struct mcs_spin_node **lock, struct > mcs_spin_node *node) > @@ -51,8 +50,8 @@ static void mcs_spin_unlock(struct mcs_spin_node > **lock, struct mcs_spin_node *n > while (!(next = ACCESS_ONCE(node->next))) > arch_mutex_cpu_relax(); > } > - ACCESS_ONCE(next->locked) = 1; > smp_wmb(); > + ACCESS_ONCE(next->locked) = 1; > } > > #endif > -- > 1.7.1 > -- 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/