From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f49.google.com (mail-pb0-f49.google.com [209.85.160.49]) by kanga.kvack.org (Postfix) with ESMTP id 467C96B0031 for ; Fri, 27 Sep 2013 15:39:02 -0400 (EDT) Received: by mail-pb0-f49.google.com with SMTP id xb4so2970216pbc.22 for ; Fri, 27 Sep 2013 12:39:01 -0700 (PDT) Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file From: Tim Chen In-Reply-To: <20130927152953.GA4464@linux.vnet.ibm.com> References: <1380147049.3467.67.camel@schen9-DESK> <20130927152953.GA4464@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Date: Fri, 27 Sep 2013 12:38:53 -0700 Message-ID: <1380310733.3467.118.camel@schen9-DESK> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: paulmck@linux.vnet.ibm.com, Waiman Long Cc: 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 On Fri, 2013-09-27 at 08:29 -0700, Paul E. McKenney wrote: > On Wed, Sep 25, 2013 at 03:10:49PM -0700, Tim Chen wrote: > > We will need the MCS lock code for doing optimistic spinning for rwsem. > > Extracting the MCS code from mutex.c and put into its own file allow us > > to reuse this code easily for rwsem. > > > > Signed-off-by: Tim Chen > > Signed-off-by: Davidlohr Bueso > > --- > > include/linux/mcslock.h | 58 +++++++++++++++++++++++++++++++++++++++++++++++ > > kernel/mutex.c | 58 +++++----------------------------------------- > > 2 files changed, 65 insertions(+), 51 deletions(-) > > create mode 100644 include/linux/mcslock.h > > > > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h > > new file mode 100644 > > index 0000000..20fd3f0 > > --- /dev/null > > +++ b/include/linux/mcslock.h > > @@ -0,0 +1,58 @@ > > +/* > > + * MCS lock defines > > + * > > + * This file contains the main data structure and API definitions of MCS lock. > > + */ > > +#ifndef __LINUX_MCSLOCK_H > > +#define __LINUX_MCSLOCK_H > > + > > +struct mcs_spin_node { > > + struct mcs_spin_node *next; > > + int locked; /* 1 if lock acquired */ > > +}; > > + > > +/* > > + * We don't inline mcs_spin_lock() so that perf can correctly account for the > > + * time spent in this lock function. > > + */ > > +static noinline > > +void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) > > +{ > > + struct mcs_spin_node *prev; > > + > > + /* Init node */ > > + node->locked = 0; > > + node->next = NULL; > > + > > + prev = xchg(lock, node); > > + if (likely(prev == NULL)) { > > + /* Lock acquired */ > > + node->locked = 1; > > + return; > > + } > > + ACCESS_ONCE(prev->next) = node; > > + smp_wmb(); BTW, is the above memory barrier necessary? It seems like the xchg instruction already provided a memory barrier. Now if we made the changes that Jason suggested: /* Init node */ - node->locked = 0; node->next = NULL; prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired */ - node->locked = 1; return; } + node->locked = 0; ACCESS_ONCE(prev->next) = node; smp_wmb(); We are probably still okay as other cpus do not read the value of node->locked, which is a local variable. Tim > > + /* Wait until the lock holder passes the lock down */ > > + while (!ACCESS_ONCE(node->locked)) > > + arch_mutex_cpu_relax(); > > +} > > + > > +static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node) > > +{ > > + struct mcs_spin_node *next = ACCESS_ONCE(node->next); > > + > > + if (likely(!next)) { > > + /* > > + * Release the lock by setting it to NULL > > + */ > > + if (cmpxchg(lock, node, NULL) == node) > > + return; > > + /* Wait until the next pointer is set */ > > + while (!(next = ACCESS_ONCE(node->next))) > > + arch_mutex_cpu_relax(); > > + } > > + ACCESS_ONCE(next->locked) = 1; > > + smp_wmb(); > > Shouldn't the memory barrier precede the "ACCESS_ONCE(next->locked) = 1;"? > Maybe in an "else" clause of the prior "if" statement, given that the > cmpxchg() does it otherwise. > > Otherwise, in the case where the "if" conditionn is false, the critical > section could bleed out past the unlock. > > Thanx, Paul > -- 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