Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754730Ab3I0Wqx (ORCPT ); Fri, 27 Sep 2013 18:46:53 -0400 Received: from mga09.intel.com ([134.134.136.24]:28832 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753073Ab3I0Wqu (ORCPT ); Fri, 27 Sep 2013 18:46:50 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.90,996,1371106800"; d="scan'208";a="410654084" Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file From: Tim Chen To: paulmck@linux.vnet.ibm.com, Jason Low Cc: 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 In-Reply-To: <20130927203858.GB9093@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> Content-Type: text/plain; charset="UTF-8" Date: Fri, 27 Sep 2013 15:46:45 -0700 Message-ID: <1380322005.3467.186.camel@schen9-DESK> Mime-Version: 1.0 X-Mailer: Evolution 2.32.3 (2.32.3-1.fc14) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4458 Lines: 131 On Fri, 2013-09-27 at 13:38 -0700, Paul E. McKenney wrote: > On Fri, Sep 27, 2013 at 12:38:53PM -0700, Tim Chen wrote: > > 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. > > I don't immediately see the need for the smp_wmb() in either case. Thinking a bit more, the following could happen in Jason's initial patch proposal. In this case variable "prev" referenced by CPU1 points to "node" referenced by CPU2 CPU 1 (calling lock) CPU 2 (calling unlock) ACCESS_ONCE(prev->next) = node *next = ACCESS_ONCE(node->next); ACCESS_ONCE(next->locked) = 1; node->locked = 0; Then we will be spinning forever on CPU1 as we overwrite the lock passed from CPU2 before we check it. The original code assign "node->locked = 0" before xchg does not have this issue. Doing the following change of moving smp_wmb immediately after node->locked assignment (suggested by Jason) node->locked = 0; smp_wmb(); ACCESS_ONCE(prev->next) = node; could avoid the problem, but will need closer scrutiny to see if there are other pitfalls if wmb happen before ACCESS_ONCE(prev->next) = node; > > > > > > + /* Wait until the lock holder passes the lock down */ > > > > + while (!ACCESS_ONCE(node->locked)) > > > > + arch_mutex_cpu_relax(); > > However, 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. Is it necessary to add a memory barrier after acquiring the lock if the previous lock holder execute smp_wmb before passing the lock? Thanks. Tim -- 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/