From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756619Ab3KFVrh (ORCPT ); Wed, 6 Nov 2013 16:47:37 -0500 Received: from mga02.intel.com ([134.134.136.20]:11331 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752930Ab3KFVrf (ORCPT ); Wed, 6 Nov 2013 16:47:35 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.93,535,1378882800"; d="scan'208";a="430938086" Subject: Re: [PATCH v3 2/5] MCS Lock: optimizations and extra comments From: Tim Chen To: Ingo Molnar Cc: 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 , Will Deacon , "Figo.zhang" In-Reply-To: <1383773824.11046.354.camel@schen9-DESK> References: <1383773824.11046.354.camel@schen9-DESK> Content-Type: text/plain; charset="UTF-8" Date: Wed, 06 Nov 2013 13:47:30 -0800 Message-ID: <1383774450.11046.361.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 On Wed, 2013-11-06 at 13:37 -0800, Tim Chen wrote: > Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node > check in mcs_spin_unlock() likely() as it is likely that a race did not occur > most of the time. > > Also add in more comments describing how the local node is used in MCS locks. > > Reviewed-by: Tim Chen > Signed-off-by: Jason Low > Signed-off-by: Tim Chen Should be Acked-by: Tim Chen . My fat fingers accidentally added my signed off for all patches. Tim > --- > 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 b5de3b0..96f14299 100644 > --- a/include/linux/mcs_spinlock.h > +++ b/include/linux/mcs_spinlock.h > @@ -18,6 +18,12 @@ struct mcs_spinlock { > }; > > /* > + * In order to acquire the lock, the caller should declare a local node and > + * pass a reference of the node to this function in addition to the lock. > + * If the lock has already been acquired, then this will proceed to spin > + * on this node->locked until the previous lock holder sets the node->locked > + * in mcs_spin_unlock(). > + * > * We don't inline mcs_spin_lock() so that perf can correctly account for the > * time spent in this lock function. > */ > @@ -33,7 +39,6 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > prev = xchg(lock, node); > if (likely(prev == NULL)) { > /* Lock acquired */ > - node->locked = 1; > return; > } > ACCESS_ONCE(prev->next) = node; > @@ -43,6 +48,10 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > arch_mutex_cpu_relax(); > } > > +/* > + * Releases the lock. The caller should pass in the corresponding node that > + * was used to acquire the lock. > + */ > static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > { > struct mcs_spinlock *next = ACCESS_ONCE(node->next); > @@ -51,7 +60,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod > /* > * Release the lock by setting it to NULL > */ > - if (cmpxchg(lock, node, NULL) == node) > + if (likely(cmpxchg(lock, node, NULL) == node)) > return; > /* Wait until the next pointer is set */ > while (!(next = ACCESS_ONCE(node->next)))