From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v5 2/4] MCS Lock: optimizations and extra comments Date: Tue, 19 Nov 2013 11:13:10 -0800 Message-ID: <20131119191310.GO4138@linux.vnet.ibm.com> References: <1383940325.11046.415.camel@schen9-DESK> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1383940325.11046.415.camel@schen9-DESK> Sender: owner-linux-mm@kvack.org 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 , Raghavendra K T , George Spelvin , "H. Peter Anvin" List-Id: linux-arch.vger.kernel.org On Fri, Nov 08, 2013 at 11:52:05AM -0800, Tim Chen wrote: > From: Jason Low > > 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 > --- > 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; Agreed, no one looks at this field in this case, so no need to initialize it, unless for debug purposes. > 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)) Agreed here as well. Takes a narrow race to hit this. So, did your testing exercise this path? If the answer is "yes", and if the issues that I called out in patch #1 are resolved: Reviewed-by: Paul E. McKenney > return; > /* Wait until the next pointer is set */ > while (!(next = ACCESS_ONCE(node->next))) > -- > 1.7.4.4 > > > -- 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 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e38.co.us.ibm.com ([32.97.110.159]:57924 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752698Ab3KSTNU (ORCPT ); Tue, 19 Nov 2013 14:13:20 -0500 Received: from /spool/local by e38.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 19 Nov 2013 12:13:19 -0700 Date: Tue, 19 Nov 2013 11:13:10 -0800 From: "Paul E. McKenney" Subject: Re: [PATCH v5 2/4] MCS Lock: optimizations and extra comments Message-ID: <20131119191310.GO4138@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1383940325.11046.415.camel@schen9-DESK> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1383940325.11046.415.camel@schen9-DESK> Sender: linux-arch-owner@vger.kernel.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 , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton , Will Deacon , "Figo.zhang" Message-ID: <20131119191310.aSXbOegWvgBdg-GcqVWBzH3txoA-U0DqM31GB2AMY-s@z> On Fri, Nov 08, 2013 at 11:52:05AM -0800, Tim Chen wrote: > From: Jason Low > > 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 > --- > 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; Agreed, no one looks at this field in this case, so no need to initialize it, unless for debug purposes. > 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)) Agreed here as well. Takes a narrow race to hit this. So, did your testing exercise this path? If the answer is "yes", and if the issues that I called out in patch #1 are resolved: Reviewed-by: Paul E. McKenney > return; > /* Wait until the next pointer is set */ > while (!(next = ACCESS_ONCE(node->next))) > -- > 1.7.4.4 > > >