From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tim Chen Subject: [PATCH v4 3/5] MCS Lock: Barrier corrections Date: Wed, 06 Nov 2013 17:26:52 -0800 Message-ID: <1383787612.11046.366.camel@schen9-DESK> References: Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: owner-linux-mm@kvack.org To: Ingo Molnar , Andrew Morton , Thomas Gleixner Cc: 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" , Tim Chen , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton Wi List-Id: linux-arch.vger.kernel.org This patch corrects the way memory barriers are used in the MCS lock and removes ones that are not needed. Also add comments on all barriers. 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 96f14299..93d445d 100644 --- a/include/linux/mcs_spinlock.h +++ b/include/linux/mcs_spinlock.h @@ -36,16 +36,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) node->locked = 0; node->next = NULL; + /* xchg() provides a memory barrier */ prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired */ 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(); + + /* Make sure subsequent operations happen after the lock is acquired */ + smp_rmb(); } /* @@ -58,6 +61,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod if (likely(!next)) { /* + * cmpxchg() provides a memory barrier. * Release the lock by setting it to NULL */ if (likely(cmpxchg(lock, node, NULL) == node)) @@ -65,9 +69,14 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod /* Wait until the next pointer is set */ while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); + } else { + /* + * Make sure all operations within the critical section + * happen before the lock is released. + */ + smp_wmb(); } ACCESS_ONCE(next->locked) = 1; - smp_wmb(); } #endif /* __LINUX_MCS_SPINLOCK_H */ -- 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 mga09.intel.com ([134.134.136.24]:42037 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753112Ab3KGB05 (ORCPT ); Wed, 6 Nov 2013 20:26:57 -0500 Subject: [PATCH v4 3/5] MCS Lock: Barrier corrections From: Tim Chen In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Wed, 06 Nov 2013 17:26:52 -0800 Message-ID: <1383787612.11046.366.camel@schen9-DESK> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Ingo Molnar , Andrew Morton , Thomas Gleixner Cc: 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" , Tim Chen , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton , Will Deacon , "Figo.zhang" Message-ID: <20131107012652.dkWFXdc_TytU2QptfmVc31N3bftDCn-LsQ06HP67cfE@z> This patch corrects the way memory barriers are used in the MCS lock and removes ones that are not needed. Also add comments on all barriers. 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 96f14299..93d445d 100644 --- a/include/linux/mcs_spinlock.h +++ b/include/linux/mcs_spinlock.h @@ -36,16 +36,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) node->locked = 0; node->next = NULL; + /* xchg() provides a memory barrier */ prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired */ 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(); + + /* Make sure subsequent operations happen after the lock is acquired */ + smp_rmb(); } /* @@ -58,6 +61,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod if (likely(!next)) { /* + * cmpxchg() provides a memory barrier. * Release the lock by setting it to NULL */ if (likely(cmpxchg(lock, node, NULL) == node)) @@ -65,9 +69,14 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod /* Wait until the next pointer is set */ while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); + } else { + /* + * Make sure all operations within the critical section + * happen before the lock is released. + */ + smp_wmb(); } ACCESS_ONCE(next->locked) = 1; - smp_wmb(); } #endif /* __LINUX_MCS_SPINLOCK_H */ -- 1.7.4.4