From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tim Chen Subject: [PATCH v7 4/6] MCS Lock: Barrier corrections Date: Thu, 16 Jan 2014 16:08:28 -0800 Message-ID: <1389917308.3138.14.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 , "Paul E.McKenney" , Will Deacon 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 , Tim Chen , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton , "Figo.zhang" List-Id: linux-arch.vger.kernel.org This patch corrects the way memory barriers are used in the MCS lock with smp_load_acquire and smp_store_release fucnction. It removes ones that are not needed. Note that using the smp_load_acquire/smp_store_release pair is not sufficient to form a full memory barrier across cpus for many architectures (except x86) for mcs_unlock and mcs_lock. For applications that absolutely need a full barrier across multiple cpus with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be used after mcs_lock if a full memory barrier needs to be guaranteed. From: Waiman Long Suggested-by: Michel Lespinasse Signed-off-by: Waiman Long Signed-off-by: Jason Low Signed-off-by: Tim Chen --- kernel/locking/mcs_spinlock.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c index 44fb092..6cdc730 100644 --- a/kernel/locking/mcs_spinlock.c +++ b/kernel/locking/mcs_spinlock.c @@ -43,9 +43,12 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) return; } ACCESS_ONCE(prev->next) = node; - smp_wmb(); - /* Wait until the lock holder passes the lock down */ - while (!ACCESS_ONCE(node->locked)) + /* + * Wait until the lock holder passes the lock down. + * Using smp_load_acquire() provides a memory barrier that + * ensures subsequent operations happen after the lock is acquired. + */ + while (!(smp_load_acquire(&node->locked))) arch_mutex_cpu_relax(); } EXPORT_SYMBOL_GPL(mcs_spin_lock); @@ -68,7 +71,12 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); } - ACCESS_ONCE(next->locked) = 1; - smp_wmb(); + /* + * Pass lock to next waiter. + * smp_store_release() provides a memory barrier to ensure + * all operations in the critical section has been completed + * before unlocking. + */ + smp_store_release(&next->locked, 1); } EXPORT_SYMBOL_GPL(mcs_spin_unlock); -- 1.7.11.7 -- 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]:17383 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752035AbaAQAIb (ORCPT ); Thu, 16 Jan 2014 19:08:31 -0500 Subject: [PATCH v7 4/6] MCS Lock: Barrier corrections From: Tim Chen In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Thu, 16 Jan 2014 16:08:28 -0800 Message-ID: <1389917308.3138.14.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 , "Paul E.McKenney" , Will Deacon 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 , Tim Chen , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton , "Figo.zhang" Message-ID: <20140117000828.G6xEocyzAgi7lH3frUZI1rJTiY3918Pcye45KDrWSH0@z> This patch corrects the way memory barriers are used in the MCS lock with smp_load_acquire and smp_store_release fucnction. It removes ones that are not needed. Note that using the smp_load_acquire/smp_store_release pair is not sufficient to form a full memory barrier across cpus for many architectures (except x86) for mcs_unlock and mcs_lock. For applications that absolutely need a full barrier across multiple cpus with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be used after mcs_lock if a full memory barrier needs to be guaranteed. From: Waiman Long Suggested-by: Michel Lespinasse Signed-off-by: Waiman Long Signed-off-by: Jason Low Signed-off-by: Tim Chen --- kernel/locking/mcs_spinlock.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c index 44fb092..6cdc730 100644 --- a/kernel/locking/mcs_spinlock.c +++ b/kernel/locking/mcs_spinlock.c @@ -43,9 +43,12 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) return; } ACCESS_ONCE(prev->next) = node; - smp_wmb(); - /* Wait until the lock holder passes the lock down */ - while (!ACCESS_ONCE(node->locked)) + /* + * Wait until the lock holder passes the lock down. + * Using smp_load_acquire() provides a memory barrier that + * ensures subsequent operations happen after the lock is acquired. + */ + while (!(smp_load_acquire(&node->locked))) arch_mutex_cpu_relax(); } EXPORT_SYMBOL_GPL(mcs_spin_lock); @@ -68,7 +71,12 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); } - ACCESS_ONCE(next->locked) = 1; - smp_wmb(); + /* + * Pass lock to next waiter. + * smp_store_release() provides a memory barrier to ensure + * all operations in the critical section has been completed + * before unlocking. + */ + smp_store_release(&next->locked, 1); } EXPORT_SYMBOL_GPL(mcs_spin_unlock); -- 1.7.11.7