From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f52.google.com (mail-pb0-f52.google.com [209.85.160.52]) by kanga.kvack.org (Postfix) with ESMTP id A0D796B01F2 for ; Fri, 8 Nov 2013 14:52:44 -0500 (EST) Received: by mail-pb0-f52.google.com with SMTP id rr4so2566184pbb.25 for ; Fri, 08 Nov 2013 11:52:44 -0800 (PST) Received: from psmtp.com ([74.125.245.188]) by mx.google.com with SMTP id dj3si7569659pbc.310.2013.11.08.11.52.42 for ; Fri, 08 Nov 2013 11:52:43 -0800 (PST) Subject: [PATCH v5 4/4] MCS Lock: Barrier corrections From: Tim Chen In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Fri, 08 Nov 2013 11:52:38 -0800 Message-ID: <1383940358.11046.417.camel@schen9-DESK> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.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" From: Waiman Long 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. It uses architecture specific load-acquire and store-release primitives for synchronization, if available. Generic implementations are provided in case they are not defined even though they may not be optimal. These generic implementation could be removed later on once changes are made in all the relevant header files. Suggested-by: Michel Lespinasse Signed-off-by: Waiman Long Signed-off-by: Jason Low Signed-off-by: Tim Chen --- kernel/locking/mcs_spinlock.c | 48 +++++++++++++++++++++++++++++++++++------ 1 files changed, 41 insertions(+), 7 deletions(-) diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c index b6f27f8..df5c167 100644 --- a/kernel/locking/mcs_spinlock.c +++ b/kernel/locking/mcs_spinlock.c @@ -23,6 +23,31 @@ #endif /* + * Fall back to use the regular atomic operations and memory barrier if + * the acquire/release versions are not defined. + */ +#ifndef xchg_acquire +# define xchg_acquire(p, v) xchg(p, v) +#endif + +#ifndef smp_load_acquire +# define smp_load_acquire(p) \ + ({ \ + typeof(*p) __v = ACCESS_ONCE(*(p)); \ + smp_mb(); \ + __v; \ + }) +#endif + +#ifndef smp_store_release +# define smp_store_release(p, v) \ + do { \ + smp_mb(); \ + ACCESS_ONCE(*(p)) = v; \ + } while (0) +#endif + +/* * 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 @@ -37,15 +62,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) node->locked = 0; node->next = NULL; - prev = xchg(lock, node); + /* xchg() provides a memory barrier */ + prev = xchg_acquire(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)) + /* + * 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); @@ -54,7 +83,7 @@ EXPORT_SYMBOL_GPL(mcs_spin_lock); * 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) +void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) { struct mcs_spinlock *next = ACCESS_ONCE(node->next); @@ -68,7 +97,12 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod 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); -- 1.7.4.4 -- 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