From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753959AbaBQVgN (ORCPT ); Mon, 17 Feb 2014 16:36:13 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:57905 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751330AbaBQVgI (ORCPT ); Mon, 17 Feb 2014 16:36:08 -0500 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, oleg@redhat.com, sbw@mit.edu, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 06/12] rcu: Define rcu_assign_pointer() in terms of smp_store_release() Date: Mon, 17 Feb 2014 13:35:53 -0800 Message-Id: <1392672959-6386-6-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1392672959-6386-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20140217213533.GA5387@linux.vnet.ibm.com> <1392672959-6386-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14021721-7164-0000-0000-000006267592 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" The new smp_store_release() function provides better guarantees than did rcu_assign_pointer(), and potentially less overhead on some architectures. The guarantee that smp_store_release() provides that rcu_assign_pointer() does that is obscure, but its lack could cause considerable confusion. This guarantee is illustrated by the following code fragment: struct foo { int a; int b; int c; struct foo *next; }; struct foo foo1; struct foo foo2; struct foo __rcu *foop; ... foo2.a = 1; foo2.b = 2; BUG_ON(foo2.c); rcu_assign_pointer(foop, &foo); ... fp = rcu_dereference(foop); fp.c = 3; The current rcu_assign_pointer() semantics permit the BUG_ON() to trigger because rcu_assign_pointer()'s smp_wmb() is not guaranteed to order prior reads against later writes. This commit therefore upgrades rcu_assign_pointer() from smp_wmb() to smp_store_release() to avoid this counter-intuitive outcome. Signed-off-by: Paul E. McKenney --- include/linux/rcupdate.h | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 278a9da69ec4..32decf1a9c6c 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -44,6 +44,7 @@ #include #include #include +#include #ifdef CONFIG_RCU_TORTURE_TEST extern int rcutorture_runnable; /* for sysctl */ @@ -580,12 +581,7 @@ static inline void rcu_preempt_sleep_check(void) * please be careful when making changes to rcu_assign_pointer() and the * other macros that it invokes. */ -#define rcu_assign_pointer(p, v) \ - do { \ - smp_wmb(); \ - ACCESS_ONCE(p) = RCU_INITIALIZER(v); \ - } while (0) - +#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v)) /** * rcu_access_pointer() - fetch RCU pointer with no dereferencing -- 1.8.1.5