From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz() Date: Sun, 16 Mar 2008 18:08:22 -0700 Message-ID: <20080317010821.GA29875@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: rostedt@goodmis.org, linux-rt-users@vger.kernel.org, mingo@elte.hu, ego@in.ibm.com, dipankar@in.ibm.com, tytso@us.ibm.com, dvhltc@us.ibm.com, oleg@tv-sign.ru, akpm@linux-foundation.org, josh@freedesktop.org, tglx@linutronix.de, niv@us.ibm.com To: linux-kernel@vger.kernel.org Return-path: Received: from e33.co.us.ibm.com ([32.97.110.151]:42810 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751476AbYCQBI0 (ORCPT ); Sun, 16 Mar 2008 21:08:26 -0400 Content-Disposition: inline Sender: linux-rt-users-owner@vger.kernel.org List-ID: Hello! In the process of writing up the mechanical proof of correctness for the dynticks/preemptable-RCU interface, I noticed misplaced memory barriers in rcu_enter_nohz() and rcu_exit_nohz(). This patch puts them in the right place and adds a comment. The key thing to keep in mind is that rcu_enter_nohz() is -exiting- the mode that can legally execute RCU read-side critical sections. The memory barrier must be between any potential RCU read-side critical sections and the increment of the per-CPU dynticks_progress_counter, and thus must come -before- this increment. And vice versa for rcu_exit_nohz(). The locking in the scheduler is probably saving us for the moment. Signed-off-by: Paul E. McKenney --- rcupreempt.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcupreempt.h linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h --- linux-2.6.25-rc6/include/linux/rcupreempt.h 2008-03-16 17:45:16.000000000 -0700 +++ linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h 2008-03-16 17:59:24.000000000 -0700 @@ -87,15 +87,15 @@ DECLARE_PER_CPU(long, dynticks_progress_ static inline void rcu_enter_nohz(void) { + mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */ __get_cpu_var(dynticks_progress_counter)++; WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1); - mb(); } static inline void rcu_exit_nohz(void) { - mb(); __get_cpu_var(dynticks_progress_counter)++; + mb(); /* CPUs seeing ++ must see subsequent RCU read-side crit sects */ WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1)); }