From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763411AbXGLBBS (ORCPT ); Wed, 11 Jul 2007 21:01:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761779AbXGLBBD (ORCPT ); Wed, 11 Jul 2007 21:01:03 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:41772 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761186AbXGLBBB (ORCPT ); Wed, 11 Jul 2007 21:01:01 -0400 Date: Wed, 11 Jul 2007 18:00:58 -0700 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@elte.hu, dipankar@in.ibm.com, josht@linux.vnet.ibm.com, akpm@linux-foundation.org Subject: [PATCH] Immunize rcu_dereference() against crazy compiler writers Message-ID: <20070712010058.GA30869@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Turns out that compiler writers are a bit more aggressive about optimizing than one might expect. This patch prevents a number of such optimizations from messing up rcu_deference(). This is not merely a theoretical problem, as evidenced by the rmb() in mce_log(). Signed-off-by: Paul E. McKenney --- rcupdate.h | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff -urpNa -X dontdiff linux-2.6.22/include/linux/rcupdate.h linux-2.6.22-volrcud/include/linux/rcupdate.h --- linux-2.6.22/include/linux/rcupdate.h 2007-07-08 16:32:17.000000000 -0700 +++ linux-2.6.22-volrcud/include/linux/rcupdate.h 2007-07-11 17:21:09.000000000 -0700 @@ -217,6 +217,18 @@ extern int rcu_needs_cpu(int cpu); local_bh_enable(); \ } while(0) +/* + * Prevent the compiler from merging or refetching accesses. The compiler + * is also forbidden from reordering successive instances of ACCESS_ONCE(), + * but only when the compiler is aware of some particular ordering. One way + * to make the compiler aware of ordering is to put the two invocations of + * ACCESS_ONCE() in different C statements. + * + * This macro does absolutely -nothing- to prevent the CPU from reordering, + * merging, or refetching absolutely anything at any time. + */ +#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) + /** * rcu_dereference - fetch an RCU-protected pointer in an * RCU read-side critical section. This pointer may later @@ -228,7 +240,7 @@ extern int rcu_needs_cpu(int cpu); */ #define rcu_dereference(p) ({ \ - typeof(p) _________p1 = p; \ + typeof(p) _________p1 = ACCESS_ONCE(p); \ smp_read_barrier_depends(); \ (_________p1); \ })