From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756573AbXGQNx3 (ORCPT ); Tue, 17 Jul 2007 09:53:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752465AbXGQNxV (ORCPT ); Tue, 17 Jul 2007 09:53:21 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:44691 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752217AbXGQNxV (ORCPT ); Tue, 17 Jul 2007 09:53:21 -0400 Date: Tue, 17 Jul 2007 06:53:18 -0700 From: "Paul E. McKenney" To: Andrew Morton Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, dipankar@in.ibm.com, josht@linux.vnet.ibm.com Subject: Re: [PATCH] Immunize rcu_dereference() against crazy compiler writers Message-ID: <20070717135318.GC8763@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20070712010058.GA30869@linux.vnet.ibm.com> <20070717024600.0aab7f36.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070717024600.0aab7f36.akpm@linux-foundation.org> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 17, 2007 at 02:46:00AM -0700, Andrew Morton wrote: > On Wed, 11 Jul 2007 18:00:58 -0700 "Paul E. McKenney" wrote: > > > 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)) > > drivers/net/hamradio/bpqether.c: In function 'bpq_seq_next': > drivers/net/hamradio/bpqether.c:421: error: invalid lvalue in unary '&' This is a bug in bpqether.c, and here is a patch that compiles. I don't have the necessary hardware to test. As you might expect, I assert that the fact that the new implementation of rcu_dereference() detects improper use on rvalues is a feature. ;-) Signed-off-by: Paul E. McKenney --- bpqether.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff -urpNa -X dontdiff linux-2.6.22/drivers/net/hamradio/bpqether.c linux-2.6.22-volrcud-hamfix/drivers/net/hamradio/bpqether.c --- linux-2.6.22/drivers/net/hamradio/bpqether.c 2007-07-08 16:32:17.000000000 -0700 +++ linux-2.6.22-volrcud-hamfix/drivers/net/hamradio/bpqether.c 2007-07-17 06:46:16.000000000 -0700 @@ -413,12 +413,12 @@ static void *bpq_seq_next(struct seq_fil ++*pos; if (v == SEQ_START_TOKEN) - p = bpq_devices.next; + p = rcu_dereference(bpq_devices.next); else - p = ((struct bpqdev *)v)->bpq_list.next; + p = rcu_dereference(((struct bpqdev *)v)->bpq_list.next); return (p == &bpq_devices) ? NULL - : rcu_dereference(list_entry(p, struct bpqdev, bpq_list)); + : list_entry(p, struct bpqdev, bpq_list); } static void bpq_seq_stop(struct seq_file *seq, void *v)