From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753551Ab3AGPuN (ORCPT ); Mon, 7 Jan 2013 10:50:13 -0500 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:47789 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753403Ab3AGPuM (ORCPT ); Mon, 7 Jan 2013 10:50:12 -0500 X-Originating-IP: 217.70.178.151 X-Originating-IP: 50.43.39.152 Date: Mon, 7 Jan 2013 07:50:02 -0800 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org Subject: Re: [PATCH tip/core/rcu 4/6] rcu: Silence compiler array out-of-bounds false positive Message-ID: <20130107155002.GA11145@leaf> References: <20130105170920.GA13766@linux.vnet.ibm.com> <1357405778-13903-1-git-send-email-paulmck@linux.vnet.ibm.com> <1357405778-13903-4-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1357405778-13903-4-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 05, 2013 at 09:09:36AM -0800, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > It turns out that gcc 4.8 warns on array indexes being out of bounds > unless it can prove otherwise. It gives this warning on some RCU > initialization code. Because this is far from any fastpath, add > an explicit check for array bounds and panic if so. This gives the > compiler enough information to figure out that the array index is never > out of bounds. > > However, if a similar false positive occurs on a fastpath, it will > probably be necessary to tell the compiler to keep its array-index > anxieties to itself. ;-) > > Markus Trippelsdorf > Signed-off-by: Paul E. McKenney > --- > kernel/rcutree.c | 4 ++++ > 1 files changed, 4 insertions(+), 0 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index d145796..e0d9815 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -2938,6 +2938,10 @@ static void __init rcu_init_one(struct rcu_state *rsp, > > BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf)); /* Fix buf[] init! */ > > + /* Silence gcc 4.8 warning about array index out of range. */ > + if (rcu_num_lvls > RCU_NUM_LVLS) > + panic("rcu_init_one: rcu_num_lvls overflow"); Why not write this as BUG_ON(rcu_num_lvls > RCU_NUM_LVLS)? Given that the condition in question can never happen, you don't really need an explanatory message. I do find it surprising, though, that the compiler can't figure this one out, given that rcu_num_lvls gets initialized right before this in the same file (and likely inlined into the same function). I wonder if it thought some other code might change it unexpectedly, since rcu_num_lvls doesn't get declared as static? Unfortunately, the loop macros in rcutree.h make it difficult to make rcu_num_lvls static, but as far as I can tell only one use of those macros ever gets expanded outside of rcutree.c: the one in rcutree_trace.c. If you compile out tracing, and declare rcu_num_lvls static, does the warning go away? - Josh Triplett