From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754619Ab3AGRTf (ORCPT ); Mon, 7 Jan 2013 12:19:35 -0500 Received: from e32.co.us.ibm.com ([32.97.110.150]:36895 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754555Ab3AGRTe (ORCPT ); Mon, 7 Jan 2013 12:19:34 -0500 Date: Mon, 7 Jan 2013 09:19:10 -0800 From: "Paul E. McKenney" To: Josh Triplett 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, markus@trippelsdorf.de Subject: Re: [PATCH tip/core/rcu 4/6] rcu: Silence compiler array out-of-bounds false positive Message-ID: <20130107171910.GA19510@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> <20130107155002.GA11145@leaf> <20130107171602.GI2525@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130107171602.GI2525@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13010717-5406-0000-0000-000003F62125 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 07, 2013 at 09:16:02AM -0800, Paul E. McKenney wrote: > On Mon, Jan 07, 2013 at 07:50:02AM -0800, Josh Triplett wrote: > > 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. > > Good point, will do! Ah, wait, BUG_ON() sometimes compiles to nothingness: #ifndef HAVE_ARCH_BUG_ON #define BUG_ON(condition) do { if (condition) ; } while(0) #endif So I do need the explicit "if". :-( Thanx, Paul > > 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? > > I found it quite surprising also, hence the "array-index anxieties" above. > > I added Marcus on CC for his thoughts on this. > > Thanx, Paul