From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755664Ab0DRDxi (ORCPT ); Sat, 17 Apr 2010 23:53:38 -0400 Received: from relay1-v.mail.gandi.net ([217.70.178.75]:53788 "EHLO relay1-v.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753837Ab0DRDxh (ORCPT ); Sat, 17 Apr 2010 23:53:37 -0400 X-Originating-IP: 217.70.178.40 X-Originating-IP: 173.50.255.69 Date: Sat, 17 Apr 2010 20:53:09 -0700 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, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com Subject: Re: [PATCH RFC tip/core/rcu 02/16] rcu: make dead code really dead Message-ID: <20100418035309.GA20129@feather> References: <20100415181255.GA19588@linux.vnet.ibm.com> <1271355219-20714-2-git-send-email-paulmck@linux.vnet.ibm.com> <20100415235251.GA2491@feather> <20100416142348.GD2615@linux.vnet.ibm.com> <20100416211610.GA8357@feather> <20100416222927.GP2615@linux.vnet.ibm.com> <20100417045326.GA11604@feather> <20100418011213.GF2876@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100418011213.GF2876@linux.vnet.ibm.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Apr 17, 2010 at 06:12:13PM -0700, Paul E. McKenney wrote: > On Fri, Apr 16, 2010 at 09:53:27PM -0700, Josh Triplett wrote: > > On Fri, Apr 16, 2010 at 03:29:27PM -0700, Paul E. McKenney wrote: > > > On Fri, Apr 16, 2010 at 02:16:10PM -0700, Josh Triplett wrote: > > > > On Fri, Apr 16, 2010 at 07:23:48AM -0700, Paul E. McKenney wrote: > > > > > On Thu, Apr 15, 2010 at 04:52:52PM -0700, Josh Triplett wrote: > > > > > > On Thu, Apr 15, 2010 at 11:13:25AM -0700, Paul E. McKenney wrote: > > > > > > > From: Lai Jiangshan > > > > > > > > > > > > > > cleanup: make dead code really dead > > > > > > > > > > > > > > Signed-off-by: Lai Jiangshan > > > > > > > Signed-off-by: Paul E. McKenney > > > > > > > --- > > > > > > > kernel/rcutree.c | 4 ++-- > > > > > > > 1 files changed, 2 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > > > > > index e54c123..6042fb8 100644 > > > > > > > --- a/kernel/rcutree.c > > > > > > > +++ b/kernel/rcutree.c > > > > > > > @@ -1236,11 +1236,11 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed) > > > > > > > break; /* grace period idle or initializing, ignore. */ > > > > > > > > > > > > > > case RCU_SAVE_DYNTICK: > > > > > > > - > > > > > > > - raw_spin_unlock(&rnp->lock); /* irqs remain disabled */ > > > > > > > if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK) > > > > > > > break; /* So gcc recognizes the dead code. */ > > > > > > > > > > > > GCC's new __builtin_unreachable would help here, though obviously we > > > > > > can't count on 4.5 or newer quite yet. A wrapper in compiler.h would > > > > > > let us use it when available though. > > > > > > > > > > So at some time when we can count on gcc 4.5 or newer, the code > > > > > would look something like the following? > > > > > > > > > > if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) > > > > > this_is_unreachable(); > > > > > > > > Yes, exactly. > > > > > > > > > I suppose that in the meantime one could supply the code to use > > > > > in the unreachable case: > > > > > > > > > > if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) > > > > > this_is_unreachable(break); > > > > > > > > > > But this is beginning to seem a bit strained to me. ;-) > > > > > > > > I'd suggest spelling that this way: > > > > > > > > if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) { > > > > unreachable(); > > > > break; > > > > } > > > > > > > > But in any case, all of these do seem excessive just to avoid the need > > > > for an ifdef. :) > > > > > > Actually, the "if" condition is a comparison of numerical constants, > > > so no #ifdef is required. > > > > I just meant that those constants get #defined based on CONFIG_NO_HZ, so > > an #ifdef on that would remove the need for the special handling of dead > > code. > > True, I could put a #ifdef CONFIG_NO_HZ around that leg of the switch > statement. Or am I still missing your point? No, you got it exactly. Hence my suggesting that all the other alternatives (the if with a break, or with __builtin_unreachable) seemed excessive just to try to convince the compiler to infer what an ifdef would tell it explicitly. :) - Josh Triplett