From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753113AbZHIVQ2 (ORCPT ); Sun, 9 Aug 2009 17:16:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752952AbZHIVQ1 (ORCPT ); Sun, 9 Aug 2009 17:16:27 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:37513 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752885AbZHIVQ0 (ORCPT ); Sun, 9 Aug 2009 17:16:26 -0400 Date: Sun, 9 Aug 2009 14:16:29 -0700 From: "Paul E. McKenney" To: Hugh Dickins Cc: Ingo Molnar , Ben Herrenschmidt , Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: CONFIG_PREEMPT_RCU in next/mmotm Message-ID: <20090809211629.GJ6866@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090808235634.GB6866@linux.vnet.ibm.com> <20090809053308.GD6866@linux.vnet.ibm.com> <20090809185753.GG6866@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Aug 09, 2009 at 09:53:53PM +0100, Hugh Dickins wrote: > On Sun, 9 Aug 2009, Paul E. McKenney wrote: > > > > I introduced the problem in commit 7fe616c5dd50a50f334edec1ea0580b90b7af0d9 > > by changing from register_cpu_notifier() to hotcpu_notifier(). The former > > lets you know when CPUs come on line unconditionally, the latter only > > when CONFIG_HOTPLUG_CPU is in effect. > > > > But hotcpu_notifier() is much nicer to use, so I propose introducing > > a cpu_notifier() that is invoked like hotcpu_notifier() is, but is > > unconditional in the same way that register_cpu_notifier(). > > > > Something like the following (untested, probably does not compile): > > > > diff --git a/include/linux/cpu.h b/include/linux/cpu.h > > index 4d668e0..d5dfc1f 100644 > > --- a/include/linux/cpu.h > > +++ b/include/linux/cpu.h > > @@ -48,6 +48,15 @@ struct notifier_block; > > > > #ifdef CONFIG_SMP > > /* Need to know about CPUs going up/down? */ > > +#if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) > > +#define cpu_notifier(fn, pri) { \ > > + static struct notifier_block fn##_nb __cpuinitdata = \ > > + { .notifier_call = fn, .priority = pri }; \ > > + register_cpu_notifier(&fn##_nb); \ > > +} > > +#else /* #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */ > > +#define cpu_notifier(fn, pri) do { (void)(fn); } while (0) > > +#endif /* #else #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */ > > #ifdef CONFIG_HOTPLUG_CPU > > extern int register_cpu_notifier(struct notifier_block *nb); > > extern void unregister_cpu_notifier(struct notifier_block *nb); > > @@ -99,11 +108,7 @@ extern struct sysdev_class cpu_sysdev_class; > > > > extern void get_online_cpus(void); > > extern void put_online_cpus(void); > > -#define hotcpu_notifier(fn, pri) { \ > > - static struct notifier_block fn##_nb __cpuinitdata = \ > > - { .notifier_call = fn, .priority = pri }; \ > > - register_cpu_notifier(&fn##_nb); \ > > -} > > +#define hotcpu_notifier(fn, pri) cpu_notifier(fn, pri) > > #define register_hotcpu_notifier(nb) register_cpu_notifier(nb) > > #define unregister_hotcpu_notifier(nb) unregister_cpu_notifier(nb) > > int cpu_down(unsigned int cpu); > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c > > index 9f0584e..c1bbfd5 100644 > > --- a/kernel/rcupdate.c > > +++ b/kernel/rcupdate.c > > @@ -251,7 +251,7 @@ void __init rcu_init(void) > > int i; > > > > __rcu_init(); > > - hotcpu_notifier(rcu_barrier_cpu_hotplug, 0); > > + cpu_notifier(rcu_barrier_cpu_hotplug, 0); > > > > /* > > * We don't need protection against CPU-hotplug here because > > > [ removed repeat of rcupdate.c patch ] > > > > Thoughts? > > That builds and works for me, with or without CONFIG_HOTPLUG_CPU. > > But I didn't get what you're achieving with the MODULE part of it; > and (I'm not a notifier buff at all) it does seems rather baroque to > me - a single callsite, why not stick with register_cpu_notifier()? > > Ah, perhaps it's your ambition to move others over to this > (or perhaps it's your ambition to leave that to someone else ;-) Actually, nothing quite that clearly thought out. I was just following the pattern set for register_cpu_notifier(). My guess at the reasoning is that when !HOTPLUG_CPU, modules cannot be loaded until all the CPUs are online, so there is no point in letting a module set itself up for notification. But whatever their reasoning, mine was that there is no point in creating a struct notifier_block that wasn't going to be used. ;-) Thanx, Paul