Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752327Ab0HTDkk (ORCPT ); Thu, 19 Aug 2010 23:40:40 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:34893 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751701Ab0HTDki (ORCPT ); Thu, 19 Aug 2010 23:40:38 -0400 Date: Thu, 19 Aug 2010 20:42:56 -0700 From: Andrew Morton To: Don Zickus Cc: Frederic Weisbecker , Len Brown , Sergey Senozhatsky , Yong Zhang , Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org, Andy Grover , "H. Peter Anvin" Subject: Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog and touch_softlockup_watchdog Message-Id: <20100819204256.3380bf6f.akpm@linux-foundation.org> In-Reply-To: <20100820025749.GB4879@redhat.com> References: <20100817025954.GA12366@nowhere> <20100817083945.GA12022@swordfish.minsk.epam.com> <20100817092407.GB12022@swordfish.minsk.epam.com> <20100817103948.GA5352@swordfish.minsk.epam.com> <20100817131320.GX4879@redhat.com> <20100818024802.GA24748@nowhere> <20100818130156.43a183d9.akpm@linux-foundation.org> <20100820025749.GB4879@redhat.com> X-Mailer: Sylpheed 2.7.1 (GTK+ 2.18.9; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4161 Lines: 110 On Thu, 19 Aug 2010 22:57:49 -0400 Don Zickus wrote: > On Wed, Aug 18, 2010 at 01:01:56PM -0700, Andrew Morton wrote: > > The surprise new requirement that touch_nmi_watchdog() be called from > > non-preemptible code does seem to make sense IMO. It's hard to see why > > anyone would be touching the watchdog unless he's spinning in irqs-off > > code. Except, of course, when we have a utility function which can be > > called from wither irqs-on or irqs-off: acpi_os_stall(). > > > > That being said, it's not good to introduce new API requirements by > > accident! An audit of all callers should first be performed, at least. > > > > > > The surprise new requirement that touch_softlockup_watchdog() be called > > from non-preemptible code doesn't make sense IMO. If I have a piece of > > code in the kernel which I expect to sit in TASK_UNINTERRUPTIBLE state > > for three minutes waiting for my egg to boil, I should be able to do > > that and I should be able to touch the softlockup detector without > > needing to go non-preemptible. > > Ok, so here is my patch that syncs the touch_*_watchdog back in line with > the old semantics. Hopefully this will undo any harm I caused. > > ------------cut -->--------------------------- > > >From b372e821c804982438db090db6b4a2f753c78091 Mon Sep 17 00:00:00 2001 > From: Don Zickus > Date: Thu, 19 Aug 2010 22:48:26 -0400 > Subject: [PATCH] [lockup detector] sync touch_*_watchdog back to old semantics > > During my rewrite, the semantics of touch_nmi_watchdog and > touch_softlockup_watchdog changed enough to break some drivers > (mostly over preemptable regions). > > This change brings those touch_*_watchdog functions back in line > to how they used to work. > > Signed-off-by: Don Zickus > --- > kernel/watchdog.c | 17 ++++++++++++----- > 1 files changed, 12 insertions(+), 5 deletions(-) > > diff --git a/kernel/watchdog.c b/kernel/watchdog.c > index 613bc1f..99e35a2 100644 > --- a/kernel/watchdog.c > +++ b/kernel/watchdog.c > @@ -122,7 +122,7 @@ static void __touch_watchdog(void) > > void touch_softlockup_watchdog(void) > { > - __get_cpu_var(watchdog_touch_ts) = 0; > + __raw_get_cpu_var(watchdog_touch_ts) = 0; > } > EXPORT_SYMBOL(touch_softlockup_watchdog); > > @@ -142,7 +142,14 @@ void touch_all_softlockup_watchdogs(void) > #ifdef CONFIG_HARDLOCKUP_DETECTOR > void touch_nmi_watchdog(void) > { > - __get_cpu_var(watchdog_nmi_touch) = true; > + if (watchdog_enabled) { > + unsigned cpu; > + > + for_each_present_cpu(cpu) { > + if (per_cpu(watchdog_nmi_touch, cpu) != true) > + per_cpu(watchdog_nmi_touch, cpu) = true; > + } > + } > touch_softlockup_watchdog(); > } > EXPORT_SYMBOL(touch_nmi_watchdog); > @@ -430,6 +437,9 @@ static int watchdog_enable(int cpu) > wake_up_process(p); > } > > + /* if any cpu succeeds, watchdog is considered enabled for the system */ > + watchdog_enabled = 1; > + > return 0; > } > > @@ -452,9 +462,6 @@ static void watchdog_disable(int cpu) > per_cpu(softlockup_watchdog, cpu) = NULL; > kthread_stop(p); > } > - > - /* if any cpu succeeds, watchdog is considered enabled for the system */ > - watchdog_enabled = 1; > } > > static void watchdog_enable_all_cpus(void) hm, the code seems a bit screwy. Maybe it was always thus. watchdog_enabled gets set in the per-cpu function but it gets cleared in the all-cpus function. Asymmetric. Also afacit the action of cpu-hotunplug+cpu-hotplug will reenable the watchdog on a CPU which was supposed to have it disabled. Perhaps you could recheck that and make sure it all makes sense - perhaps we need a separate state variable which is purely "current setting of /proc/sys/kernel/nmi_watchdog" and doesn't get altered internally. Anyway, I'll be disappearing for a few days so perhaps Frederic or hpa can help get this all fixed/merged up? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/