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Date:	Wed, 30 Jul 2014 09:43:42 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Andrew Jones <drjones@...hat.com>
Cc:	Ulrich Obergfell <uobergfe@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	pbonzini@...hat.com, akpm@...ux-foundation.org, mingo@...hat.com
Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default

On Fri, Jul 25, 2014 at 01:25:11PM +0200, Andrew Jones wrote:
> > to enable hard lockup detection explicitly.
> > 
> > I think changing the 'watchdog_thresh' while 'watchdog_running' is true should
> > _not_ enable hard lockup detection as a side-effect, because a user may have a
> > 'sysctl.conf' entry such as
> > 
> >    kernel.watchdog_thresh = ...
> > 
> > or may only want to change the 'watchdog_thresh' on the fly.
> > 
> > I think the following flow of execution could cause such undesired side-effect.
> > 
> >    proc_dowatchdog
> >      if (watchdog_user_enabled && watchdog_thresh) {
> > 
> >          watchdog_enable_hardlockup_detector
> >            hardlockup_detector_enabled = true
> > 
> >          watchdog_enable_all_cpus
> >            if (!watchdog_running) {
> >                ...
> >            } else if (sample_period_changed)
> >                       update_timers_all_cpus
> >                         for_each_online_cpu
> >                             update_timers
> >                               watchdog_nmi_disable
> >                               ...
> >                               watchdog_nmi_enable
> > 
> >                                 watchdog_hardlockup_detector_is_enabled
> >                                   return true
> > 
> >                                 enable perf counter for hard lockup detection
> > 
> > Regards,
> > 
> > Uli
> 
> Nice catch. Looks like this will need a v2. Paolo, do we have a
> consensus on the proc echoing? Or should that be revisited in the v2 as
> well?

As discussed privately, how about something like this to handle that case:
(applied on top of these patches)

Cheers,
Don

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 34eca29..027fb6c 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -666,7 +666,12 @@ int proc_dowatchdog(struct ctl_table *table, int write,
 	 * watchdog_*_all_cpus() function takes care of this.
 	 */
 	if (watchdog_user_enabled && watchdog_thresh) {
-		watchdog_enable_hardlockup_detector(true);
+		/*
+		 * Prevent a change in watchdog_thresh accidentally overriding
+		 * the enablement of the hardlockup detector.
+		 */
+		if (watchdog_user_enabled != old_enabled)
+			watchdog_enable_hardlockup_detector(true);
 		err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
 	} else
 		watchdog_disable_all_cpus();

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