linux-kernel - Re: [PATCH v2] watchdog: touch_nmi_watchdog should only touch local cpu not every one

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Message-ID: <20101108182804.GR4823@redhat.com>
Date:	Mon, 8 Nov 2010 13:28:04 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, akpm@...ux-foundation.org,
	sergey.senozhatsky@...il.com
Subject: Re: [PATCH v2] watchdog:  touch_nmi_watchdog should only touch local
 cpu not every one

On Mon, Nov 08, 2010 at 07:05:23PM +0100, Frederic Weisbecker wrote:
> On Mon, Nov 08, 2010 at 11:44:35AM -0500, Don Zickus wrote:
> > I ran into a scenario where while one cpu was stuck and should have panic'd
> > because of the NMI watchdog, it didn't.  The reason was another cpu was spewing
> > stack dumps on to the console.  Upon investigation, I noticed that when writing
> > to the console and also when dumping the stack, the watchdog is touched.
> > 
> > This causes all the cpus to reset their NMI watchdog flags and the 'stuck' cpu
> > just spins forever.
> > 
> > This change causes the semantics of touch_nmi_watchdog to be changed slightly.
> > Previously, I accidentally changed the semantics and we noticed there was a
> > codepath in which touch_nmi_watchdog could be touched from a preemtible area.
> > That caused a BUG() to happen when CONFIG_DEBUG_PREEMPT was enabled.  I believe
> > it was the acpi code.
> > 
> > My attempt here re-introduces the change to have the touch_nmi_watchdog() code
> > only touch the local cpu instead of all of the cpus.  But instead of using
> > __get_cpu_var(), I use the __raw_get_cpu_var() version.
> > 
> > This avoids the preemption problem.  However my reasoning wasn't because I was
> > trying to be lazy.  Instead I rationalized it as, well if preemption is enabled
> > then interrupts should be enabled to and the NMI watchdog will have no reason
> > to trigger.  So it won't matter if the wrong cpu is touched because the percpu
> > interrupt counters the NMI watchdog uses should still be incrementing.
> > 
> > V2:  remove touch_all_nmi_watchdog code
> 
> 
> Are you sure you did? :)

Hmm.. Odd.  I remember making and committing the changes.  But now I can't
find them.

Thanks for catching that! I'll send out a version 3.

Cheers,
Don
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