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

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Date:	Mon, 8 Nov 2010 19:05:23 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Don Zickus <dzickus@...hat.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 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? :)


> +void touch_all_nmi_watchdogs(void)
> +{
>  	if (watchdog_enabled) {
>  		unsigned cpu;
>  
> @@ -151,7 +166,7 @@ void touch_nmi_watchdog(void)
>  	}
>  	touch_softlockup_watchdog();
>  }
> -EXPORT_SYMBOL(touch_nmi_watchdog);
> +EXPORT_SYMBOL(touch_all_nmi_watchdogs);
>  
>  #endif
>  
> -- 
> 1.7.2.3
> 

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