linux-kernel - Re: [PATCH 1/1] Fix HARD Lockup Firing off while in debugger

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Message-ID: <20151214172840.GB42652@redhat.com>
Date:	Mon, 14 Dec 2015 12:28:40 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Jeff Merkey <linux.mdb@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, akpm@...ux-foundation.org,
	uobergfe@...hat.com, atomlin@...hat.com, cmetcalf@...hip.com,
	fweisbec@...il.com
Subject: Re: [PATCH 1/1] Fix HARD Lockup Firing off while in debugger

On Sat, Dec 12, 2015 at 02:08:13PM -0700, Jeff Merkey wrote:
> The current touch_nmi_watchdog() function in /kernel/watchdog.c does
> not always catch all cases when a processor is spinning in the nmi
> handler inside either KGDB, KDB, or MDB.  The hrtimer_interrupts_saved
> count can still end up matching the previous value in some cases,
> resulting in the hard lockup detector tagging processors inside a

Hi Jeff,

I am confused here, the 'touch_nmi_watchdog()' was supposed to block the
check for hrtimer_interrupts from happening.  So if the check is still being
executed _after_ you executed touch_nmi_watchdog(), it would imply there was
about 10 seconds or so of time elapse from the touch command to the hrtimer
check.

So I am not sure how the below patch would fix this, other than just add
another 10 second delay (for a total of 20 seconds) to your timeout?


> debugger and executing a panic.  The patch below corrects this
> problem.  I did not add this to the touch_nmi_function directly
> becuase of possible affects on timing issues.
> 
> I have tested this patch and it fixes the problem for kernel debuggers
> stopping errant hard lockup events when processors are spinning inside
> the debugger.

The kernel doesn't normal take patches like this without a corresponding
user, which I didn't see attached in this patch or a patch series.

Cheers,
Don

> 
> 
> Signed-off-by:  Jeff V. Merkey <jeffmerkey@...il.com>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 18f34cf..b682aab 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -283,6 +283,13 @@ static bool is_hardlockup(void)
>  	__this_cpu_write(hrtimer_interrupts_saved, hrint);
>  	return false;
>  }
> +
> +void touch_hardlockup_watchdog(void)
> +{
> +	__this_cpu_write(hrtimer_interrupts_saved, 0);
> +}
> +EXPORT_SYMBOL_GPL(touch_hardlockup_watchdog);
> +
>  #endif
> 
>  static int is_softlockup(unsigned long touch_ts)
--
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