linux-kernel - Re: [PATCH] Fix spurious hard lockup events while in debugger

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Message-ID: <CAO6TR8WWq103=Kz+tKv_0PGCbWFiPF_p+EZcD3ZnsABT_Z2iwQ@mail.gmail.com>
Date:	Wed, 16 Dec 2015 10:22:18 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	akpm@...ux-foundation.org, uobergfe@...hat.com, dzickus@...hat.com,
	atomlin@...hat.com, cmetcalf@...hip.com,
	fweisbec <fweisbec@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	peterz@...radead.org, luto@...nel.org
Subject: Re: [PATCH] Fix spurious hard lockup events while in debugger

On 12/14/15, Jeff Merkey <linux.mdb@...il.com> 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, in particular, the case where
> a processor is being held by a debugger inside an int1 handler.
>
> The hrtimer_interrupts_saved count can still end up matching the
> hrtime value in some cases, resulting in the hard lockup detector
> tagging processors inside a 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 since the function is widely used by drivers and
> modules.
>
> 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.
>
> Signed-off-by: Jeff Merkey <linux.mdb@...il.com>
> ---
>  kernel/watchdog.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> 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)
> --
> 1.8.3.1
>
>

I got to the bottom of it.  It's related to the hardware I am using.
One of the processors is faulting and hanging due to an existing bug
in the hw_breakpoint handler not setting the resume flag (I have
previously reported it and submitted a patch).  This breaks your code,
but there's nothing you can do about it.

There is a severe bug in hw_breakpoint.c that causes int1 recursion
and this whole "lazy debug register switching" nonsense does not work
properly.  I am probably the first person to actually test this code
path robustly.  I applied the patch that fixes this bug in
hw_breakpoint.c and the problem with your code firing off and ignoring
the touch flag
went away.

Jeff
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