On 12/14/15, Jeff Merkey <linux....@gmail.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....@gmail.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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