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

[Jeff Merkey]

On 12/16/15, Jeff Merkey <linux.mdb@xxxxxxxxx> wrote:
> On 12/14/15, Jeff Merkey <linux.mdb@xxxxxxxxx> 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@xxxxxxxxx>
>> ---
>>  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
>

Wow, I figured it out.  What's really needed here is the ability to
touch all the processors from just one processor.  That's what's
missing.  This per processor nonsense doesn't fly here.  A debugger
needs to be able to turn off your stuff (it needs an on/off switch)
completely when needed.

I'll submit a patch for that.  I'll just maintain the working version
in my patch for the debugger so people can get a working, stable,
debugable kernel and not a broken one until fixes start showing up in
the tree.

Jeff
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