Subject: Re: [PATCH v3 1/4] powerpc/watchdog: Fix missed watchdog reset due to memory ordering race

From: Laurent Dufour <[email-masked]>
Date: Fri, 19 Nov 2021 10:25:15 +0100

Le 19/11/2021 à 10:05, Nicholas Piggin a écrit :
> Excerpts from Laurent Dufour's message of November 16, 2021 1:09 am:
>> Le 10/11/2021 à 03:50, Nicholas Piggin a écrit :
>>> It is possible for all CPUs to miss the pending cpumask becoming clear,
>>> and then nobody resetting it, which will cause the lockup detector to
>>> stop working. It will eventually expire, but watchdog_smp_panic will
>>> avoid doing anything if the pending mask is clear and it will never be
>>> reset.
>>>
>>> Order the cpumask clear vs the subsequent test to close this race.
>>>
>>> Add an extra check for an empty pending mask when the watchdog fires and
>>> finds its bit still clear, to try to catch any other possible races or
>>> bugs here and keep the watchdog working. The extra test in
>>> arch_touch_nmi_watchdog is required to prevent the new warning from
>>> firing off.
>>>
>>> Debugged-by: Laurent Dufour <[email-masked]>
>>> Signed-off-by: Nicholas Piggin <[email-masked]>
>>> ---
>>>    arch/powerpc/kernel/watchdog.c | 41 +++++++++++++++++++++++++++++++++-
>>>    1 file changed, 40 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
>>> index f9ea0e5357f9..3c60872b6a2c 100644
>>> --- a/arch/powerpc/kernel/watchdog.c
>>> +++ b/arch/powerpc/kernel/watchdog.c
>>> @@ -135,6 +135,10 @@ static void set_cpumask_stuck(const struct cpumask *cpumask, u64 tb)
>>>    {
>>>    	cpumask_or(&wd_smp_cpus_stuck, &wd_smp_cpus_stuck, cpumask);
>>>    	cpumask_andnot(&wd_smp_cpus_pending, &wd_smp_cpus_pending, cpumask);
>>> +	/*
>>> +	 * See wd_smp_clear_cpu_pending()
>>> +	 */
>>> +	smp_mb();
>>>    	if (cpumask_empty(&wd_smp_cpus_pending)) {
>>>    		wd_smp_last_reset_tb = tb;
>>>    		cpumask_andnot(&wd_smp_cpus_pending,
>>> @@ -215,13 +219,44 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
>>>    
>>>    			cpumask_clear_cpu(cpu, &wd_smp_cpus_stuck);
>>>    			wd_smp_unlock(&flags);
>>> +		} else {
>>> +			/*
>>> +			 * The last CPU to clear pending should have reset the
>>> +			 * watchdog so we generally should not find it empty
>>> +			 * here if our CPU was clear. However it could happen
>>> +			 * due to a rare race with another CPU taking the
>>> +			 * last CPU out of the mask concurrently.
>>> +			 *
>>> +			 * We can't add a warning for it. But just in case
>>> +			 * there is a problem with the watchdog that is causing
>>> +			 * the mask to not be reset, try to kick it along here.
>>> +			 */
>>> +			if (unlikely(cpumask_empty(&wd_smp_cpus_pending)))
>>> +				goto none_pending;
>>
>> If I understand correctly, that branch is a security in case the code is not
>> working as expected. But I'm really wondering if that's really needed, and we
>> will end up with a contention on the watchdog lock while this path should be
>> lockless, and I'd say that in most of the case there is nothing to do after
>> grabbing that lock. Am I missing something risky here?
> 
> I'm thinking it should not hit very much because that first test
> 
>      if (!cpumask_test_cpu(cpu, &wd_smp_cpus_pending)) {
> 
> I think it should not be true too often, it would mean a CPU has taken
> two timer interrupts while another one has not taken any, so hopefully
> that's pretty rare in normal operation.

Thanks, Nick, for the clarification.

Reviewed-by: Laurent Dufour <[email-masked]>