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

[Nicholas Piggin]

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 <ldufour at linux.ibm.com>
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>>   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