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 <lduf...@linux.ibm.com>
>> Signed-off-by: Nicholas Piggin <npig...@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
