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SubjectRe: [RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt among all monitored CPUs
On Tue, 12 Jun 2018, Ricardo Neri wrote:
> + /* There are no CPUs to monitor. */
> + if (!cpumask_weight(&hdata->monitored_mask))
> + return NMI_HANDLED;
> +
> inspect_for_hardlockups(regs);
>
> + /*
> + * Target a new CPU. Keep trying until we find a monitored CPU. CPUs
> + * are addded and removed to this mask at cpu_up() and cpu_down(),
> + * respectively. Thus, the interrupt should be able to be moved to
> + * the next monitored CPU.
> + */
> + spin_lock(&hld_data->lock);

Yuck. Taking a spinlock from NMI ...

> + for_each_cpu_wrap(cpu, &hdata->monitored_mask, smp_processor_id() + 1) {
> + if (!irq_set_affinity(hld_data->irq, cpumask_of(cpu)))
> + break;

... and then calling into generic interrupt code which will take even more
locks is completely broken.

Guess what happens when the NMI hits a section where one of those locks is
held? Then you need another watchdog to decode the lockup you just ran into.

Thanks,

tglx


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