Subject: Re: [RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt among all monitored CPUs

From: Ricardo Neri <[email-masked]>
Date: Thu, 14 Jun 2018 19:16:29 -0700

On Wed, Jun 13, 2018 at 11:48:09AM +0200, Thomas Gleixner wrote:
> 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 ...

I am sorry. I will look into other options for locking. Do you think rcu_lock
would help in this case? I need this locking because the CPUs being monitored
changes as CPUs come online and offline.

> 
> > +	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.

I will into reworking how the destination of the interrupt is set.

Thanks and BR,

Ricardo