Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency

[Waiman Long]

On 8/4/23 12:59, Will Deacon wrote:
> On Fri, Aug 04, 2023 at 12:51:47PM -0400, Waiman Long wrote:
> > On 8/4/23 12:28, Will Deacon wrote:
> > > > > >     struct dmc620_pmu {
> > > > > > @@ -423,9 +424,14 @@ static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num)
> > > > > >     	struct dmc620_pmu_irq *irq;
> > > > > >     	int ret;
> > > > > > -	list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node)
> > > > > > -		if (irq->irq_num == irq_num && refcount_inc_not_zero(&irq->refcount))
> > > > > > +	list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node) {
> > > > > > +		if (irq->irq_num != irq_num)
> > > > > > +			continue;
> > > > > > +		if (!irq->valid)
> > > > > > +			return ERR_PTR(-EAGAIN);	/* Try again later */
> > > > > It looks like this can bubble up to the probe() routine. Does the driver
> > > > > core handle -EAGAIN coming back from a probe routine?
> > > > Right, I should add code to handle this error condition. I think it can be
> > > > handled in dmc620_pmu_get_irq(). The important thing is to release the
> > > > mutex, wait a few ms and try again. What do you think?
> > > I don't really follow, but waiting a few ms and trying again sounds like
> > > a really nasty hack for something which doesn't appear to be constrained
> > > by broken hardware. In other words, we got ourselves into this mess, so
> > > we should be able to resolve it properly.
> >  From my point of view, the proper way to solve the problem is to reverse the
> > locking order. Since you don't to add a EXPORT statement to the core kernel
> > code, we will have to find a way around it by not holding the
> > dmc620_pmu_irqs_lock when cpuhp_state_add_instance_nocalls() is called.
> > Another alternative that I can think of is to add one more mutex that we
> > will hold just for the entirety of  __dmc620_pmu_get_irq() and take
> > dmc620_pmu_irqs_lock only when the linked list is being modified. That will
> > eliminate the need to introduce arbitrary wait as other caller of
> > __dmc620_pmu_get_irq() will wait in the new mutex. Will this work for you?
> Yes. To be honest, I think we've both spent far too much time trying to
> fix this (and I admire your persistence!), so adding a mutex to make it
> "obviously" correct sounds like the right thing to me. We can look at
> optimisations later if anybody cares.

Sorry to be too persistent sometimes:-) Will send out a new version soon.

Cheers,
Longman