Messages in this thread | | | Date | Fri, 4 Aug 2023 17:59:44 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency |
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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.
Cheers,
Will
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