Messages in this thread | | | Date | Fri, 4 Aug 2023 13:03:36 -0400 | Subject | Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency | From | Waiman Long <> |
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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
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