Messages in this thread | | | Date | Fri, 4 Aug 2023 12:51:47 -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 12:28, 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?
Cheers, Longman
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