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