Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state

Jan Kiszka
2010-12-17T10:32:31Z

Am 17.12.2010 11:23, Thomas Gleixner wrote:
> On Fri, 17 Dec 2010, Jan Kiszka wrote:
>> Am 16.12.2010 21:26, Jan Kiszka wrote:
>>> Am 16.12.2010 14:13, Thomas Gleixner wrote:
>>>> On Mon, 13 Dec 2010, Jan Kiszka wrote:
>>>>> + if (old_action && (old_action->flags & IRQF_ADAPTIVE) &&
>>>>> + !(desc->irq_data.drv_status & IRQS_SHARED)) {
>>>>> + /*
>>>>> + * Signal the old handler that is has to switch to shareable
>>>>> + * handling mode. Disable the line to avoid any conflict with
>>>>> + * a real IRQ.
>>>>> + */
>>>>> + disable_irq(irq);
>>>>
>>>> This is weird, really. I thought you wanted to avoid waiting for the
>>>> threaded handler to finish if it's on the fly. So this should be
>>>> disable_irq_nosync() or did you change your mind ?
>>>
>>> No, I did not. I wanted to avoid that we set MAKE_SHAREABLE while there
>>> might be another IRQ in flight. The handler that is called due to a real
>>> IRQ might misinterpret MAKE_SHAREABLE as "there is no real event" and
>>> perform the wrong steps (at least the current implementation for KVM would).
>>
>> Actually, the requirement we have to fulfill here is to avoid that the
>> hardirq handler sees !SHARED while the threaded one reads "SHARED". To
>> achieve this without disabling the line, I'm still searching for a way
>> to couple the sharing state of associated hard and threaded handler runs
>> - but I think there is no reliable association, is there?
>
> Unfortunately not. So the only way to solve that is disabling the
> interrupt which makes sure that all handlers have completed.

Hmm, what a pity.

>
> OTOH, if we have to disable anyway, then we could simply keep it
> disabled across the installation of a new handler. That would make the
> notification business go away, wouldn't it ?

No, the notification is still necessary in case the registered handler
keeps the line off after returning from both hard and threaded handler.

Jan

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