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

Jan Kiszka
2010-12-15T15:49:57Z

Am 15.12.2010 16:41, Thomas Gleixner wrote:
> On Wed, 15 Dec 2010, Jan Kiszka wrote:
>
>> Am 15.12.2010 14:04, Thomas Gleixner wrote:
>>> On Wed, 15 Dec 2010, Jan Kiszka wrote:
>>>> Am 14.12.2010 21:54, Thomas Gleixner wrote:
>>>>> On Mon, 13 Dec 2010, Jan Kiszka wrote:
>>>>>> @@ -943,6 +950,9 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
>>>>>> /* Make sure it's not being used on another CPU: */
>>>>>> synchronize_irq(irq);
>>>>>>
>>>>>> + if (single_handler)
>>>>>> + desc->irq_data.drv_status &= ~IRQS_SHARED;
>>>>>> +
>>>>>
>>>>> What's the reason to clear this flag outside of the desc->lock held
>>>>> region.
>>>>
>>>> We need to synchronize the irq first before clearing the flag.
>>>>
>>>> The problematic scenario behind this: An IRQ started in shared mode,
>>>> this the line was unmasked after the hardirq. Now we clear IRQS_SHARED
>>>> before calling into the threaded handler. And that handler may now think
>>>> that the line is still masked as IRQS_SHARED is set.
>>>
>>> That should read "not set" I guess.
>>
>> Can't remember who wrote this, but that guy might have been too tired
>> for clear sentences: Yes, of course, we could run into troubles, if
>> IRQS_SHARED was _not_ set while the IRQ line is unmasked between hard
>> and threaded handler.
>>
>>> Hmm, needs more thoughts :(
>>
>> Be warned, might be painful.
>
> Talking about headache. Your solution above does not prevent that
> scenario.
>
> CPU 0 CPU 1
>
> synchronize_irq();
> hard irq comes in sees shared and unmasks

Nope, IRQ_ONESHOT is already cleared at that point.

> clear IRQS_SHARED
> thread handler runs and sees !SHARED
>
> Same scenario, just moved by a few lines :)

The same, just the other way around - and mostly harmless, I hope. :)

Jan

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