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

Thomas Gleixner
2010-12-15T15:42:18Z

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
clear IRQS_SHARED
thread handler runs and sees !SHARED

Same scenario, just moved by a few lines :)

Thanks,

tglx