Re: [PATCH 2/3] genirq/irqdomain: Re-check mapping after associate in irq_create_mapping()

["Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@nokia.com>]

Hi Marc,

On 20/09/2019 17:52, Marc Zyngier wrote:
>> If two irq_create_mapping() calls perform a mapping of the same hwirq on
>> two CPU cores in parallel they both will get 0 from irq_find_mapping(),
>> both will allocate unique virq using irq_domain_alloc_descs() and both
>> will finally irq_domain_associate() it. Giving different virq numbers
>> to their callers.
>>
>> In practice the first caller is usually an interrupt controller driver and
>> the seconds is some device requesting the interrupt providede by the above
>> interrupt controller.
> I disagree with this "In practice". An irqchip controller should *very
> rarely* call irq_create_mapping on its own. It usually indicates some
> level of brokenness, unless the mapped interrupt is exposed by the
> irqchip itself (the GIC maintenance interrupt, for example).

I also didn't understand the reason the irqchip in question calls
irq_create_mapping(), but as 9 upstream irqchips do this as well I was
not really interested in the reasons for this.

>> In this case either the interrupt controller driver configures virq which
>> is not the one being "associated" with hwirq, or the "slave" device
>> requests the virq which is never being triggered.
> Why should the interrupt controller configure that interrupt? On any
> sane platform, the mapping should be created by the user of the
> interrupt, and not by the provider.
> 
> This doesn't mean we shouldn't fix the irqdomain races, but I tend to
> disagree with the analysis here.

That's in fact what happens in our case and may happen with 9 upstream
irqchips as well. Same race would however happen with any IRQ client
driver calling of_irq_get(), if they share same HW IRQ line.

-- 
Best regards,
Alexander Sverdlin.