On the x86 because the interrupt may be shared. There are other platforms
with interrupt sources that are not disableable - so you cannot return
to user space until the IRQ is cleared - or you get another IRQ. Thankfully
that bogosity is rare.
> > The card is on IRQ 9 you disable IRQ 9 you return to user space. IRQ 9 is shared with the
> > disk. You do a disk I/O. Goodbye computer
> It's true you can't do shared interrupts with my driver. But it'll
> never try that: It requests the interrupt without SA_SHIRQ so your
> scenario could never happen. You just get -EBUSY when you open the
> device and the interrupt happens to be in use.
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