> If you have distinct interrupt priorities, you can
> 
> 1) provide an interrupt stack for each priority
> 2) mask all lower priorities when handling one
> 
> Would that not work?

The PIC does that already. IE. it will only interrupt again before
->eoi() for an interrupt of a higher priority. But by using
IRQF_DISABLED, you mask interrupts in the core, and thus effectively
completely prevents the whole thing.

> The problems with enabling irqs in hardirq handlers are that you get
> unlimited irq nesting, which is bad for your stack, furthermore, somehow
> people thing it makes things 'faster' because the irq-off latency goes
> down.

No, you don't get unlimited IRQ nesting, at least not on sane archs with
a decent PIC that does things like what I described above :-)

> The latter just isn't true, as you still have preemption disabled, so
> everything but irqs still suffers.
> 
> The only way to make things low-latency is to pull work out of
> non-preemptable context. Using threaded IRQs is one way to do that.

Cheers,
Ben.