Interrupt threads

[Greg 'groggy' Lehey]

On Tuesday, 22 April 2003 at 17:49:03 -0400, Aniruddha Bohra wrote:
> Hello
>    Reading the implementation of interrupt handling on i386,
> and reading Greg Lehey's Usenix paper on FreeBSD 5.0 SMP
> implementation, I understand that interrupt handling is done in
> process context. This is needed partly to support sleeping while
> handling the interrupt because GIANT needs to be locked and
> a sleep is possible there.
>
>    My question is after all subsystems that lock giant are made
> INTR_MPSAFE will the implementation go back to non-process
> context interrupt handling (as in older versions) ?

No.  INTR_MPSAFE means that the ISRs use their own locking scheme.
They can still sleep.

>    Moreover, will the locking be per-IRQ or there will be a global
> IRQ lock for ensuring atomic access to the ISRs ?

No.  The locks will be finer grained, generally per device, not per
IRQ.

>    Finally why do we need to create a kernel thread for each IRQ -
> why can't one kernel thread handle all the IRQs ?

If the kernel thread sleeps on one lock, everything would stop.  Even
if you accept that you're only processing one interrupt at a time, I
can't think of a way to do this without instant deadlock.  That's the
whole point of having the threads in the first place.

Greg
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