Re: [patch] PPC/PPC64 port of voluntary preempt patch

[Scott Wood <scott@timesys.com>]

On Mon, Aug 23, 2004 at 06:18:16PM -0400, Scott Wood wrote:
> I have attached a port of the voluntary preempt patch to PPC and
> PPC64.  The patch is against P7, but it applies against P8 as well.
> 
> I've tested it on a dual G5 Mac, both in uniprocessor and SMP.
> 
> Some notes on changes to the generic part of the patch/existing
> generic code:

Another thing that I forgot to mention is that I have some doubts as
to the current generic_synchronize_irq() implementation.  Given that
IRQs are now preemptible, a higher priority RT thread calling
synchronize_irq can't just spin waiting for the IRQ to complete, as
it never will (and it wouldn't be a great idea for non-RT tasks
either).  I see that a do_hardirq() call was added, presumably to
hurry completion of the interrupt, but is that really safe?  It looks
like that could end up re-entering handlers, and you'd still have a
partially executed handler after synchronize_irq() finishes (causing
not only an extra end() call, but possibly code being executed after
it's been unloaded, and other synchronization violations).

If I'm missing something, please let me know, but I don't see a good
way to implement it without blocking for the IRQ thread's completion
(such as with the per-IRQ waitqueues in M5).

-Scott