Re: [Xenomai-core] [BUG] IO-APIC stall due to broken fasteoi handling

[Philippe Gerum <rpm@xenomai.org>]

On Thu, 2007-10-04 at 16:06 +0200, Jan Kiszka wrote:
> Philippe Gerum wrote:
> > On Thu, 2007-10-04 at 14:42 +0200, Jan Kiszka wrote:
> >> Philippe Gerum wrote:
> >>> On Thu, 2007-10-04 at 11:34 +0200, Philippe Gerum wrote: 
> >>>>> Well, this trace also reveals a second bug that can cause nasty priority
> >>>>> inversion: a high-prio domains executes when a fasteoi-IRQ arrives for a
> >>>>> low-prio domain. This will now block all IRQs until the low-prio domain
> >>>>> was able to run its IRQ handler completely. Thus we must _mask_ fasteoi
> >>>>> IRQs for low-prio domains while high-prio ones are running!
> >>>>>
> >>>> This code was actually there up to 2.6.17-1.5-02, and was removed at
> >>>> some point in the 2.6.19 series, due to some severe conflicts with the
> >>>> vanilla IO-APIC support which used to be a hell of a moving target at
> >>>> that time. I guess it's time to bring this code back.
> >>>>
> >>> Does the following work for you?
> >> Will give it a try later. Meanwhile...
> >>
> >>> diff --git a/arch/i386/kernel/io_apic.c b/arch/i386/kernel/io_apic.c
> >>> index 2ae79e9..517937b 100644
> >>> --- a/arch/i386/kernel/io_apic.c
> >>> +++ b/arch/i386/kernel/io_apic.c
> >>> @@ -2022,6 +2022,8 @@ static void ack_ioapic_quirk_irq(unsigned int irq)
> >>>  		__unmask_and_level_IO_APIC_irq(irq);
> >>>  		spin_unlock(&ioapic_lock);
> >>>  	}
> >>> +
> >>> +	__mask_IO_APIC_irq(irq);
> >>>  }
> >> ...I have problems understanding this hunk. Typo? Should this read
> >> __unmask_IO_APIC_irq?
> >>
> > 
> > No, you want to mask it here. EOI in the IO-APIC case goes through some
> > quirks which you want to apply immediately on behalf of the primary
> > I-pipe ack handler, basically to work around some IO-APIC errata. Then,
> > either the high priority domain (__ipipe_end_fasteoi_irq) or the root
> > one (handle_fasteoi_irq) will unmask the IRQ as needed, whichever comes
> > first (and only).
> 
> ack_ioapic_quirk_irq == eio for fasteoi, so it is specifically executed
> on exit of handle_fasteoi_irq. I still don't see why you want to leave
> the IRQ masked here.
> 

It is not executed on IRQ exit anymore when the I-pipe is enabled. The
EOI handler is called earlier in the latter case to ack the LAPIC, then
mask the interrupt source from the IO-APIC, waiting for the Linux
handler to process the device which triggered the interrupt. The source
is eventually unmasked when either the high priority domain or Linux is
done with the interrupt.

> Jan
> 
-- 
Philippe.