PATCH/RFC: [kdump] fix APIC shutdown sequence

[Eric W. Biederman]

Martin Wilck <martin.wilck at fujitsu-siemens.com> writes:

> Vivek Goyal wrote:
>
>> Got this oops while testing your patch when I did
>> "echo c > /proc/sysrq-trigger"
>
> That's bad :-(
>
> ...
>> Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP:
>  [<0000000000000000>]
>>  <IRQ>  [<ffffffff8025afd0>] handle_edge_irq+0x5c/0x127
>>  [<ffffffff8020e1ae>] do_IRQ+0xf1/0x15f
>>  [<ffffffff8020c191>] ret_from_intr+0x0/0xa
>>  <EOI>  <NMI>  [<ffffffff80351ae3>] __delay+0x6/0x10
>>  [<ffffffff8021e328>] crash_nmi_callback+0x4b/0x77
>
> Obviously, it the oops occurs after the IRQs are re-enabled in
> crash_nmi_callback().
> It appears that desc->chip->mask (in mask_ack_irq(), kernel/irq/chip.c:462) was
> NULL.
> I have no idea how my patch could cause that.
>
> This proves that re-enabling IRQs in this situation is too dangerous :-(.
> I really did hundreds of dumps in different ways (echo c>/proc/sysrq-trigger,
> forcing Oops and panic(), actually hititng Alt-Sysrq) and this never happened to
> me.

This reminds me. 

There is currently an issue that no has finished converting
the lapic to the new generic irq way.  Which can have this effect.

Eric