linux-kernel - Re: [tip:x86/debug] x86/kdump: No need to disable ioapic/ lapic in crash path

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Message-ID: <m1y5s07rn3.fsf@fess.ebiederm.org>
Date:	Fri, 17 Feb 2012 19:21:52 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Don Zickus <dzickus@...hat.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, linux-kernel@...r.kernel.org,
	mingo@...hat.com, hpa@...or.com, torvalds@...ux-foundation.org,
	kexec@...ts.infradead.org, vgoyal@...hat.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/debug] x86/kdump: No need to disable ioapic/ lapic in crash path

Don Zickus <dzickus@...hat.com> writes:

> On Fri, Feb 17, 2012 at 04:41:01AM -0800, Eric W. Biederman wrote:
>> 
>> The fix with a guarantee of no more scope creep is to just disable the
>> nmi watchdog on the kexec on panic path.
>> 
>> Don if you have time please figure out is needed to ignore nmi's and
>> possible record and/or report them while we boot, otherwise please cook
>> up a patch that just disables the nmi watchdog wherever we are sending
>> it from (the local apic or the ioapic).
>
> Can I keep things even simpler?  The original problem was the deadlock
> with the ioapic lock.  We fixed that by removing the call to
> disable_IO_APIC().  Can we just leave the disable_local_APIC calls in
> there for now?  Is there any real harm?

> All this rewrite is going to take time which will delay fixing a current
> problem with kexec on panic, the ioapic deadlock.

Hmm.

My apologies I just realized that we can not disable the nmi watchdog
safely in all cases.  To avoid the deadlock we fundamentally can not
write to the io_apic, because the locks are the io_apic write path.
The nmi watchdog can be sourced from either the local apics or the
io_apics.  To disable the nmi_watchdog we need at least potentially
to write io_apic.

So it appears to me that the only reasonable and robust thing we can
do is to ignore nmis in the kexec on panic path.

So it looks to me that the only path forward at this point is to fix
the other bug where an unexpected nmi will kill the kexec on panic boot.

I just took a look at the code in /sbin/kexec and that code does not in
fact change the idt except when we switch to 16bit mode, which we
definitely do not do in the kexec on panic case.  So it appears that we
don't need to coordinate an /sbin/kexec release with a kernel release to
ignore nmis.

In fact it looks like we only need to fix the interrupt descriptors
loaded in machine_kexec_64.c and head64.c to ignore nmis.

At which point we will have fixed two bugs and have a much more reliable
kexec on panic implementation.

Eric
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