linux-kernel - Re: [PATCH] [BUGFIX] crash/ioapic: Prevent crash_kexec() from deadlocking of ioapic

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Date:	Tue, 20 Aug 2013 10:27:40 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Gleb Natapov <gleb@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Joerg Roedel <joro@...tes.org>, x86@...nel.org,
	stable@...r.kernel.org, Marcelo Tosatti <mtosatti@...hat.com>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
	Ingo Molnar <mingo@...hat.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	yrl.pp-manager.tt@...achi.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Seiji Aguchi <seiji.aguchi@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] [BUGFIX] crash/ioapic: Prevent crash_kexec() from
 deadlocking of ioapic_lock

On Tue, Aug 20, 2013 at 03:12:32AM -0700, Eric W. Biederman wrote:
> Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com> writes:
> 
> > Hi Ingo,
> >
> > Thank you for fixing typos!
> > OK, I'll fix them and rename to ioapic_zap_locks().
> >
> > Thank you again!
> 
> 
> The better fix for this would be to remove the disable_IO_APIC call from
> crash_kexec.
> 
> I know last time it was investigated the kernel was very close to
> working without needing that, and the code will be much more robust in
> the long term if we can avoid disabling them in the crashing kernel.
> 
> Yoshihiro is there any chance you can look into removing the
> disable_IO_APIC entirely?
> 
> The apic disablement and the disable_IO_APIC exists entirely due to
> limitations in the kernel boot path.

Yup.  We went down this path a year ago:

https://lkml.org/lkml/2012/2/2/331

Then we got sidetracked and talked about removing the lapic stuff at
shutdown too:

http://lists.infradead.org/pipermail/kexec/2012-February/006017.html
(sorry couldn't find lkml link for some reason)

And the second patch was committed.

However, it was quickly reverted when Yinghai Lu noticed a problem:

https://lkml.org/lkml/2012/2/11/143

The problem stemmed from the fact that the nmi_watchdog caused an NMI in
the middle of transitioning between the two kernels (we didn't shutdown
the lapic) and caused a reset (there is no NMI handler in purgatory).

I think I dropped the ball in investigating how to write an idt for the
purgatory code to handle spurious NMIs.

Regardless of all that, I think if we stick to just removing the ioapic
shutdown code (ie the first patch linked above), we should be ok.  I
believe my testing went smoothly.  It was the lapic stuff that needed more
tweaking.

So, I agree with Eric, let's remove the disable_IO_APIC() stuff and keep
the code simpler.

Cheers,
Don
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