linux-kernel - Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

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Date:	Wed, 18 Mar 2015 23:38:33 +0100
From:	Stefan Seyfried <stefan.seyfried@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Jiri Kosina <jkosina@...e.cz>
CC:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Takashi Iwai <tiwai@...e.de>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

Am 18.03.2015 um 23:29 schrieb Andy Lutomirski:
> On Wed, Mar 18, 2015 at 3:22 PM, Jiri Kosina <jkosina@...e.cz> wrote:
>> On Wed, 18 Mar 2015, Andy Lutomirski wrote:
>>
>>> sysret64 can only fail with #GP, and we're totally screwed if that
>>> happens,
>>
>> But what if the GPF handler pagefaults afterwards? It'd be operating on
>> user stack already.
> 
> Good point.
> 
> Stefan, can you try changing the first "jne
> opportunistic_sysret_failed" to "jmp opportunistic_sysret_failed" in
> entry_64.S and seeing if you can reproduce this?  (Is it easy enough
> to reproduce that this would tell us anything?)

I have no good way of reproducing the issue (happens once per week...)
but apparently Takashi has, so I'd like to hand this task over to him.

> It's a shame that double_fault doesn't record what gs was on entry.
> If we did sysret -> general_protection -> page_fault -> double_fault,
> then we'd enter double_fault with usergs, whereas syscall ->
> page_fault -> double_fault would enter double_fault with kernelgs.
> 
> Hmm.  We may be able to answer this more directly.  Stefan, can you
> dump a couple hundred bytes starting at 0x00007fffa55eafb8 (i.e. your
> page_fault stack at the time of the failure)?  That will tell us the
> faulting address.  If that fails, try starting at 00007fffa55eb000
> instead.

Unfortunately not, is this userspace memory? It's not in the dump I have.
This issue is the first I have seen where having a full dump would be
really helpful apart from cosmetic reasons...
-- 
Stefan Seyfried
Linux Consultant & Developer -- GPG Key: 0x731B665B

B1 Systems GmbH
Osterfeldstraße 7 / 85088 Vohburg / http://www.b1-systems.de
GF: Ralph Dehner / Unternehmenssitz: Vohburg / AG: Ingolstadt,HRB 3537
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