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 15:18:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Stefan Seyfried <stefan.seyfried@...glemail.com>,
	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?

On Wed, Mar 18, 2015 at 2:55 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> On Xen, it goes to xen_sysret64, which touches the same percpu
> variables that we touch on entry.  So I still like my percpu vmap
> fault hypothesis, even though I don't understand what would trigger
> it.

I don't dislike the theory per se, but not only don't I see how it
could happen on regular execution on a laptop, but I also don't see
why this fault behavior would be new to 4.0.

(And I do believe that we should make sure that CPU bringup ends up
faulting in the percpu area, even if I don't really see why that would
be the issue here)

Afaik, the system call entry code hasn't changed at all.

What *has* changed is the "paranoid" handling (double-fault has that
magical "paranoid=2" thing, for example) and the return to user-space
code.

Which is really why I don't believe in that syscall thing. Not because
it isn't the obvious culprit, but simply because it hasn't *changed*.

Or is there something subtle I've missed?

                        Linus
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