linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Wed, 26 Nov 2014 07:52:56 +0100
From:	Juergen Gross <jgross@...e.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	the arch/x86 maintainers <x86@...nel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: frequent lockups in 3.18rc4

On 11/26/2014 07:21 AM, Linus Torvalds wrote:
> On Tue, Nov 25, 2014 at 9:52 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> And leave it running for a while, and see if the trace is always the
>> same, or if there are variations on it...
>
> Amusing.
>
> Lookie here:
>
>     http://lists.xenproject.org/archives/html/xen-changelog/2005-08/msg00310.html
>
> That's from 2005.

:-)

>
> Anyway, I don't see why the cr3 issue matters, *unless* there is some
> situation where the scheduler can run with interrupts enabled. And why
> this is Xen-related, I have no idea.
>
> The Xen patches seem to have lost that
>
>   /* On Xen the line below does not always work. Needs investigating! */
>
> line when backporting the 2.6.29 patches to Xen. And clearly nobody
> investigated.
>
> So please do get me back-traces, and we'll investigate. Better late
> than never. But it does sound Xen-specific - although it's possible
> that Xen just triggers some timing (and has apparently been able to
> trigger it since 2005) that DaveJ now triggers on his one machine.

Yeah, this sounds plausible.

I'm working on the back traces right now, hope to have them soon.


Juergen

>
> So DaveJ, even though this does appear Xen-centric (Xentric?) and
> you're running on bare hardware, maybe you could do the same thing in
> that x86-64 vmalloc_fault(). The timing with Jürgen is kind of
> intriguing - if 3.18-rc made it happen much more often for him, maybe
> it really is very timing-sensitive, and you actually are seeing a
> non-Xen version of the same thing...
>
>                             Linus
>

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