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Date:	Mon, 07 Apr 2014 21:57:00 +0200
From:	Toralf Förster <toralf.foerster@....de>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Michele Ballabio <barra_cuda@...amail.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Bisected KVM hang on x86-32 between v3.12 and v3.13

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On 04/07/2014 08:59 PM, Frederic Weisbecker wrote:
> On Mon, Apr 07, 2014 at 08:16:24PM +0200, Toralf Förster wrote:
>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256
>> 
>> On 04/07/2014 05:07 PM, Peter Zijlstra wrote:
>>> On Mon, Apr 07, 2014 at 05:03:37PM +0200, Peter Zijlstra
>>> wrote:
>>>> So what I suspect at this point is that because i386 and
>>>> x86_64 have a difference in current_thread_info() (i386 is
>>>> stack based), we end up setting the TIF_NEED_RESCHED bit on
>>>> the wrong stack.
>>>> 
>>>> Now I have some vague memories of propagating the TIF flags
>>>> on stack switch, but I cannot remember what arch we did that
>>>> for. Let me stare at this a little more.
>>>> 
>>>> Also, IFF this is the case, then the fingered patch above
>>>> (and your suggested 'fix') aren't the real curlpit/cure but
>>>> simply make it more/less likely to happen.
>>>> 
>>>> Now, Steve had a patch somewhere that would make i386 use 
>>>> per-cpu variables for current_thread_info() just like x86_64 
>>>> already does I think. Let me go find them too.
>>> 
>>> Ohh, goodie, they're already in Linus' tree. Could you see if 
>>> current git still suffers this problem?
>>> 
>> v3.14-10353-g2b3a8fd works fine AFAICS (BTW the fix is stable
>> material, right ?)
> 
> If we are reffering to 198d208df4371734ac4728f69cb585c284d20a15 
> (x86: Keep thread_info on thread stack in x86_32) it doesn't carry
> a stable tag.
> 
> So to be clear, you are saying that v3.14 is fine but other
> release are buggy? Which ones are these?
> 
No, 3.13.x and 3.14 have the issue, latest git is fine and 3.12.x

> If a backport is needed, 198d208df4371734ac4728f69cb585c284d20a15
> looks too large...
> 


- -- 
MfG/Sincerely
Toralf Förster
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