linux-kernel - Re: Bisected KVM hang on x86-32 between v3.12 and v3.13

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Message-ID: <20140407224308.GD4106@localhost.localdomain>
Date:	Tue, 8 Apr 2014 00:43:10 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Toralf Förster <toralf.foerster@....de>
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

On Mon, Apr 07, 2014 at 09:57:00PM +0200, Toralf Förster wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
> 
> 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
> 

Ah ok. Hmm, Peter seemed to have an idea on a fix to backport so I'll let
him handle that :)

Thanks.
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