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

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Message-ID: <20140407185949.GC4106@localhost.localdomain>
Date:	Mon, 7 Apr 2014 20:59:51 +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 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?

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