linux-kernel - Re: Please revert 709f744 (x86: bitops asm constraint fixes)

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Message-Id: <482175EF.76E4.0078.0@novell.com>
Date:	Wed, 07 May 2008 08:27:11 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	<odie@...aau.dk>, "Ingo Molnar" <mingo@...e.hu>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>, <tglx@...utronix.de>,
	"Chuck Ebbert" <cebbert@...hat.com>,
	<linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: Please revert 709f744 (x86: bitops asm constraint fixes)

>>> Ingo Molnar <mingo@...e.hu> 06.05.08 14:01 >>>
>
>* Simon Holm Thøgersen <odie@...aau.dk> wrote:
>
>> [CC'ing all that commented on the patch on LKML]
>> 
>> 709f744 causes my computer to freeze during the start up of X and my 
>> login manger (GDM). It gets to the point where it has shown the 
>> default X mouse cursor logo (a big X / cross) and does not respond to 
>> anything from that point on.
>> 
>> This worked fine before 709f744, and it works fine with 709f744 
>> reverted on top of Linus' current tree (f74d505). The revert had 
>> conflicts, as far as I can tell due to white space changes. The diff I 
>> ended up with is below.
>> 
>> It is 100% reproducible.
>
>thanks Simon for tracking this down.
>
>I've applied your revert (see the patch below), we'll do it unless the 
>real bug is found and confirmed by you. What exact compiler version are 
>you using to build the kernel?
>
>Jan, any ideas what's wrong with your commit?

No, I have no idea at all (apart from considering mis-compilation as you
did. The best path I could suggest is to try and nail this down to one
(or more, if that happens to be the case) function(s) having been
changed - this is mostly because part of the changes are really
tightening things (which therefore I would think ought to be kept),
while the change to __test_and_change_bit() really weakens things
(which I nevertheless continue to think is correct and consistent with
other functions, but which then would be the primary suspect). Of
course, since no-one else has seen this so far, this would need to be
done by Simon.
Once down to a single (hopefully) function, it might be possible to just
statically compare the two vmlinux-es to perhaps spot whether this
indeed is mis-compilation.

Jan
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