linux-kernel - Re: [crash, bisected] Re: [PATCH 3/4] x86

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Message-ID: <48480DFB.7000404@sgi.com>
Date:	Thu, 05 Jun 2008 09:02:03 -0700
From:	Mike Travis <travis@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area

Ingo Molnar wrote:
> * Mike Travis <travis@....com> wrote:
> 
>>   * Declare the pda as a per cpu variable.
>>
>>   * Make the x86_64 per cpu area start at zero.
>>
>>   * Since the pda is now the first element of the per_cpu area, cpu_pda()
>>     is no longer needed and per_cpu() can be used instead.  This also makes
>>     the _cpu_pda[] table obsolete.
>>
>>   * Since %gs is pointing to the pda, it will then also point to the per cpu
>>     variables and can be accessed thusly:
>>
>> 	%gs:[&per_cpu_xxxx - __per_cpu_start]
>>
>> Based on linux-2.6.tip
> 
> -tip testing found an instantaneous reboot crash on 64-bit x86, with 
> this config:
> 
>   http://redhat.com/~mingo/misc/config-Thu_Jun__5_11_43_51_CEST_2008.bad
> 
> there is no boot log as the instantaneous reboot happens before anything 
> is printed to the (early-) serial console. I have bisected it down to:
> 
> | 7670dc09e89a2b151a1cf49eccebc07c41c2ce9f is first bad commit
> | commit 7670dc09e89a2b151a1cf49eccebc07c41c2ce9f
> | Author: Mike Travis <travis@....com>
> | Date:   Tue Jun 3 17:30:21 2008 -0700
> |
> |     x86_64: Fold pda into per cpu area
> 
> the big problem is not just this crash, but that the patch is _way_ too 
> big:
> 
>  arch/x86/Kconfig                 |    3 +
>  arch/x86/kernel/head64.c         |   34 ++++++--------
>  arch/x86/kernel/irq_64.c         |   36 ++++++++-------
>  arch/x86/kernel/setup.c          |   90 ++++++++++++---------------------------
>  arch/x86/kernel/setup64.c        |    5 --
>  arch/x86/kernel/smpboot.c        |   51 ----------------------
>  arch/x86/kernel/traps_64.c       |   11 +++-
>  arch/x86/kernel/vmlinux_64.lds.S |    1 
>  include/asm-x86/percpu.h         |   48 ++++++--------------
>  9 files changed, 89 insertions(+), 190 deletions(-)
> 
> considering the danger involved, this is just way too large, and there's 
> no reasonable debugging i can do in the bisection to narrow it down any 
> further.
> 
> Please resubmit with the bug fixed and with a proper splitup, the more 
> patches you manage to create, the better. For a dangerous code area like 
> this, with a track record of frequent breakages in the past, i would not 
> mind a "one line of code changed per patch" splitup either. (Feel free 
> to send a git tree link for us to try as well.)
> 
> 	Ingo

Thanks for the feedback Ingo.  I'll test the above config and look at
splitting up the patch.  The difficulty is making each patch independently
compilable and testable.

Mike
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