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

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Date:	Tue, 01 Jul 2008 18:51:02 -0700
From:	Mike Travis <travis@....com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area

Eric W. Biederman wrote:
> Mike Travis <travis@....com> writes:
> 
>> H. Peter Anvin wrote:
>>> Jeremy Fitzhardinge wrote:
>>>> Yes, and there's no reason we couldn't do the same on 64-bit, aside
>>>> from the stack-protector's use of %gs:40.  There's no code-size cost
>>>> in large offsets, since they're always 32-bits anyway (there's no
>>>> short absolute addressing mode).
>>>>
>>>> If we manually generate %gs-relative references to percpu data, then
>>>> it's no different to what we do with 32-bit, whether it be a specific
>>>> symbol address or using the TLS relocations.
>>>>
>>> If we think the problem is the zero-basing triggering linker bugs, we
>>> should probably just use a small offset, like 64 (put a small dummy
>>> section before the .percpu.data section to occupy this section.)
>>>
>>> I'm going to play with this a bit and see if I come up with something
>>> sanish.
>>>
>>>     -hpa
>> One interesting thing I've discovered is the gcc --version may make a
>> difference.
>>
>> The kernel panic that occurred from Ingo's config, I was able to replicate
>> with GCC 4.2.0 (which is on our devel server).  But this one complained
>> about not being able to handle the STACK-PROTECTOR option so I moved
>> everything to another machine that has 4.2.4, and now it seems that it
>> works fine.  I'm still re-verifying that the source bits and config options 
>> are identical (it was a later git-remote update), and that in fact it is
>> the gcc --version, but that may be the conclusion.  (My code also has some
>> patches submitted but not yet included in the tip/master tree.  Curiously
>> just enabling some debug options changed the footprint of the panic.)
>>
>> Are we allowed to insist on a specific level of GCC for compiling the
>> kernel?
> 
> Depends on the root cause.  If it turns out to be something that is buggy
> in gcc and we can't work around.  We might do something.  I don't recall
> that kind of thing happening often.  I think our minimum gcc is currently
> gcc-3.4.
> 
> Eric

Ouch.  How far into it do we need to investigate?  I can surely compare the
vmlinux object files, but I'm not cognizant enough about the linker internals
to examine much more than that.

But hey, maybe gcc-3.4 will work ok...?  ;-)

[Or it may be the stack-protector thing is introducing better code?  I'll try
some more config options tomorrow to see if that affects anything.]

Cheers,
Mike
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