Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

Eric W. Biederman wrote:
Mike Travis <travis@xxxxxxx> 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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