Re: [GIT PULL] Additional x86 fixes for 2.6.31-rc5

H. Peter Anvin
2009-08-02T03:53:21Z

On 08/01/2009 06:20 PM, Paul Mackerras wrote:
> When I tried using __thread for per-cpu variables on ppc64, I found
> that gcc would sometimes precompute and cache the address of a per-cpu
> variable even though it could always access the variable using an
> offset from r13. The cached address was wrong if the task got moved
> to a different cpu, of course, but there was no way to tell gcc that.
> Compiler barriers don't help because they say that the contents of
> variables might have changed, but not their addresses.
>
> So on x86 the concern would be that gcc might do lea %gs:foo,%rbx and
> then use (%rbx) to refer to foo later on. It would be possible to use
> __thread for per-task variables rather than having to put all per-task
> things in the task_struct, but __thread doesn't work for per-cpu
> variables in my experience.

You can't actually do "lea %gs:foo,%rbx" (lea doesn't include the
segment offset, unfortunately; I asked AMD to change that on 64 bits
but, not too surprisingly, they weren't able to make that change.)
Getting the address is expensive, but it's obviously possible -- which
may have similar bad results. It would be worth experimenting with a
little bit, though.

-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.