Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Arvind Sankar]

On Sat, Aug 22, 2020 at 02:43:08AM +0200, Thomas Gleixner wrote:
> On Fri, Aug 21 2020 at 16:16, Nick Desaulniers wrote:
> > On Fri, Aug 21, 2020 at 4:04 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> >> On Fri, Aug 21, 2020 at 02:37:48AM +0200, Thomas Gleixner wrote:
> >> The gcc bug I linked to earlier is only fixed in gcc-6 onwards. Is that
> >
> > (based on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602#c14)
> >
> >> good enough to remove force_order? I can test gcc-4.9 and gcc-5 to check
> >> if it would currently have any impact.
> 
> And that test tells you what exactly? That your particular build of
> those compilers does not have the problem. A truly scientific approach.

More that the current kernel code doesn't have that problem, but yeah,
it might creep in later.

> 
> > I think checking the disassemblies with a pre-gcc-6 would be good
> > enough then; that bug isn't specific to this particular case.
> 
> What? I clearly want a statement from the GCC people that this won't
> happen on pre gcc6 compilers and not just some 'works for me' statement
> based on a randomly picked compiler build.

Presumably also from clang that the compiler does have protections
against this, as opposed to doesn't happen today.

> 
> Thanks,
> 
>         tglx

Cc Segher.

Segher, we were looking at gcc PR82602, where IRA could reorder volatile
asm's (reported on ARM). The fix was backported to gcc-6. Do you know if
there is any reason the problem couldn't occur on x86 on older gcc
without the fix?

Thanks.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602