Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Jan 24, 2022 at 3:26 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Tue, Jan 18, 2022 at 3:01 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > Apparently this patch isn't going to work after all :-(
> >
> >   https://lkml.kernel.org/r/202201190632.lhlaiCBk-lkp@intel.com
>
> I noticed in that report and
> https://lore.kernel.org/lkml/202201190702.XNSXrMTK-lkp@intel.com/
> that gcc-9 was used.  I wonder if %= has been fixed in gcc-10+? Have
> there been other reports with gcc-10+ for my patch?
>
> Boris' case of xfrm_output_resume is yet a third case; Boris, what
> version of gcc did you spot that with?
>
> If this is fixed in gcc-10, then we can probably add a comment with a
> FIXME link to the issue or commit to replace __COUNTER__ with %= one
> day.  If not, then we can probably come up with a reduced test case
> for the GCC devs to take a look at, then add the FIXME comment to
> kernel sources.


$ wget https://download.01.org/0day-ci/archive/20220119/202201190702.XNSXrMTK-lkp@intel.com/config
-O .config
$ make -j72 -s olddefconfig drivers/net/wireless/mac80211_hwsim.o
drivers/net/wireless/mac80211_hwsim.o: warning: objtool:
mac80211_hwsim_tx()+0x9aa: unreachable instruction
$ gcc --version
gcc (Debian 11.2.0-12) 11.2.0

:(

Let me see if I can come up with a reduced test case that I will
report upstream to https://gcc.gnu.org/bugzilla/.
-- 
Thanks,
~Nick Desaulniers