linux-kernel - Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER_

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Message-ID: <CAKwvOdm+pnKb2PU7H5W1ks8eFd3DPEtN5kzjhnz5TLLwpBLOsA@mail.gmail.com>
Date:   Mon, 24 Jan 2022 15:38:38 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        x86@...nel.org, llvm@...ts.linux.dev, linux-sparse@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

On Mon, Jan 24, 2022 at 3:26 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Jan 18, 2022 at 3:01 PM Josh Poimboeuf <jpoimboe@...hat.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