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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 24 Jan 2022 15:38:38 -0800
Message-ID: <CAKwvOdm+pnKb2PU7H5W1ks8eFd3DPEtN5kzjhnz5TLLwpBLOsA@mail.gmail.com>
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__
To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Borislav Petkov <bp@alien8.de>
Cc:     Vasily Gorbik <gor@linux.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        x86@kernel.org, llvm@lists.linux.dev, linux-sparse@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel test robot <lkp@intel.com>,
        Nathan Chancellor <nathan@kernel.org>,
        linux-toolchains@vger.kernel.org
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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