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

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Message-ID: <CAKwvOdnBT74081nCw2hcm2dxCacV0ume_noZ0=Q6e0UiBDF7wg@mail.gmail.com>
Date:   Tue, 25 Jan 2022 18:12:20 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        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, apinski@...vell.com
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

On Tue, Jan 25, 2022 at 4:59 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Jan 25, 2022 at 3:34 PM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
> >
> > Hi!
> >
> > On Mon, Jan 24, 2022 at 03:26:36PM -0800, Nick Desaulniers wrote:
> >
> > > 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.
> >
> > Please open a PR?
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104236

Andrew clarified (thanks Andrew!) that %= can't be used as I imagined
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104236#c4
and that I think was alluded to in
commit 3d1e236022cc ("objtool: Prevent GCC from merging annotate_unreachable()")
which is fine, so I'll just need to keep usage of __COUNTER__. I'll
try out a revised approach for this patch tomorrow.
-- 
Thanks,
~Nick Desaulniers