linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Message-ID: <87eenzqzmr.fsf@nanos.tec.linutronix.de>
Date:   Sat, 22 Aug 2020 02:43:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Will Deacon <will@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Fri, Aug 21 2020 at 16:16, Nick Desaulniers wrote:
> On Fri, Aug 21, 2020 at 4:04 PM Arvind Sankar <nivedita@...m.mit.edu> 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.

> 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.

Thanks,

        tglx