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

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Message-ID: <875z9dioll.fsf@nanos.tec.linutronix.de>
Date:   Thu, 20 Aug 2020 12:44:06 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        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 Thu, Aug 13 2020 at 14:09, Arvind Sankar wrote:
> On Thu, Aug 13, 2020 at 10:37:01AM -0700, Paul E. McKenney wrote:
>> > Let me ask (hopefully) useful questions this time:
>> > 
>> >   Is a compiler allowed to reorder two 'asm volatile()'?
>> > 
>> >   Are there compilers (gcc >= 4.9 or other supported ones) which do that?
>> 
>> I would hope that the answer to both of these questions is "no"!
>> 
>> But I freely confess that I have been disappointed before on this sort
>> of thing.  :-/
>> 
>> 							Thanx, Paul
>
> Ok, I found this, so gcc developers consider re-ordering volatile asm
> wrt each other a bug at least.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

Yes. It prevents reordering of volatiles, but it does not necessarily
prevent reorder of something like this:

        asm volatile(...);
        foo();
        asm volatile(...);

it might turn that into

        foo();
        asm volatile(...);
        asm volatile(...);

if I understood their discussion correctly. So removing this magic is
not really straight forward.

Thanks,

        tglx