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Date:   Fri, 21 Aug 2020 02:37:48 +0200
From:   Thomas Gleixner <>
To:     Arvind Sankar <>
Cc:     Arvind Sankar <>,
        "Paul E. McKenney" <>,
        Nick Desaulniers <>,
        Ingo Molnar <>, Arnd Bergmann <>,
        Borislav Petkov <>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <>,
        "H. Peter Anvin" <>,
        "Kirill A. Shutemov" <>,
        Zhenzhong Duan <>,
        Kees Cook <>,
        Peter Zijlstra <>,
        Juergen Gross <>,
        Andy Lutomirski <>,
        Andrew Cooper <>,
        LKML <>,
        clang-built-linux <>,
        Will Deacon <>,
        Linus Torvalds <>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Thu, Aug 20 2020 at 09:06, Arvind Sankar wrote:
> I don't think that's an issue, or at least, not one where force_order
> helps.
> If the source for foo() is not visible to the compiler, the only reason
> force_order prevents the reordering is because foo() might have
> references to it, but equally foo() might have volatile asm, so the
> reordering isn't possible anyway.
> If the source is visible, force_order won't prevent any reordering
> except across references to force_order, but the only references are
> from the volatile asm's which already prevent reordering.
> I think force_order can only help with buggy compilers, and for those it
> should really have been an input-output operand -- it wouldn't currently
> do anything to prevent cr writes from being reordered.

Fair enough. Care to provide a patch which has the collected wisdom of
this thread in the changelog?



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