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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 21 May 2020 13:21:09 -0700
Message-ID: <CAKwvOdnR7BXw_jYS5PFTuUamcwprEnZ358qhOxSu6wSSSJhxOA@mail.gmail.com>
Subject: Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression
To:     Marco Elver <elver@google.com>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Borislav Petkov <bp@alien8.de>
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On Thu, May 21, 2020 at 7:22 AM 'Marco Elver' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> It appears that compilers have trouble with nested statement
> expressions. Therefore remove one level of statement expression nesting
> from the data_race() macro. This will help us avoid potential problems
> in future as its usage increases.
>
> Link: https://lkml.kernel.org/r/20200520221712.GA21166@zn.tnic
> Acked-by: Will Deacon <will@kernel.org>
> Signed-off-by: Marco Elver <elver@google.com>

Thanks Marco, I can confirm this series fixes the significant build
time regressions.

Tested-by: Nick Desaulniers <ndesaulniers@google.com>

More measurements in: https://github.com/ClangBuiltLinux/linux/issues/1032

Might want:
Reported-by: Borislav Petkov <bp@suse.de>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
too.

> ---
> v3:
> * Fix for 'const' non-scalar expressions.
> v2:
> * Add patch to series in response to above linked discussion.
> ---
>  include/linux/compiler.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 7444f026eead..379a5077e9c6 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -211,12 +211,12 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   */
>  #define data_race(expr)                                                        \
>  ({                                                                     \
> -       __kcsan_disable_current();                                      \
> -       ({                                                              \
> -               __unqual_scalar_typeof(({ expr; })) __v = ({ expr; });  \
> -               __kcsan_enable_current();                               \
> -               __v;                                                    \
> +       __unqual_scalar_typeof(({ expr; })) __v = ({                    \
> +               __kcsan_disable_current();                              \
> +               expr;                                                   \
>         });                                                             \
> +       __kcsan_enable_current();                                       \
> +       __v;                                                            \
>  })
>
>  /*
> --
> 2.26.2.761.g0e0b3e54be-goog
>
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-- 
Thanks,
~Nick Desaulniers