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SubjectRe: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression
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

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