Re: [PATCH -tip v2 09/11] data_race: Avoid nested statement expression

Will Deacon
2020-05-21T13:31:57Z

On Thu, May 21, 2020 at 01:08:52PM +0200, Marco Elver wrote:
> It appears that compilers have trouble with nested statements
> expressions, as such make the data_race() macro be only a single
> statement expression. This will help us avoid potential problems in
> future as its usage increases.
>
> Link: https://lkml.kernel.org/r/20200520221712.GA21166@zn.tnic
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v2:
> * Add patch to series in response to above linked discussion.
> ---
> include/linux/compiler.h | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 7444f026eead..1f9bd9f35368 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -211,12 +211,11 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> */
> #define data_race(expr) \
> ({ \
> + __unqual_scalar_typeof(({ expr; })) __v; \
> __kcsan_disable_current(); \
> - ({ \
> - __unqual_scalar_typeof(({ expr; })) __v = ({ expr; }); \
> - __kcsan_enable_current(); \
> - __v; \
> - }); \
> + __v = ({ expr; }); \
> + __kcsan_enable_current(); \
> + __v; \

Hopefully it doesn't matter, but this will run into issues with 'const'
non-scalar expressions.

Will