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

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Date:   Thu, 21 May 2020 15:39:38 +0200
From:   Marco Elver <elver@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH -tip v2 09/11] data_race: Avoid nested statement expression

On Thu, 21 May 2020 at 15:31, Will Deacon <will@...nel.org> wrote:
>
> 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@...gle.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.

Good point. We could move the kcsan_disable_current() into ({
__kcsan_disable_current(); expr; }).

Will fix for v3.

Thanks,
-- Marco

> Will
>
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