linux-kernel - Re: [PATCH -tip 08/10] READ_ONCE, WRITE_ONCE: Remove data

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Date:   Thu, 21 May 2020 12:18:14 +0200
From:   Marco Elver <elver@...gle.com>
To:     Marco Elver <elver@...gle.com>
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>,
        Will Deacon <will@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH -tip 08/10] READ_ONCE, WRITE_ONCE: Remove data_race() wrapping

On Thu, 21 May 2020 at 11:47, Marco Elver <elver@...gle.com> wrote:
>
> On Fri, 15 May 2020 at 17:04, Marco Elver <elver@...gle.com> wrote:
> >
> > The volatile access no longer needs to be wrapped in data_race(),
> > because we require compilers that emit instrumentation distinguishing
> > volatile accesses.
> >
> > Signed-off-by: Marco Elver <elver@...gle.com>
> > ---
> >  include/linux/compiler.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 17c98b215572..fce56402c082 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -229,7 +229,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >  #define __READ_ONCE_SCALAR(x)                                          \
> >  ({                                                                     \
> >         typeof(x) *__xp = &(x);                                         \
> > -       __unqual_scalar_typeof(x) __x = data_race(__READ_ONCE(*__xp));  \
> > +       __unqual_scalar_typeof(x) __x = __READ_ONCE(*__xp);             \
> >         kcsan_check_atomic_read(__xp, sizeof(*__xp));                   \
>
> Some self-review: We don't need kcsan_check_atomic anymore, and this
> should be removed.
>
> I'll send v2 to address this (together with fix to data_race()
> removing nested statement expressions).

The other thing here is that we no longer require __xp, and can just
pass x into __READ_ONCE.

> >         smp_read_barrier_depends();                                     \
> >         (typeof(x))__x;                                                 \
> > @@ -250,7 +250,7 @@ do {                                                                        \
> >  do {                                                                   \
> >         typeof(x) *__xp = &(x);                                         \
> >         kcsan_check_atomic_write(__xp, sizeof(*__xp));                  \
>
> Same.

__xp can also be removed.

Note that this effectively aliases __WRITE_ONCE_SCALAR to
__WRITE_ONCE. To keep the API consistent with READ_ONCE, I assume we
want to keep __WRITE_ONCE_SCALAR, in case it is meant to change in
future?

> > -       data_race(({ __WRITE_ONCE(*__xp, val); 0; }));                  \
> > +       __WRITE_ONCE(*__xp, val);                                       \
> >  } while (0)
> >
> >  #define WRITE_ONCE(x, val)                                             \
> > --
> > 2.26.2.761.g0e0b3e54be-goog
> >