linux-kernel - Re: [PATCH] compiler.h: simplify data

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Message-ID: <CANpmjNMZU=T6J5OBpELxB=ZqOnrkou2iRG7zaqoNy7bCGgH9hA@mail.gmail.com>
Date: Mon, 24 Jun 2024 17:09:02 +0200
From: Marco Elver <elver@...gle.com>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org, 
	akpm@...ux-foundation.org
Subject: Re: [PATCH] compiler.h: simplify data_race() macro

On Mon, 24 Jun 2024 at 16:43, Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> On Mon, Jun 24, 2024 at 02:27:43PM +0200, Marco Elver wrote:
> > On Mon, 24 Jun 2024 at 13:49, Alexey Dobriyan <adobriyan@...il.com> wrote:
> > >
> > > Use auto type deduction and comma expression to decrease macro expansion
> > > size.
> > >
> > > __unqual_scalar_typeof() is quite wordy macro by itself.
> > >
> > > "expr" can be arbitrary complex so not expanding it twice is good.
> > > Should be faster too because type is deduced only once
> > > from the initializer.
> > >
> > > Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> >
> > Thanks for cleaning up.  That code certainly predates the availability
> > of __auto_type. Although if I recall correctly, __unqual_scalar_typeof
> > became the first user of _Generic (the first C11 keyword we used in
> > the kernel?), but we used some ifdef to still support ancient
> > compilers initially (that definitely also didn't have __auto_type).
> >
> > Reviewed-by: Marco Elver <elver@...gle.com>
> >
> > Which tree is this for?
> >
> > > ---
> > >
> > >  include/linux/compiler.h |    5 +----
> > >  1 file changed, 1 insertion(+), 4 deletions(-)
> > >
> > > --- a/include/linux/compiler.h
> > > +++ b/include/linux/compiler.h
> > > @@ -200,10 +200,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> > >   */
> > >  #define data_race(expr)                                                        \
> > >  ({                                                                     \
> > > -       __unqual_scalar_typeof(({ expr; })) __v = ({                    \
> > > -               __kcsan_disable_current();                              \
> > > -               expr;                                                   \
> > > -       });                                                             \
> > > +       __auto_type __v = (__kcsan_disable_current(), expr);            \
> > >         __kcsan_enable_current();                                       \
> > >         __v;                                                            \
> > >  })
>
> I just realized, comma expression should not be necesary.
> -Wdeclaration-after-statement prohibited simple
>
>         ({
>                 __kcsan_disable_current
>                 auto v = (expr);
>                 __kcsan_enable_current
>                 v;
>         })

Even better.