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

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Message-ID: <2aab04d1-c16b-44e4-a283-7bbf8cba28e7@p183>
Date: Mon, 24 Jun 2024 17:42:54 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: Marco Elver <elver@...gle.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, 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;
	})