linux-kernel - Re: [PATCH rcu 3/6] srcu,notifier: Remove #ifdefs in favor of SRCU Tiny srcu

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Date:   Mon, 17 Jul 2023 11:16:46 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...a.com, rostedt@...dmis.org,
        atthias Brugger <matthias.bgg@...il.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Sachin Sant <sachinp@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH rcu 3/6] srcu,notifier: Remove #ifdefs in favor of SRCU
 Tiny srcu_usage

On Mon, Jul 17, 2023 at 11:09:56AM -0700, Linus Torvalds wrote:
> On Mon, 17 Jul 2023 at 11:03, Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > +// Dummy structure for srcu_notifier_head.
> > +struct srcu_usage {
> > +       char srcuu_dummy;
> > +};
> > +
> > +#define __SRCU_USAGE_INIT(name) { .srcuu_dummy = 0, }
> 
> You really should be able to just do
> 
>    struct srcu_usage { };
>    #define __SRCU_USAGE_INIT(name) { }
> 
> which is something we've done for ages for spinlocks in
> 
>     include/linux/spinlock_types_up.h
> 
> because while we had a gcc bug wrt empty structures, that was ages ago
> (ie "gcc-2.x").
> 
> See commit a1365647022e ("[PATCH] remove gcc-2 checks") from 2006.
> 
> So we've already had these kinds of empty dummy structs for literally
> over a decade in active use. Exactly so that you can avoid having to
> use #ifdef's etc, and can just always assume you have a spinlock, even
> if it doesn't generate any code or any data overhead.

Showing my age again, I guess.  ;-)

Thank you for the hint!  I will rework this as you suggest.

							Thanx, Paul