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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 17 Jul 2023 11:09:56 -0700
Message-ID: <CAHk-=whqX8r3EaaO48dgi=wEBZxJETg1m67HrScRbH1FK7GTRw@mail.gmail.com>
Subject: Re: [PATCH rcu 3/6] srcu,notifier: Remove #ifdefs in favor of SRCU
 Tiny srcu_usage
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org,
        atthias Brugger <matthias.bgg@gmail.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        Dmitry Osipenko <dmitry.osipenko@collabora.com>,
        Sachin Sant <sachinp@linux.ibm.com>,
        Joel Fernandes <joel@joelfernandes.org>
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On Mon, 17 Jul 2023 at 11:03, Paul E. McKenney <paulmck@kernel.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.

               Linus