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

[Linus Torvalds]

On Mon, 17 Jul 2023 at 11:03, Paul E. McKenney <paulmck@xxxxxxxxxx> 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