Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

From: Peter Zijlstra
Date: Fri Apr 19 2019 - 15:31:00 EST

On Fri, Apr 19, 2019 at 01:21:45PM -0400, Alan Stern wrote:
> Index: usb-devel/Documentation/atomic_t.txt
> ===================================================================
> --- usb-devel.orig/Documentation/atomic_t.txt
> +++ usb-devel/Documentation/atomic_t.txt
> @@ -171,7 +171,10 @@ The barriers:
> smp_mb__{before,after}_atomic()
> only apply to the RMW ops and can be used to augment/upgrade the ordering
> -inherent to the used atomic op. These barriers provide a full smp_mb().
> +inherent to the used atomic op. Unlike normal smp_mb() barriers, they order
> +only the RMW op itself against the instructions preceding the
> +smp_mb__before_atomic() or following the smp_mb__after_atomic(); they do
> +not order instructions on the other side of the RMW op at all.

Now it is I who is confused; what?

x = 1;
atomic_add(1, &a);
y = 1;

the stores to both x and y will be ordered as if an smp_mb() where
there. There is no order between a and y otoh.