Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

[David Howells]

Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> -	node = result.terminal_node.node;
> -	smp_read_barrier_depends();
> +	node = READ_ONCE(result.terminal_node.node); /* Address dependency. */

The main problem I have with this method of annotation is that it's not
obvious there's a barrier there or which side the barrier is.

I think one of the trickiest issues is that a barrier is typically between two
things and we're not making it clear what those two things actually are.

Also, I would say that the most natural interpretation of READ_ONCE() is that
the implicit barrier comes after the read, e.g.:

	f = READ_ONCE(stuff->foo);
	/* Implied barrier */
	look_at(f->a);
	look_at(f->b);

I.e. READ_ONCE() prevents stuff->foo from being reread whilst you access f and
orders LOAD(stuff->foo) before LOAD(f->a) and LOAD(f->b).

David