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SubjectRe: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()
Date
Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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

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