| From | David Howells <> | Subject | Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() | Date | Wed, 11 Oct 2017 13:19:59 +0100 |
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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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