On Wed, Oct 11, 2017 at 01:19:59PM +0100, David Howells wrote: > 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). FWIW, that's exactly what my patches do, this fixup looks a bit weird because it removes a prior barrier which suggests that either (a) it's in the wrong place to start with, or (b) we're annotating the wrong load. Will
