On Wed, Oct 11, 2017 at 01:22:17PM +0100, Will Deacon wrote: > 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. You lost me on this one. Here is the side-by-side change, minus the comment: node = result.terminal_node.node; node = READ_ONCE(result.terminal_node.node); smp_read_barrier_depends(); The barrier was after the load that got annotated. Or are you talking about some other fixup? Thanx, Paul
