On Wed, Oct 11, 2017 at 03:18:17PM +0100, Will Deacon wrote: > On Wed, Oct 11, 2017 at 05:54:51AM -0700, Paul E. McKenney wrote: > > 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. > > Yes, sorry, I completely lost my ability to read diff. Looking again, I > don't actually know what's being ordered by the smp_read_barrier_depends() > in the snippet above, given that assigning "node" is a load from the stack > afaict. Good point, and in fact the required READ_ONCE() already exists off in assoc_array_walk(). Updated. Thanx, Paul
