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 <paul...@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).
> > >
> > > 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
