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.
Or are you talking about some other fixup?
Thanx, Paul
