Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

[Will Deacon]

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.

Will