rhashtable: Add barrier to ensure we see new tables in walker

The walker is a lockless reader so it too needs an smp_rmb before
reading the future_tbl field in order to see any new tables that
may contain elements that we should have walked over.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/rhashtable.c

@@ -477,6 +477,9 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
 		iter->skip = 0;
 	}
 
+	/* Ensure we see any new tables. */
+	smp_rmb();
+
 	iter->walker->tbl = rht_dereference_rcu(tbl->future_tbl, ht);
 	if (iter->walker->tbl) {
 		iter->slot = 0;