rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls

When the objects managed by rculist_nulls are allocated with
SLAB_TYPESAFE_BY_RCU, old readers may still hold references to an object
even though it is just now being added, which means the modification of
->next is visible to readers.  This patch therefore uses WRITE_ONCE()
for assignments to ->next.
Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

include/linux/rculist_nulls.h

@@ -101,7 +101,7 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
 {
 	struct hlist_nulls_node *first = h->first;
 
-	n->next = first;
+	WRITE_ONCE(n->next, first);
 	WRITE_ONCE(n->pprev, &h->first);
 	rcu_assign_pointer(hlist_nulls_first_rcu(h), n);
 	if (!is_a_nulls(first))
@@ -137,7 +137,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
 		last = i;
 
 	if (last) {
-		n->next = last->next;
+		WRITE_ONCE(n->next, last->next);
 		n->pprev = &last->next;
 		rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
 	} else {