rcu/nocb: Allow lockless use of rcu_segcblist_empty()

Currently, rcu_segcblist_empty() assumes that the callback list is not
being changed by other CPUs, but upcoming changes will require it to
operate locklessly.  This commit therefore adds the needed READ_ONCE()
call, along with the WRITE_ONCE() calls when updating the callback list's
->head field.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/rcu_segcblist.c

@@ -213,7 +213,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
 	if (!rcu_segcblist_ready_cbs(rsclp))
 		return; /* Nothing to do. */
 	*rclp->tail = rsclp->head;
-	rsclp->head = *rsclp->tails[RCU_DONE_TAIL];
+	WRITE_ONCE(rsclp->head, *rsclp->tails[RCU_DONE_TAIL]);
 	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
 	rclp->tail = rsclp->tails[RCU_DONE_TAIL];
 	for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--)
@@ -268,7 +268,7 @@ void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
 	if (!rclp->head)
 		return; /* No callbacks to move. */
 	*rclp->tail = rsclp->head;
-	rsclp->head = rclp->head;
+	WRITE_ONCE(rsclp->head, rclp->head);
 	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
 		if (&rsclp->head == rsclp->tails[i])
 			WRITE_ONCE(rsclp->tails[i], rclp->tail);

kernel/rcu/rcu_segcblist.h

@@ -36,7 +36,7 @@ struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp);
  */
 static inline bool rcu_segcblist_empty(struct rcu_segcblist *rsclp)
 {
-	return !rsclp->head;
+	return !READ_ONCE(rsclp->head);
 }
 
 /* Return number of callbacks in segmented callback list. */