rcu/segcblist: Add debug checks for segment lengths

This commit adds debug checks near the end of rcu_do_batch() that emit
warnings if an empty rcu_segcblist structure has non-zero segment counts,
or, conversely, if a non-empty structure has all-zero segment counts.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Fix queue/segment-length checks. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcu_segcblist.c

@@ -94,6 +94,18 @@ static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
 	return READ_ONCE(rsclp->seglen[seg]);
 }
 
+/* Return number of callbacks in segmented callback list by summing seglen. */
+long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp)
+{
+	long len = 0;
+	int i;
+
+	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
+		len += rcu_segcblist_get_seglen(rsclp, i);
+
+	return len;
+}
+
 /* Set the length of a segment of the rcu_segcblist structure. */
 static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
 {

kernel/rcu/rcu_segcblist.h

@@ -15,6 +15,9 @@ static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
 	return READ_ONCE(rclp->len);
 }
 
+/* Return number of callbacks in segmented callback list by summing seglen. */
+long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp);
+
 void rcu_cblist_init(struct rcu_cblist *rclp);
 void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
 void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,

kernel/rcu/tree.c

@@ -2434,6 +2434,7 @@ int rcutree_dead_cpu(unsigned int cpu)
 static void rcu_do_batch(struct rcu_data *rdp)
 {
 	int div;
+	bool __maybe_unused empty;
 	unsigned long flags;
 	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 	struct rcu_head *rhp;
@@ -2548,9 +2549,12 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	 * The following usually indicates a double call_rcu().  To track
 	 * this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
 	 */
-	WARN_ON_ONCE(count == 0 && !rcu_segcblist_empty(&rdp->cblist));
+	empty = rcu_segcblist_empty(&rdp->cblist);
+	WARN_ON_ONCE(count == 0 && !empty);
 	WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-		     count != 0 && rcu_segcblist_empty(&rdp->cblist));
+		     count != 0 && empty);
+	WARN_ON_ONCE(count == 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) != 0);
+	WARN_ON_ONCE(!empty && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
 
 	rcu_nocb_unlock_irqrestore(rdp, flags);