rcu-tasks: Check for abandoned callbacks

This commit adds a debugging scan for callbacks that got lost during a
callback-queueing transition.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -439,6 +439,11 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 			WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
 			pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
 		}
+		for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
+			struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+			WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
+		}
 		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
 	}