rcu: Report callbacks enqueued on offline CPU blind spot

Callbacks enqueued after rcutree_report_cpu_dead() fall into RCU barrier
blind spot. Report any potential misuse.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Author: Frederic Weisbecker

kernel/rcu/tree.c

@@ -3084,8 +3084,11 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
 	head->func = func;
 	head->next = NULL;
 	kasan_record_aux_stack_noalloc(head);
+
 	local_irq_save(flags);
 	rdp = this_cpu_ptr(&rcu_data);
+	RCU_LOCKDEP_WARN(!rcu_rdp_cpu_online(rdp), "Callback enqueued on offline CPU!");
+
 	lazy = lazy_in && !rcu_async_should_hurry();
 
 	/* Add the callback to our list. */