rcu/nocb: Don't deoffload an offline CPU with pending work

Offloaded CPUs do not migrate their callbacks, instead relying on
their rcuo kthread to invoke them.  But if the CPU is offline, it
will be running neither its RCU_SOFTIRQ handler nor its rcuc kthread.
This means that de-offloading an offline CPU that still has pending
callbacks will strand those callbacks.  This commit therefore refuses
to toggle offline CPUs having pending callbacks.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -2227,6 +2227,15 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 	printk("De-offloading %d\n", rdp->cpu);
 
 	rcu_nocb_lock_irqsave(rdp, flags);
+	/*
+	 * If there are still pending work offloaded, the offline
+	 * CPU won't help much handling them.
+	 */
+	if (cpu_is_offline(rdp->cpu) && !rcu_segcblist_empty(&rdp->cblist)) {
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		return -EBUSY;
+	}
+
 	rcu_segcblist_offload(cblist, false);
 
 	if (rdp->nocb_cb_sleep) {