rcu-tasks: Handle the running-offline idle-task special case

The idle task corresponding to an offline CPU can appear to be running
while that CPU is offline.  This commit therefore adds checks for this
situation, treating it as a quiescent state.  Because the tasklist scan
and the holdout-list scan now exclude CPU-hotplug operations, readers
on the CPU-hotplug paths are still waited for.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -818,16 +818,20 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 {
 	int cpu = task_cpu(t);
 	bool in_qs = false;
+	bool ofl = cpu_is_offline(cpu);
 
 	if (task_curr(t)) {
+		WARN_ON_ONCE(ofl & !is_idle_task(t));
+
 		// If no chance of heavyweight readers, do it the hard way.
-		if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+		if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
 			return false;
 
 		// If heavyweight readers are enabled on the remote task,
 		// we can inspect its state despite its currently running.
 		// However, we cannot safely change its state.
-		if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
+		if (!ofl && // Check for "running" idle tasks on offline CPUs.
+		    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
 			return false; // No quiescent state, do it the hard way.
 		in_qs = true;
 	} else {