rcu: Assume rcu_init() is called before smp

The rcu_init() function is called way before SMP is initialized and
therefore only the boot CPU should be online at this stage.

Simplify the boot per-cpu initialization accordingly.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -4782,7 +4782,7 @@ static void __init kfree_rcu_batch_init(void)
 
 void __init rcu_init(void)
 {
-	int cpu;
+	int cpu = smp_processor_id();
 
 	rcu_early_boot_tests();
 
@@ -4802,11 +4802,10 @@ void __init rcu_init(void)
 	 * or the scheduler are operational.
 	 */
 	pm_notifier(rcu_pm_notify, 0);
-	for_each_online_cpu(cpu) {
-		rcutree_prepare_cpu(cpu);
-		rcu_cpu_starting(cpu);
-		rcutree_online_cpu(cpu);
-	}
+	WARN_ON(num_online_cpus() > 1); // Only one CPU this early in boot.
+	rcutree_prepare_cpu(cpu);
+	rcu_cpu_starting(cpu);
+	rcutree_online_cpu(cpu);
 
 	/* Create workqueue for Tree SRCU and for expedited GPs. */
 	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);