rcu: Change return type of rcu_spawn_one_boost_kthread()

The return value of rcu_spawn_one_boost_kthread() is not used any longer.
This commit therefore changes its return type from int to void, and
removes the cast to void from its callers.
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1123,7 +1123,7 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
  * already exist.  We only create this kthread for preemptible RCU.
  * Returns zero if all is well, a negated errno otherwise.
  */
-static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
+static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 {
 	int rnp_index = rnp - rcu_get_root();
 	unsigned long flags;
@@ -1131,25 +1131,27 @@ static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 	struct task_struct *t;
 
 	if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
-		return 0;
+		return;
 
 	if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
-		return 0;
+		return;
 
 	rcu_state.boost = 1;
+
 	if (rnp->boost_kthread_task != NULL)
-		return 0;
+		return;
+
 	t = kthread_create(rcu_boost_kthread, (void *)rnp,
 			   "rcub/%d", rnp_index);
-	if (IS_ERR(t))
-		return PTR_ERR(t);
+	if (WARN_ON_ONCE(IS_ERR(t)))
+		return;
+
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	rnp->boost_kthread_task = t;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	sp.sched_priority = kthread_prio;
 	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
-	return 0;
 }
 
 /*
@@ -1190,7 +1192,7 @@ static void __init rcu_spawn_boost_kthreads(void)
 	struct rcu_node *rnp;
 
 	rcu_for_each_leaf_node(rnp)
-		(void)rcu_spawn_one_boost_kthread(rnp);
+		rcu_spawn_one_boost_kthread(rnp);
 }
 
 static void rcu_prepare_kthreads(int cpu)
@@ -1200,7 +1202,7 @@ static void rcu_prepare_kthreads(int cpu)
 
 	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
 	if (rcu_scheduler_fully_active)
-		(void)rcu_spawn_one_boost_kthread(rnp);
+		rcu_spawn_one_boost_kthread(rnp);
 }
 
 #else /* #ifdef CONFIG_RCU_BOOST */