Merge branch 'smp-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull smp/hotplug fix from Thomas Gleixner:
 "A single fix for a brown paperbag bug:

  The unparking of the initial percpu threads of an upcoming CPU happens
  right now on the idle task, but that's wrong as the unpark function
  might sleep. Move it to the control CPU."

* 'smp-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  smp/hotplug: Move unparking of percpu threads to the control CPU

kernel/cpu.c

@@ -271,11 +271,25 @@ void cpu_hotplug_enable(void)
 EXPORT_SYMBOL_GPL(cpu_hotplug_enable);
 #endif	/* CONFIG_HOTPLUG_CPU */
 
+static void __cpuhp_kick_ap_work(struct cpuhp_cpu_state *st);
+
 static int bringup_wait_for_ap(unsigned int cpu)
 {
 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 
+	/* Wait for the CPU to reach CPUHP_AP_ONLINE_IDLE */
+	wait_for_completion(&st->done);
+	BUG_ON(!cpu_online(cpu));
+
+	/* Unpark the stopper thread and the hotplug thread of the target cpu */
+	stop_machine_unpark(cpu);
+	kthread_unpark(st->thread);
+
+	/* Should we go further up ? */
+	if (st->target > CPUHP_AP_ONLINE_IDLE) {
+		__cpuhp_kick_ap_work(st);
 		wait_for_completion(&st->done);
+	}
 	return st->result;
 }
 
@@ -296,9 +310,7 @@ static int bringup_cpu(unsigned int cpu)
 	irq_unlock_sparse();
 	if (ret)
 		return ret;
-	ret = bringup_wait_for_ap(cpu);
-	BUG_ON(!cpu_online(cpu));
-	return ret;
+	return bringup_wait_for_ap(cpu);
 }
 
 /*
@@ -767,30 +779,19 @@ void notify_cpu_starting(unsigned int cpu)
 }
 
 /*
- * Called from the idle task. We need to set active here, so we can kick off
- * the stopper thread and unpark the smpboot threads. If the target state is
- * beyond CPUHP_AP_ONLINE_IDLE we kick cpuhp thread and let it bring up the
- * cpu further.
+ * Called from the idle task. Wake up the controlling task which brings the
+ * stopper and the hotplug thread of the upcoming CPU up and then delegates
+ * the rest of the online bringup to the hotplug thread.
  */
 void cpuhp_online_idle(enum cpuhp_state state)
 {
 	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
-	unsigned int cpu = smp_processor_id();
 
 	/* Happens for the boot cpu */
 	if (state != CPUHP_AP_ONLINE_IDLE)
 		return;
 
 	st->state = CPUHP_AP_ONLINE_IDLE;
-
-	/* Unpark the stopper thread and the hotplug thread of this cpu */
-	stop_machine_unpark(cpu);
-	kthread_unpark(st->thread);
-
-	/* Should we go further up ? */
-	if (st->target > CPUHP_AP_ONLINE_IDLE)
-		__cpuhp_kick_ap_work(st);
-	else
 	complete(&st->done);
 }