cpu/hotplug: Split do_cpu_down()

Split out the inner workings of do_cpu_down() to allow reuse of that
function for the upcoming SMT disabling mechanism.

No functional change.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>

CVE-2018-3620
CVE-2018-3646

[smb: Minor context adjustments, drop target argument]
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/cpu.c

@@ -430,20 +430,19 @@ static int _cpu_down(unsigned int cpu, int tasks_frozen)
 	return err;
 }
 
+static int cpu_down_maps_locked(unsigned int cpu)
+{
+	if (cpu_hotplug_disabled)
+		return -EBUSY;
+	return _cpu_down(cpu, 0);
+}
+
 int cpu_down(unsigned int cpu)
 {
 	int err;
 
 	cpu_maps_update_begin();
-
-	if (cpu_hotplug_disabled) {
-		err = -EBUSY;
-		goto out;
-	}
-
-	err = _cpu_down(cpu, 0);
-
-out:
+	err = cpu_down_maps_locked(cpu);
 	cpu_maps_update_done();
 	return err;
 }