sched,livepatch: Use wake_up_if_idle()

Make sure to prod idle CPUs so they call klp_update_patch_state().

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Acked-by: Miroslav Benes <[e-mail]>
Acked-by: Vasily Gorbik <[e-mail]>
Tested-by: Petr Mladek <[e-mail]>
Tested-by: Vasily Gorbik <[e-mail]> # on s390
Link: https://lkml.kernel.org/r/[e-mail]

include/linux/sched/idle.h

···
 	CPU_MAX_IDLE_TYPES
 };
 
+#ifdef CONFIG_SMP
 extern void wake_up_if_idle(int cpu);
+#else
+static inline void wake_up_if_idle(int cpu) { }
+#endif
 
 /*
  * Idle thread specific functions to determine the need_resched

kernel/livepatch/transition.c

···
 	for_each_possible_cpu(cpu) {
 		task = idle_task(cpu);
 		if (cpu_online(cpu)) {
-			if (!klp_try_switch_task(task))
+			if (!klp_try_switch_task(task)) {
 				complete = false;
+				/* Make idle task go through the main loop. */
+				wake_up_if_idle(cpu);
+			}
 		} else if (task->patch_state != klp_target_state) {
 			/* offline idle tasks can be switched immediately */
 			clear_tsk_thread_flag(task, TIF_PATCH_PENDING);