[PATCH v2 3/5] kthread: don't use to_live_kthread() in kthread_stop()

[Oleg Nesterov <oleg@redhat.com>]

kthread_stop() had to use to_live_kthread() simply because it was not
possible to access kthread->exited after the exiting kthread clears
task_struct->vfork_done. Now that to_kthread() is always valid we can
do wake_up_process() + wait_for_completion() unconditionally, we don't
care if it has already passed complete_vfork_done() or even dead.

The exiting kthread can get the spurious wakeup after mm_release() but
this is possible without this change too and this is fine, do_task_dead()
ensures that this can't make any harm.

Note: we can even change this function to use task_work_add() and avoid
->vfork_done altogether, probably we will do this later.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/kthread.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 7891a94..4dcbc8b 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -532,13 +532,11 @@ int kthread_stop(struct task_struct *k)
 	trace_sched_kthread_stop(k);
 
 	get_task_struct(k);
-	kthread = to_live_kthread(k);
-	if (kthread) {
-		set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
-		__kthread_unpark(k, kthread);
-		wake_up_process(k);
-		wait_for_completion(&kthread->exited);
-	}
+	kthread = to_kthread(k);
+	set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
+	__kthread_unpark(k, kthread);
+	wake_up_process(k);
+	wait_for_completion(&kthread->exited);
 	ret = k->exit_code;
 	put_task_struct(k);
 
-- 
2.5.0