[PATCH 2/2] kthread: don't use to_live_kthread() in kthread_park() and kthread_unpark()

[Oleg Nesterov <oleg@redhat.com>]

Now that to_kthread() is always valid we can change kthread_park() and
kthread_unpark() to use it and kill to_live_kthread().

kthread_unpark() is trivial, if KTHREAD_IS_PARKED is set we know that this
kthread has called complete(&self->parked), we do not care if it exits after
that if we race with kthread_stop().

kthread_park() is more tricky, but only because its semantics is not well
defined. It returns -ENOSYS if the thread exited but this can never happen
and as Roman pointed out kthread_park() can obviously block forever if it
could race with the exiting kthread.

I think we need to unexport kthread_park/unpark, and either make it return
"void" or actually fix the race with kthred_stop/exit. This patch just adds
WARN_ON(PF_EXITING) for now.

The usage of kthread_park() in cpuhp code (cpu.c, smpboot.c, stop_machine.c)
is fine. It can never see an exiting/exited kthread, smpboot_destroy_threads()
clears *ht->store, smpboot_park_thread() checks it is not NULL under the same
smpboot_threads_lock. cpuhp_threads and cpu_stop_threads never exit, so other
callers are fine too.

But it has two more users:

- watchdog_park_threads() and it does not look nice. The code is actually
  correct, get_online_cpus() ensures that kthread_park() can't race with
  itself (note that kthread_park() can't handle this race correctly), but
  imo it should not use kthread_park() directly.

- drivers/gpu/drm/amd/scheduler/gpu_scheduler.c and I think it should not
  use kthread_park() too.

  But this patch should not break this code. kthread_park() must not be
  called after amd_sched_fini() which does kthread_stop(), otherwise even
  to_live_kthread() is not safe because task_struct can be already freed
  and sched->thread can point to nowhere.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/kthread.c | 69 ++++++++++++++++++++------------------------------------
 1 file changed, 24 insertions(+), 45 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 4dcbc8b..01d2716 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -78,21 +78,6 @@ void free_kthread_struct(struct task_struct *k)
 	kfree(to_kthread(k));
 }
 
-#define __to_kthread(vfork)	\
-	container_of(vfork, struct kthread, exited)
-
-/*
- * TODO: kill it and use to_kthread(). But we still need the users
- * like kthread_stop() which has to sync with the exiting kthread.
- */
-static struct kthread *to_live_kthread(struct task_struct *k)
-{
-	struct completion *vfork = ACCESS_ONCE(k->vfork_done);
-	if (likely(vfork))
-		return __to_kthread(vfork);
-	return NULL;
-}
-
 /**
  * kthread_should_stop - should this kthread return now?
  *
@@ -441,8 +426,18 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 	return p;
 }
 
-static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
+/**
+ * kthread_unpark - unpark a thread created by kthread_create().
+ * @k:		thread created by kthread_create().
+ *
+ * Sets kthread_should_park() for @k to return false, wakes it, and
+ * waits for it to return. If the thread is marked percpu then its
+ * bound to the cpu again.
+ */
+void kthread_unpark(struct task_struct *k)
 {
+	struct kthread *kthread = to_kthread(k);
+
 	clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
 	/*
 	 * We clear the IS_PARKED bit here as we don't wait
@@ -460,22 +455,6 @@ static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
 		wake_up_state(k, TASK_PARKED);
 	}
 }
-
-/**
- * kthread_unpark - unpark a thread created by kthread_create().
- * @k:		thread created by kthread_create().
- *
- * Sets kthread_should_park() for @k to return false, wakes it, and
- * waits for it to return. If the thread is marked percpu then its
- * bound to the cpu again.
- */
-void kthread_unpark(struct task_struct *k)
-{
-	struct kthread *kthread = to_live_kthread(k);
-
-	if (kthread)
-		__kthread_unpark(k, kthread);
-}
 EXPORT_SYMBOL_GPL(kthread_unpark);
 
 /**
@@ -492,20 +471,20 @@ EXPORT_SYMBOL_GPL(kthread_unpark);
  */
 int kthread_park(struct task_struct *k)
 {
-	struct kthread *kthread = to_live_kthread(k);
-	int ret = -ENOSYS;
-
-	if (kthread) {
-		if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
-			set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
-			if (k != current) {
-				wake_up_process(k);
-				wait_for_completion(&kthread->parked);
-			}
+	struct kthread *kthread = to_kthread(k);
+
+	if (WARN_ON(k->flags & PF_EXITING))
+		return -ENOSYS;
+
+	if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
+		set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+		if (k != current) {
+			wake_up_process(k);
+			wait_for_completion(&kthread->parked);
 		}
-		ret = 0;
 	}
-	return ret;
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(kthread_park);
 
@@ -534,7 +513,7 @@ int kthread_stop(struct task_struct *k)
 	get_task_struct(k);
 	kthread = to_kthread(k);
 	set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
-	__kthread_unpark(k, kthread);
+	kthread_unpark(k);
 	wake_up_process(k);
 	wait_for_completion(&kthread->exited);
 	ret = k->exit_code;
-- 
2.5.0