[PATCH v2 4/4] kthread: Simplify kthread_park() completion

[Peter Zijlstra]

Oleg explains the reason we could hit park+park is that
smpboot_update_cpumask_percpu_thread()'s

  for_each_cpu_and(cpu, &tmp, cpu_online_mask)
        smpboot_park_kthread();

turns into:

  for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)
        smpboot_park_kthread();

on UP, ignoring the mask. But since we just completely removed that
function, this is no longer relevant.

So revert commit:

  b1f5b378e126 ("kthread: Allow kthread_park() on a parked kthread")

Suggested-by: Oleg Nesterov <o...@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 kernel/kthread.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -190,7 +190,7 @@ static void __kthread_parkme(struct kthr
                if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
                        break;
 
-               complete_all(&self->parked);
+               complete(&self->parked);
                schedule();
        }
        __set_current_state(TASK_RUNNING);
@@ -465,7 +465,6 @@ void kthread_unpark(struct task_struct *
        if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
                __kthread_bind(k, kthread->cpu, TASK_PARKED);
 
-       reinit_completion(&kthread->parked);
        clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
        /*
         * __kthread_parkme() will either see !SHOULD_PARK or get the wakeup.
@@ -493,6 +492,9 @@ int kthread_park(struct task_struct *k)
        if (WARN_ON(k->flags & PF_EXITING))
                return -ENOSYS;
 
+       if (WARN_ON_ONCE(test_bit(KTHREAD_SHOULD_PARK, &kthread->flags)))
+               return -EBUSY;
+
        set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
        if (k != current) {
                wake_up_process(k);