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Subject[PATCH v2 4/4] kthread: Simplify kthread_park() completion
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 <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@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);

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