patches/kthread-Allow-kthread_park-on-a-parked-kthread.patch

From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 4 May 2018 11:11:42 +0200
Subject: [PATCH] kthread: Allow kthread_park() on a parked kthread

[ Upstream commit b1f5b378e126133521df668379249fb8265121f1 ]

The following commit:

  85f1abe0019f ("kthread, sched/wait: Fix kthread_parkme() completion issue")

added a WARN() in the case where we call kthread_park() on an already
parked thread, because the old code wasn't doing the right thing there
and it wasn't at all clear that would happen.

It turns out, this does in fact happen, so we have to deal with it.

Instead of potentially returning early, also wait for the completion.
This does however mean we have to use complete_all() and re-initialize
the completion on re-use.

Reported-by: LKP <lkp@01.org>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel test robot <lkp@intel.com>
Cc: wfg@linux.intel.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 85f1abe0019f ("kthread, sched/wait: Fix kthread_parkme() completion issue")
Link: http://lkml.kernel.org/r/20180504091142.GI12235@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/kthread.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 2017a39ab490..481951bf091d 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -193,7 +193,7 @@ EXPORT_SYMBOL_GPL(kthread_parkme);
 
 void kthread_park_complete(struct task_struct *k)
 {
-	complete(&to_kthread(k)->parked);
+	complete_all(&to_kthread(k)->parked);
 }
 
 static int kthread(void *_create)
@@ -459,6 +459,7 @@ void kthread_unpark(struct task_struct *k)
 	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);
 	wake_up_state(k, TASK_PARKED);
 }
@@ -483,9 +484,6 @@ 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);
-- 
2.18.0