[PATCH]: kthread: Fix memory ordering in __kthread_parkme

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 07, 2014 at 10:39:48AM +0100, Thomas Gleixner wrote:
> > 2) the usage of __set_current_state(TASK_PARKED) in __kthread_parkme()
> >    is wrong AFAICT, one should always use set_current_state() for
> >    setting !TASK_RUNNING state. The comment with set_current_state()
> >    explains why.
> > 
> >    This would've allowed the test_bit(KTHREAD_SHOULD_PARK) load to have
> >    been satisfied before the store of TASK_PARKED.
> 
> My bad. Can you send a proper patch addressing that issue please? That
> should be tagged stable as well I guess.

Sure thing, something like so then?

---
Subject: kthread: Fix memory ordering in __kthread_parkme

One should always use set_current_state() to set !TASK_RUNNING states.

	set_current_state(TASK_*);		cond = true;
	/* mb */				/* wmb */
	if (!cond)				wake_up_state(, TASK_*);
		schedule();

By not having the mb we allow for the cond load to be satisfied before
the state store, this can result in:

	if (!cond)
						cond = true;
						wake_up_state(, TASK_*);
	__set_current_state(TASK_*);
		schedule();

Which would block 'forever', since the cond is still false and the
wakeup would not have seen the !TASK_RUNNING state.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/kthread.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 10e489c448fe..9787244d43ec 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -156,12 +156,12 @@ void *probe_kthread_data(struct task_struct *task)
 
 static void __kthread_parkme(struct kthread *self)
 {
-	__set_current_state(TASK_PARKED);
+	set_current_state(TASK_PARKED);
 	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
 		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
 			complete(&self->parked);
 		schedule();
-		__set_current_state(TASK_PARKED);
+		set_current_state(TASK_PARKED);
 	}
 	clear_bit(KTHREAD_IS_PARKED, &self->flags);
 	__set_current_state(TASK_RUNNING);