[PATCH v2] psi: get poll_work to run when calling poll syscall next time

[Jason Xing <kerneljasonxing@linux.alibaba.com>]

Only when calling the poll syscall the first time can user
receive POLLPRI correctly. After that, user always fails to
acquire the event signal.

Reproduce case:
1. Get the monitor code in Documentation/accounting/psi.txt
2. Run it, and wait for the event triggered.
3. Kill and restart the process.

If the user doesn't kill the monitor process, it seems the
poll_work works fine. After killing and restarting the monitor,
the poll_work in kernel will never run again due to the wrong
value of poll_scheduled. Therefore, we should reset the value
as group_init() does after the last trigger is destroyed.

[PATCH V2]
In the patch v2, I put the atomic_set(&group->poll_scheduled, 0);
into the right place.
Here I quoted from Johannes as the best explaination:
"The question is why we can end up with poll_scheduled = 1 but the work
not running (which would reset it to 0). And the answer is because the
scheduling side sees group->poll_kworker under RCU protection and then
schedules it, but here we cancel the work and destroy the worker. The
cancel needs to pair with resetting the poll_scheduled flag."

Signed-off-by: Jason Xing <kerneljasonxing@linux.alibaba.com>
Reviewed-by: Caspar Zhang <caspar@linux.alibaba.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 kernel/sched/psi.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 7acc632..acdada0 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1131,7 +1131,14 @@ static void psi_trigger_destroy(struct kref *ref)
 	 * deadlock while waiting for psi_poll_work to acquire trigger_lock
 	 */
 	if (kworker_to_destroy) {
+		/*
+		 * After the RCU grace period has expired, the worker
+		 * can no longer be found through group->poll_kworker.
+		 * But it might have been already scheduled before
+		 * that - deschedule it cleanly before destroying it.
+		 */
 		kthread_cancel_delayed_work_sync(&group->poll_work);
+		atomic_set(&group->poll_scheduled, 0);
 		kthread_destroy_worker(kworker_to_destroy);
 	}
 	kfree(t);
-- 
1.8.3.1