[PATCH] 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.

Signed-off-by: Jason Xing <kerneljasonxing@linux.alibaba.com>
---
 kernel/sched/psi.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 7acc632..66f4385 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1133,6 +1133,12 @@ static void psi_trigger_destroy(struct kref *ref)
 	if (kworker_to_destroy) {
 		kthread_cancel_delayed_work_sync(&group->poll_work);
 		kthread_destroy_worker(kworker_to_destroy);
+		/*
+		 * The poll_work should have the chance to be put into the
+		 * kthread queue when calling poll syscall next time. So
+		 * reset poll_scheduled to zero as group_init() does
+		 */
+		atomic_set(&group->poll_scheduled, 0);
 	}
 	kfree(t);
 }
-- 
1.8.3.1