[PATCH sched/urgent] sched/deadline: Fix stale rq->dl_server after a server pick returns RETRY_TASK

[Tejun Heo]

__pick_task_dl() records the server in rq->dl_server whenever the server's
pick returns non-NULL, RETRY_TASK included. __pick_next_task() clears
rq->dl_server at its top, but a RETRY_TASK restarts the pick below that
clear. put_prev_set_next_task() then hands the stale server to whatever the
restarted pick takes.

Since b3a2dfa8b42e ("sched/fair: Add newidle balance to pick_task_fair()"),
fair_server_pick_task() returns RETRY_TASK when the newidle balance released
the rq lock and a higher class task was enqueued meanwhile.

That task, an RT task for example, then carries the fair server in
p->dl_server until it is next put. The RV deadline monitors see the fair
server switch in with it, and core scheduling, if enabled meanwhile, treats
the task as deadline priority.

Don't record the server when its pick returns RETRY_TASK. The restarted pick
records whichever server it goes through, if any.

Verified with a synthetic reproducer.

Fixes: b3a2dfa8b42e ("sched/fair: Add newidle balance to pick_task_fair()")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/deadline.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index c0ebdcde5fe5..cc9c7cb7dc42 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2836,6 +2836,13 @@ static struct task_struct *__pick_task_dl(struct rq *rq, struct rq_flags *rf)
 			dl_server_stop(dl_se);
 			goto again;
 		}
+		/*
+		 * RETRY_TASK restarts the pick. Don't record @dl_se, or
+		 * put_prev_set_next_task() hands it to whatever the restarted
+		 * pick takes.
+		 */
+		if (p == RETRY_TASK)
+			return p;
 		rq->dl_server = dl_se;
 	} else {
 		p = dl_task_of(dl_se);
-- 
2.56.0