sched/rt: pick_next_rt_entity(): check list_entry

Commit 326587b8 ("sched: fix goto retry in pick_next_task_rt()")
removed any path which could make pick_next_rt_entity() return NULL.
However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
pick_next_rt_entity()) still checks the error condition, which can
never happen, since list_entry() never returns NULL.
Remove the BUG_ON check, and instead emit a warning in the only
possible error condition here: the queue being empty which should
never happen.

Fixes: 326587b8 ("sched: fix goto retry in pick_next_task_rt()")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v3-1-b1a71bd1ac6b@diag.uniroma1.it

kernel/sched/rt.c

@@ -1777,6 +1777,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rt_rq *rt_rq)
 	BUG_ON(idx >= MAX_RT_PRIO);
 
 	queue = array->queue + idx;
+	if (SCHED_WARN_ON(list_empty(queue)))
+		return NULL;
 	next = list_entry(queue->next, struct sched_rt_entity, run_list);
 
 	return next;
@@ -1789,7 +1791,8 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
 
 	do {
 		rt_se = pick_next_rt_entity(rt_rq);
-		BUG_ON(!rt_se);
+		if (unlikely(!rt_se))
+			return NULL;
 		rt_rq = group_rt_rq(rt_se);
 	} while (rt_rq);