From:   Pietro Borrello <borrello@diag.uniroma1.it>
Date:   Mon, 06 Feb 2023 22:33:54 +0000
Subject: [PATCH v3] sched: pick_next_rt_entity(): check list_entry
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Message-Id: <20230128-list-entry-null-check-sched-v3-1-b1a71bd1ac6b@diag.uniroma1.it>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Phil Auld <pauld@redhat.com>, Ben Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc:     Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>, Jakob Koschel <jkl820.git@gmail.com>,
        Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
        Pietro Borrello <borrello@diag.uniroma1.it>
Precedence: bulk

Commit 326587b84078 ("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: 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
---
Changes in v3:
- return NULL if the list is empty
- Link to v2: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v2-1-d8e010cce91b@diag.uniroma1.it

Changes in v2:
- pick_next_rt_entity(): emit warning instead of crashing
- Link to v1: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v1-1-c93085ee0055@diag.uniroma1.it
---
 kernel/sched/rt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ed2a47e4ddae..0a11f44adee5 100644
--- a/kernel/sched/rt.c
+++ b/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);
 

---
base-commit: 2241ab53cbb5cdb08a6b2d4688feb13971058f65
change-id: 20230128-list-entry-null-check-sched-a3f3dfd6d468

Best regards,
-- 
Pietro Borrello <borrello@diag.uniroma1.it>