linux-kernel - [PATCH] sched/deadline: Fix BUG_ON condition for deboosted tasks

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Message-Id: <20220713075014.411739-1-juri.lelli@redhat.com>
Date:   Wed, 13 Jul 2022 09:50:14 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: [PATCH] sched/deadline: Fix BUG_ON condition for deboosted tasks

Tasks the are being deboosted from SCHED_DEADLINE might enter
enqueue_task_dl() one last time and hit an erroneous BUG_ON condition:
since they are not boosted anymore, the if (is_dl_boosted()) branch is
not taken, but the else if (!dl_prio) is and inside this one we
BUG_ON(!is_dl_boosted), which is of course false (BUG_ON triggered)
otherwise we had entered the if branch above. Long story short, the
current condition doesn't make sense and always leads to triggering of a
BUG.

Fix this by only checking enqueue flags, properly: ENQUEUE_REPLENISH has
to be present, but additional flags are not a problem.

Fixes: 2279f540ea7d ("sched/deadline: Fix priority inheritance with multiple scheduling classes")
Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
---
 kernel/sched/deadline.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 5867e186c39a..0447d46f4718 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1703,7 +1703,7 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 		 * the throttle.
 		 */
 		p->dl.dl_throttled = 0;
-		BUG_ON(!is_dl_boosted(&p->dl) || flags != ENQUEUE_REPLENISH);
+		BUG_ON(!(flags & ENQUEUE_REPLENISH));
 		return;
 	}
 
-- 
2.36.1