linux-kernel - [PATCH v2] sched/deadline: Fix BUG_ON condition for deboosted tasks

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Date:   Thu, 14 Jul 2022 17:19:08 +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:     "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        Juri Lelli <juri.lelli@...hat.com>, stable@...r.kernel.org,
        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 v2] 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: 64be6f1f5f71 ("sched/deadline: Don't replenish from a !SCHED_DEADLINE entity")
Cc: stable@...r.kernel.org
Signed-off-by: Juri Lelli <juri.lelli@...hat.com>

---
v1->v2
 - Make detection of faulty condition less fatal [Peter Zijlstra]
 - Cc stable and update fixes tag [Srivatsa S. Bhat]
---
 kernel/sched/deadline.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 5867e186c39a..0ab79d819a0d 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1703,7 +1703,10 @@ 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);
+		if (!(flags & ENQUEUE_REPLENISH))
+			printk_deferred_once("sched: DL de-boosted task PID %d: REPLENISH flag missing\n",
+					     task_pid_nr(p));
+
 		return;
 	}
 
-- 
2.36.1