linux-kernel - Re: [v6.12] WARNING: at kernel/sched/deadline.c:1995 enqueue_dl_entity (task blocked for more than 28262 seconds)

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Message-ID: <20241209105514.GK21636@noisy.programming.kicks-ass.net>
Date: Mon, 9 Dec 2024 11:55:14 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Vineeth Remanan Pillai <vineeth@...byteword.org>
Cc: Joel Fernandes <joel@...lfernandes.org>,
	Ilya Maximets <i.maximets@....org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>, vineethrp@...gle.com,
	shraash@...gle.com
Subject: Re: [v6.12] WARNING: at kernel/sched/deadline.c:1995
 enqueue_dl_entity (task blocked for more than 28262 seconds)

On Fri, Dec 06, 2024 at 11:57:30AM -0500, Vineeth Remanan Pillai wrote:

> I was able to reproduce this WARN_ON couple of days back with
> syzkaller. dlserver's dl_se gets enqueued during a update_curr while
> the dlserver is stopped. And subsequent dlserver start will cause a
> double enqueue. 

Right, I spotted that hole late last week. There is this thread:

  https://lore.kernel.org/all/20241209094941.GF21636@noisy.programming.kicks-ass.net/T/#u

Where I just added this thunk:

  @@ -1674,6 +1679,12 @@ void dl_server_start(struct sched_dl_entity *dl_se)
 
 void dl_server_stop(struct sched_dl_entity *dl_se)
 {
+	if (current->dl_server == dl_se) {
+		struct rq *rq = rq_of_dl_se(dl_se);
+		trace_printk("stop fair server %d\n", cpu_of(rq));
+		current->dl_server = NULL;
+	}
+
 	if (!dl_se->dl_runtime)
 		return;
 
Which was my attempt at plugging said hole. But since I do not have
means of reproduction, I'm not at all sure it is sufficient :/