linux-kernel - [PATCH 4.19 057/100] sched/deadline: Correctly handle active 0-lag timers

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Date:   Tue, 30 Apr 2019 13:38:26 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, luca abeni <luca.abeni@...tannapisa.it>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "chengjian (D)" <cj.chengjian@...wei.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 4.19 057/100] sched/deadline: Correctly handle active 0-lag timers

From: luca abeni <luca.abeni@...tannapisa.it>

commit 1b02cd6a2d7f3e2a6a5262887d2cb2912083e42f upstream.

syzbot reported the following warning:

   [ ] WARNING: CPU: 4 PID: 17089 at kernel/sched/deadline.c:255 task_non_contending+0xae0/0x1950

line 255 of deadline.c is:

	WARN_ON(hrtimer_active(&dl_se->inactive_timer));

in task_non_contending().

Unfortunately, in some cases (for example, a deadline task
continuosly blocking and waking immediately) it can happen that
a task blocks (and task_non_contending() is called) while the
0-lag timer is still active.

In this case, the safest thing to do is to immediately decrease
the running bandwidth of the task, without trying to re-arm the 0-lag timer.

Signed-off-by: luca abeni <luca.abeni@...tannapisa.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Juri Lelli <juri.lelli@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: chengjian (D) <cj.chengjian@...wei.com>
Link: https://lkml.kernel.org/r/20190325131530.34706-1-luca.abeni@santannapisa.it
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/sched/deadline.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -252,7 +252,6 @@ static void task_non_contending(struct t
 	if (dl_entity_is_special(dl_se))
 		return;
 
-	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
 	WARN_ON(dl_se->dl_non_contending);
 
 	zerolag_time = dl_se->deadline -
@@ -269,7 +268,7 @@ static void task_non_contending(struct t
 	 * If the "0-lag time" already passed, decrease the active
 	 * utilization now, instead of starting a timer
 	 */
-	if (zerolag_time < 0) {
+	if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) {
 		if (dl_task(p))
 			sub_running_bw(dl_se, dl_rq);
 		if (!dl_task(p) || p->state == TASK_DEAD) {