[PATCH] sched/deadline: Fix switched_from_dl

[Juri Lelli <juri.lelli@redhat.com>]

Mark noticed that syzkaller is able to reliably trigger the following

  dl_rq->running_bw > dl_rq->this_bw
  WARNING: CPU: 1 PID: 153 at kernel/sched/deadline.c:124 switched_from_dl+0x454/0x608
  Kernel panic - not syncing: panic_on_warn set ...

  CPU: 1 PID: 153 Comm: syz-executor253 Not tainted 4.18.0-rc3+ #29
  Hardware name: linux,dummy-virt (DT)
  Call trace:
   dump_backtrace+0x0/0x458
   show_stack+0x20/0x30
   dump_stack+0x180/0x250
   panic+0x2dc/0x4ec
   __warn_printk+0x0/0x150
   report_bug+0x228/0x2d8
   bug_handler+0xa0/0x1a0
   brk_handler+0x2f0/0x568
   do_debug_exception+0x1bc/0x5d0
   el1_dbg+0x18/0x78
   switched_from_dl+0x454/0x608
   __sched_setscheduler+0x8cc/0x2018
   sys_sched_setattr+0x340/0x758
   el0_svc_naked+0x30/0x34

syzkaller reproducer runs a bunch of threads that constantly switch
between DEADLINE and NORMAL classes while interacting through futexes.

The splat above is caused by the fact that if a DEADLINE task is setattr
back to NORMAL while in non_contending state (blocked on a futex -
inactive timer armed), its contribution to running_bw is not removed
before sub_rq_bw() gets called (!task_on_rq_queued() branch) and the
latter sees running_bw > this_bw.

Fix it by removing a task contribution from running_bw if the task is
not queued and in non_contending state while switched to a different
class.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/sched/deadline.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index fbfc3f1d368a..10c7b51c0d1f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2290,8 +2290,17 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 	if (task_on_rq_queued(p) && p->dl.dl_runtime)
 		task_non_contending(p);
 
-	if (!task_on_rq_queued(p))
+	if (!task_on_rq_queued(p)) {
+		/*
+		 * Inactive timer is armed. However, p is leaving DEADLINE and
+		 * might migrate away from this rq while continuing to run on
+		 * some other class. We need to remove its contribution from
+		 * this rq running_bw now, or sub_rq_bw (below) will complain.
+		 */
+		if (p->dl.dl_non_contending)
+			sub_running_bw(&p->dl, &rq->dl);
 		sub_rq_bw(&p->dl, &rq->dl);
+	}
 
 	/*
 	 * We cannot use inactive_task_timer() to invoke sub_running_bw()
-- 
2.14.4