lockdep-related warning in kernel/sched/deadline.c::find_lock_later_rq()

[Luca Abeni <luca.abeni@unitn.it>]

Hi all,

after fixing task migrations for SCHED_DEADLINE, I started to see some
lockdep-related warnings that look like this:
[  794.428081] WARNING: CPU: 1 PID: 0 at /home/luca/Src/GRUB/linux-reclaiming/kernel/locking/lockdep.c:3407 lock_release+0x3f4/0x440()
[  794.428439] releasing a pinned lock
[  794.428439] Modules linked in:
[  794.428439] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.3.0-rc5+ #32
[  794.428439] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Bochs 01/01/2011
[  794.428439]  ffffffff81c946d0 ffff880007b03cc8 ffffffff8135abd2 ffff880007b03d10
[  794.428439]  ffff880007b03d00 ffffffff810546f1 ffff880007b159d8 ffffffff81096663
[  794.428439]  ffff88000754c558 0000000000000001 ffff88000754bd80 ffff880007b03d60
[  794.428439] Call Trace:
[  794.428439]  <IRQ>  [<ffffffff8135abd2>] dump_stack+0x4b/0x69
[  794.428439]  [<ffffffff810546f1>] warn_slowpath_common+0x81/0xc0
[  794.428439]  [<ffffffff81096663>] ? find_lock_later_rq+0x103/0x180
[  794.428439]  [<ffffffff81054777>] warn_slowpath_fmt+0x47/0x50
[  794.428439]  [<ffffffff8109feb4>] lock_release+0x3f4/0x440
[  794.428439]  [<ffffffff8194215a>] _raw_spin_unlock+0x1a/0x30
[  794.428439]  [<ffffffff81096663>] find_lock_later_rq+0x103/0x180
[  794.428439]  [<ffffffff81096741>] push_dl_task.part.33+0x61/0x190
[  794.428439]  [<ffffffff81096a94>] dl_task_timer+0x194/0x2e0
[  794.428439]  [<ffffffff81096900>] ? task_woken_dl+0x60/0x60
[  794.428439]  [<ffffffff810c3a87>] __hrtimer_run_queues+0x107/0x470
[  794.428439]  [<ffffffff810c3f82>] ? hrtimer_interrupt+0x82/0x1b0
[  794.428439]  [<ffffffff810c3fa6>] hrtimer_interrupt+0xa6/0x1b0
[  794.428439]  [<ffffffff81039570>] local_apic_timer_interrupt+0x30/0x60
[  794.428439]  [<ffffffff81039ee8>] smp_apic_timer_interrupt+0x38/0x50
[  794.428439]  [<ffffffff819436c4>] apic_timer_interrupt+0x84/0x90
[  794.428439]  <EOI>  [<ffffffff8100ea3b>] ? default_idle+0x1b/0x140
[  794.428439]  [<ffffffff8100ea39>] ? default_idle+0x19/0x140
[  794.428439]  [<ffffffff8100f25a>] arch_cpu_idle+0xa/0x10
[  794.428439]  [<ffffffff81097e45>] default_idle_call+0x25/0x40
[  794.428439]  [<ffffffff810981d0>] cpu_startup_entry+0x310/0x390
[  794.428439]  [<ffffffff810380c3>] start_secondary+0xf3/0x100
[  794.428439] ---[ end trace 1d35bf076299f814 ]---
(this happended on KVM, but I can trigger similar warnings on real hardware too).

This was not happening before my last patch, because push_dl_task() never migrated
any task, and always returned before calling find_lock_later_rq()...

Now, if I understand correctly the issue is that dl_task_timer() does:
	rq = task_rq_lock(p, &flags);
[...]
	if (has_pushable_dl_tasks(rq))
		push_dl_task(rq);
with task_rq_lock() that pins rq->lock and push_tl_task() that invokes
find_lock_later_rq() that unlocks rq->lock() while it is pinned.

I am not sure about how to fix this issue: as a first try, I did
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 142df26..5b1ba95 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -668,8 +668,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
  	 * Queueing this task back might have overloaded rq, check if we need
  	 * to kick someone away.
  	 */
-	if (has_pushable_dl_tasks(rq))
+	if (has_pushable_dl_tasks(rq)) {
+		lockdep_unpin_lock(&rq->lock);
  		push_dl_task(rq);
+		lockdep_pin_lock(&rq->lock);
+	}
  #endif

  unlock:

This removes the warning, but I am not sure if it is the correct fix (is it
valid to unpin rq->lock, here?).

If someone can confirm that this is the correct approach, I'll test the patch a
little bit more and then I'll send a properly signed-off patch; otherwise, if
someone can suggest the correct approach I'll try it.



			Thanks,
				Luca