======================================
| [ 30.477535][ T66] #3: ffffffff9c207e88 (rtnl_mutex){+.+.}-{4:4}, at: default_device_exit_batch (net/core/dev.c:12109)
| [ 30.477849][ T66] #4: ffffffff9c1f6a70 ((netdev_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
| [   30.478161][   T66]
| [   30.478161][   T66] stack backtrace:
[   30.478622][   T66] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   30.478820][   T66] Workqueue: netns cleanup_net
[   30.478978][   T66] Call Trace:
[   30.479095][   T66]  <TASK>
[ 30.479175][ T66] dump_stack_lvl (lib/dump_stack.c:123)
[ 30.479331][ T66] print_deadlock_bug (kernel/locking/lockdep.c:3040)
[ 30.479481][ T66] validate_chain (kernel/locking/lockdep.c:3894)
[ 30.479632][ T66] ? __pfx_validate_chain (kernel/locking/lockdep.c:3860)
[ 30.479786][ T66] ? hlock_class (./arch/x86/include/asm/bitops.h:227 ./arch/x86/include/asm/bitops.h:239 ./include/asm-generic/bitops/instrumented-non-atomic.h:142 kernel/locking/lockdep.c:228)
[ 30.479936][ T66] ? mark_lock (kernel/locking/lockdep.c:4727 (discriminator 3))
[ 30.480056][ T66] __lock_acquire (kernel/locking/lockdep.c:5226)
[ 30.480204][ T66] ? spin_bug (./include/linux/pid.h:232 kernel/locking/spinlock_debug.c:64 kernel/locking/spinlock_debug.c:78)
[ 30.480318][ T66] ? debug_object_assert_init (./include/linux/err.h:81 lib/debugobjects.c:1010)
[ 30.480471][ T66] lock_acquire.part.0 (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5851)
[ 30.480625][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.480814][ T66] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 30.480966][ T66] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 30.481117][ T66] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 30.481271][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.481456][ T66] down_read (./arch/x86/include/asm/preempt.h:79 kernel/locking/rwsem.c:1246 kernel/locking/rwsem.c:1261 kernel/locking/rwsem.c:1526)
[ 30.481571][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.481758][ T66] ? __pfx_down_read (kernel/locking/rwsem.c:1522)
[ 30.481911][ T66] blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.482061][ T66] __dev_close_many (net/core/dev.c:1537 (discriminator 3))
[ 30.482211][ T66] ? __pfx___dev_close_many (net/core/dev.c:1525)
[ 30.482371][ T66] dev_close_many (net/core/dev.c:1588)
[ 30.482524][ T66] ? __pfx_dev_close_many (net/core/dev.c:1578)
[ 30.482678][ T66] ? ipv6_mc_down (net/ipv6/mcast.c:1140 net/ipv6/mcast.c:2777)
[ 30.482841][ T66] vlan_device_event (net/8021q/vlan.c:451)
[ 30.482995][ T66] ? __pfx_vlan_device_event (net/8021q/vlan.c:362)
[ 30.483150][ T66] ? __pfx_br_device_event (net/bridge/br.c:29)
[ 30.483306][ T66] ? packet_notifier (net/packet/af_packet.c:4282)
[ 30.483465][ T66] notifier_call_chain (kernel/notifier.c:85 (discriminator 2))
[ 30.483626][ T66] blocking_notifier_call_chain (kernel/notifier.c:381 kernel/notifier.c:368)
[ 30.483777][ T66] dev_close_many (net/core/dev.c:1591)
[ 30.483926][ T66] ? fou_exit_net (net/ipv4/fou_core.c:1234)
[ 30.484077][ T66] ? __pfx_dev_close_many (net/core/dev.c:1578)
[ 30.484226][ T66] ? __pfx___mutex_trylock_common (kernel/locking/mutex.c:79)
[ 30.484413][ T66] unregister_netdevice_many_notify (net/core/dev.c:11549)
[ 30.484602][ T66] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11516)
[ 30.484796][ T66] ? default_device_exit_batch (net/core/dev.c:12109)
[ 30.484952][ T66] ? mutex_is_locked (./arch/x86/include/asm/atomic64_64.h:15 ./include/linux/atomic/atomic-arch-fallback.h:2583 ./include/linux/atomic/atomic-long.h:38 ./include/linux/atomic/atomic-instrumented.h:3189 kernel/locking/mutex.h:47 kernel/locking/mutex.c:66)
[ 30.485106][ T66] ? rtnl_is_locked (net/core/rtnetlink.c:163)
[ 30.485262][ T66] ? unregister_netdevice_queue (net/core/dev.c:11501)
[ 30.485456][ T66] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11500)
[ 30.485652][ T66] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11500)
[ 30.485846][ T66] default_device_exit_batch (net/core/dev.c:12123)
[ 30.486009][ T66] ? __pfx_default_device_exit_batch (net/core/dev.c:12098)
[ 30.486204][ T66] ? ops_exit_list (net/core/net_namespace.c:171 (discriminator 3))
[ 30.486360][ T66] cleanup_net (net/core/net_namespace.c:647 (discriminator 3))
[ 30.486519][ T66] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 30.486670][ T66] ? __pfx_cleanup_net (net/core/net_namespace.c:592)
[ 30.486825][ T66] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 30.486970][ T66] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 30.487123][ T66] ? process_one_work (kernel/workqueue.c:3205)
[ 30.487279][ T66] process_one_work (kernel/workqueue.c:3229)
[ 30.487430][ T66] ? __pfx___lock_release (kernel/locking/lockdep.c:5501)
[ 30.487581][ T66] ? __pfx_process_one_work (kernel/workqueue.c:3131)
[ 30.487736][ T66] ? assign_work (kernel/workqueue.c:1200)
[ 30.487888][ T66] worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391)
[ 30.488039][ T66] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4347 kernel/locking/lockdep.c:4406)
[ 30.488227][ T66] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 30.488376][ T66] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 30.488524][ T66] kthread (kernel/kthread.c:389)
[ 30.488639][ T66] ? __pfx_kthread (kernel/kthread.c:342)
[ 30.488786][ T66] ret_from_fork (arch/x86/kernel/process.c:147)
[ 30.488938][ T66] ? __pfx_kthread (kernel/kthread.c:342)


Finger prints:
print_deadlock_bug:validate_chain:__lock_acquire:down_read:blocking_notifier_call_chain