======================================
| [ 30.150123][ T66] #3: ffffffffaf9f1088 (rtnl_mutex){+.+.}-{4:4}, at: default_device_exit_batch (net/core/dev.c:12109)
| [ 30.150433][ T66] #4: ffffffffaf9dfc30 ((netdev_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
| [   30.150751][   T66]
| [   30.150751][   T66] stack backtrace:
[   30.151206][   T66] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   30.151401][   T66] Workqueue: netns cleanup_net
[   30.151560][   T66] Call Trace:
[   30.151683][   T66]  <TASK>
[ 30.151763][ T66] dump_stack_lvl (lib/dump_stack.c:123)
[ 30.151929][ T66] print_deadlock_bug (kernel/locking/lockdep.c:3040)
[ 30.152085][ T66] validate_chain (kernel/locking/lockdep.c:3894)
[ 30.152240][ T66] ? __pfx_validate_chain (kernel/locking/lockdep.c:3860)
[ 30.152396][ 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)
[ 30.152550][ T66] ? mark_lock (kernel/locking/lockdep.c:4727 (discriminator 3))
[ 30.152669][ T66] __lock_acquire (kernel/locking/lockdep.c:5226)
[ 30.152830][ T66] lock_acquire.part.0 (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5851)
[ 30.152982][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.153171][ T66] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 30.153327][ T66] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 30.153484][ T66] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 30.153638][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.153828][ 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)
[ 30.153952][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.154143][ T66] ? __pfx_down_read (kernel/locking/rwsem.c:1522)
[ 30.154304][ T66] ? __pfx___lock_release (kernel/locking/lockdep.c:5501)
[ 30.154459][ T66] blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 30.154614][ T66] __netdev_upper_dev_unlink (net/core/dev.c:8000 (discriminator 11) net/core/dev.c:8017 (discriminator 11) net/core/dev.c:8167 (discriminator 11))
[ 30.154767][ T66] ? __pfx___netdev_upper_dev_unlink (net/core/dev.c:8151)
[ 30.154956][ T66] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 30.155115][ T66] ? del_nbp (net/bridge/br_if.c:215 net/bridge/br_if.c:359) bridge
[ 30.155305][ 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)
[ 30.155461][ T66] netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 30.155615][ T66] ? __pfx_netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 30.155808][ T66] ? mutex_spin_on_owner (kernel/locking/mutex.c:353)
[ 30.155963][ T66] ? rtnl_is_locked (net/core/rtnetlink.c:163)
[ 30.156127][ T66] del_nbp (net/bridge/br_if.c:365) bridge
[ 30.156315][ T66] br_del_if (net/bridge/br_if.c:741) bridge
[ 30.156501][ T66] br_device_event (net/bridge/br.c:141) bridge
[ 30.156683][ T66] ? __pfx_br_device_event (net/bridge/br.c:29) bridge
[ 30.156899][ T66] ? netconsole_netdev_event (drivers/net/netconsole.c:1082)
[ 30.157056][ T66] ? packet_notifier (net/packet/af_packet.c:4282)
[ 30.157213][ T66] notifier_call_chain (kernel/notifier.c:85 (discriminator 2))
[ 30.157367][ T66] blocking_notifier_call_chain (kernel/notifier.c:381 kernel/notifier.c:368)
[ 30.157527][ T66] unregister_netdevice_many_notify (net/core/dev.c:11575)
[ 30.157720][ T66] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11516)
[ 30.157913][ T66] ? default_device_exit_batch (net/core/dev.c:12109)
[ 30.158067][ T66] ? rtnl_is_locked (net/core/rtnetlink.c:163)
[ 30.158219][ T66] ? unregister_netdevice_queue (net/core/dev.c:11501)
[ 30.158409][ T66] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11500)
[ 30.158601][ T66] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11500)
[ 30.158797][ T66] default_device_exit_batch (net/core/dev.c:12123)
[ 30.158949][ T66] ? __pfx_default_device_exit_batch (net/core/dev.c:12098)
[ 30.159144][ 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)
[ 30.159307][ T66] ? nexthop_net_exit_batch_rtnl (net/ipv4/nexthop.c:585 net/ipv4/nexthop.c:2158 net/ipv4/nexthop.c:2666 net/ipv4/nexthop.c:4010)
[ 30.159503][ T66] cleanup_net (net/core/net_namespace.c:647 (discriminator 3))
[ 30.159657][ T66] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 30.159815][ T66] ? __pfx_cleanup_net (net/core/net_namespace.c:592)
[ 30.159966][ T66] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 30.160129][ T66] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 30.160284][ T66] ? process_one_work (kernel/workqueue.c:3205)
[ 30.160440][ T66] process_one_work (kernel/workqueue.c:3229)
[ 30.160597][ T66] ? __pfx___lock_release (kernel/locking/lockdep.c:5501)
[ 30.160751][ T66] ? __pfx_process_one_work (kernel/workqueue.c:3131)
[ 30.160914][ T66] ? assign_work (kernel/workqueue.c:1200)
[ 30.161068][ T66] worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391)
[ 30.161222][ T66] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4347 kernel/locking/lockdep.c:4406)
[ 30.161414][ T66] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 30.161570][ T66] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 30.161724][ T66] kthread (kernel/kthread.c:389)
[ 30.161840][ T66] ? __pfx_kthread (kernel/kthread.c:342)
[ 30.161995][ T66] ret_from_fork (arch/x86/kernel/process.c:147)
[ 30.162155][ T66] ? __pfx_kthread (kernel/kthread.c:342)


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