======================================
| [ 2885.939668][ T67] #3: ffffffff98407e88 (rtnl_mutex){+.+.}-{4:4}, at: default_device_exit_batch (net/core/dev.c:12109)
| [ 2885.939995][ T67] #4: ffffffff983f6a70 ((netdev_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
| [ 2885.940352][   T67]
| [ 2885.940352][   T67] stack backtrace:
[ 2885.940894][   T67] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 2885.941149][   T67] Workqueue: netns cleanup_net
[ 2885.941354][   T67] Call Trace:
[ 2885.941476][   T67]  <TASK>
[ 2885.941574][ T67] dump_stack_lvl (lib/dump_stack.c:123)
[ 2885.941772][ T67] print_deadlock_bug (kernel/locking/lockdep.c:3040)
[ 2885.941938][ T67] validate_chain (kernel/locking/lockdep.c:3894)
[ 2885.942141][ T67] ? __pfx_validate_chain (kernel/locking/lockdep.c:3860)
[ 2885.942305][ T67] ? 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)
[ 2885.942465][ T67] ? mark_lock (kernel/locking/lockdep.c:4727 (discriminator 3))
[ 2885.942592][ T67] __lock_acquire (kernel/locking/lockdep.c:5226)
[ 2885.942765][ T67] lock_acquire.part.0 (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5851)
[ 2885.942925][ T67] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2885.943156][ T67] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 2885.943348][ T67] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 2885.943512][ T67] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 2885.943716][ T67] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 2885.943872][ T67] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2885.944072][ T67] down_read (./arch/x86/include/asm/preempt.h:79 kernel/locking/rwsem.c:1246 kernel/locking/rwsem.c:1261 kernel/locking/rwsem.c:1526)
[ 2885.944217][ T67] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2885.944499][ T67] ? __pfx_down_read (kernel/locking/rwsem.c:1522)
[ 2885.944668][ T67] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4339 kernel/locking/lockdep.c:4406)
[ 2885.944868][ T67] blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2885.945064][ T67] __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))
[ 2885.945228][ T67] ? __pfx___netdev_upper_dev_unlink (net/core/dev.c:8151)
[ 2885.945470][ T67] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 2885.945639][ T67] ? del_nbp (net/bridge/br_if.c:215 net/bridge/br_if.c:359)
[ 2885.945771][ T67] ? 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)
[ 2885.945960][ T67] netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 2885.946120][ T67] ? __pfx_netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 2885.946327][ T67] ? mutex_is_locked (./include/linux/instrumented.h:68 ./include/linux/atomic/atomic-instrumented.h:3188 kernel/locking/mutex.h:47 kernel/locking/mutex.c:66)
[ 2885.946508][ T67] ? rtnl_is_locked (net/core/rtnetlink.c:163)
[ 2885.946710][ T67] del_nbp (net/bridge/br_if.c:365)
[ 2885.946841][ T67] br_del_if (net/bridge/br_if.c:741)
[ 2885.946965][ T67] br_device_event (net/bridge/br.c:141)
[ 2885.947123][ T67] ? __pfx_br_device_event (net/bridge/br.c:29)
[ 2885.947289][ T67] ? packet_notifier (net/packet/af_packet.c:4282)
[ 2885.947499][ T67] notifier_call_chain (kernel/notifier.c:85 (discriminator 2))
[ 2885.947674][ T67] blocking_notifier_call_chain (kernel/notifier.c:381 kernel/notifier.c:368)
[ 2885.947837][ T67] unregister_netdevice_many_notify (net/core/dev.c:11575)
[ 2885.948032][ T67] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11516)
[ 2885.948231][ T67] ? default_device_exit_batch (net/core/dev.c:12109)
[ 2885.948428][ T67] ? rtnl_is_locked (net/core/rtnetlink.c:163)
[ 2885.948590][ T67] ? unregister_netdevice_queue (net/core/dev.c:11501)
[ 2885.948840][ T67] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11500)
[ 2885.949039][ T67] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11500)
[ 2885.949269][ T67] default_device_exit_batch (net/core/dev.c:12123)
[ 2885.949433][ T67] ? __pfx_default_device_exit_batch (net/core/dev.c:12098)
[ 2885.949685][ T67] ? ops_exit_list (net/core/net_namespace.c:171 (discriminator 3))
[ 2885.949855][ T67] cleanup_net (net/core/net_namespace.c:647 (discriminator 3))
[ 2885.950033][ T67] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 2885.950215][ T67] ? __pfx_cleanup_net (net/core/net_namespace.c:592)
[ 2885.950376][ T67] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 2885.950551][ T67] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 2885.950738][ T67] ? process_one_work (kernel/workqueue.c:3205)
[ 2885.950899][ T67] process_one_work (kernel/workqueue.c:3229)
[ 2885.951095][ T67] ? __pfx___lock_release (kernel/locking/lockdep.c:5501)
[ 2885.951262][ T67] ? __pfx_process_one_work (kernel/workqueue.c:3131)
[ 2885.951450][ T67] ? assign_work (kernel/workqueue.c:1200)
[ 2885.951623][ T67] worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391)
[ 2885.951787][ T67] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4347 kernel/locking/lockdep.c:4406)
[ 2885.951983][ T67] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 2885.952146][ T67] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 2885.952307][ T67] kthread (kernel/kthread.c:389)
[ 2885.952440][ T67] ? __pfx_kthread (kernel/kthread.c:342)
[ 2885.952639][ T67] ret_from_fork (arch/x86/kernel/process.c:147)
[ 2885.952802][ T67] ? __pfx_kthread (kernel/kthread.c:342)


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