======================================
| [ 1180.875060][ T67] #3: ffffffffb6007e88 (rtnl_mutex){+.+.}-{4:4}, at: default_device_exit_batch (net/core/dev.c:12103)
| [ 1180.875341][ T67] #4: ffffffffb5ff6a70 ((netdev_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
| [ 1180.875619][   T67]
| [ 1180.875619][   T67] stack backtrace:
[ 1180.876035][   T67] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 1180.876210][   T67] Workqueue: netns cleanup_net
[ 1180.876353][   T67] Call Trace:
[ 1180.876459][   T67]  <TASK>
[ 1180.876532][ T67] dump_stack_lvl (lib/dump_stack.c:123)
[ 1180.876676][ T67] print_deadlock_bug (kernel/locking/lockdep.c:3040)
[ 1180.876818][ T67] validate_chain (kernel/locking/lockdep.c:3894)
[ 1180.876959][ T67] ? __pfx_validate_chain (kernel/locking/lockdep.c:3860)
[ 1180.877101][ 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)
[ 1180.877239][ T67] ? mark_lock (kernel/locking/lockdep.c:4727 (discriminator 3))
[ 1180.877346][ T67] __lock_acquire (kernel/locking/lockdep.c:5226)
[ 1180.877487][ T67] lock_acquire.part.0 (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5851)
[ 1180.877624][ T67] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 1180.877796][ T67] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 1180.877937][ T67] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 1180.878077][ T67] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 1180.878215][ T67] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 1180.878351][ T67] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 1180.878523][ 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)
[ 1180.878630][ T67] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 1180.878802][ T67] ? __pfx_down_read (kernel/locking/rwsem.c:1522)
[ 1180.878938][ T67] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4339 kernel/locking/lockdep.c:4406)
[ 1180.879111][ T67] blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 1180.879252][ 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))
[ 1180.879391][ T67] ? __pfx___netdev_upper_dev_unlink (net/core/dev.c:8151)
[ 1180.879561][ T67] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 1180.879706][ T67] ? del_nbp (net/bridge/br_if.c:215 net/bridge/br_if.c:359)
[ 1180.879814][ 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)
[ 1180.879952][ T67] netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 1180.880090][ T67] ? __pfx_netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 1180.880261][ 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)
[ 1180.880401][ T67] ? rtnl_is_locked (net/core/rtnetlink.c:164)
[ 1180.880540][ T67] del_nbp (net/bridge/br_if.c:365)
[ 1180.880647][ T67] br_del_if (net/bridge/br_if.c:741)
[ 1180.880753][ T67] br_device_event (net/bridge/br.c:141)
[ 1180.880895][ T67] ? __pfx_br_device_event (net/bridge/br.c:29)
[ 1180.881033][ T67] ? packet_notifier (net/packet/af_packet.c:4282)
[ 1180.881170][ T67] notifier_call_chain (kernel/notifier.c:85 (discriminator 2))
[ 1180.881313][ T67] blocking_notifier_call_chain (kernel/notifier.c:381 kernel/notifier.c:368)
[ 1180.881454][ T67] unregister_netdevice_many_notify (net/core/dev.c:11571)
[ 1180.881632][ T67] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11512)
[ 1180.881804][ T67] ? default_device_exit_batch (net/core/dev.c:12103)
[ 1180.881941][ T67] ? rtnl_is_locked (net/core/rtnetlink.c:164)
[ 1180.882078][ T67] ? unregister_netdevice_queue (net/core/dev.c:11497)
[ 1180.882250][ T67] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11496)
[ 1180.882422][ T67] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11496)
[ 1180.882597][ T67] default_device_exit_batch (net/core/dev.c:12117)
[ 1180.882736][ T67] ? __pfx_default_device_exit_batch (net/core/dev.c:12092)
[ 1180.882909][ T67] ? ops_exit_list (net/core/net_namespace.c:171 (discriminator 3))
[ 1180.883050][ T67] cleanup_net (net/core/net_namespace.c:647 (discriminator 3))
[ 1180.883188][ T67] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 1180.883330][ T67] ? __pfx_cleanup_net (net/core/net_namespace.c:592)
[ 1180.883468][ T67] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 1180.883606][ T67] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 1180.883742][ T67] ? process_one_work (kernel/workqueue.c:3205)
[ 1180.883882][ T67] process_one_work (kernel/workqueue.c:3229)
[ 1180.884023][ T67] ? __pfx___lock_release (kernel/locking/lockdep.c:5501)
[ 1180.884166][ T67] ? __pfx_process_one_work (kernel/workqueue.c:3131)
[ 1180.884305][ T67] ? assign_work (kernel/workqueue.c:1200)
[ 1180.884446][ T67] worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391)
[ 1180.884585][ T67] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4347 kernel/locking/lockdep.c:4406)
[ 1180.884761][ T67] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 1180.884897][ T67] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 1180.885034][ T67] kthread (kernel/kthread.c:389)
[ 1180.885140][ T67] ? __pfx_kthread (kernel/kthread.c:342)
[ 1180.885277][ T67] ret_from_fork (arch/x86/kernel/process.c:147)
[ 1180.885415][ T67] ? __pfx_kthread (kernel/kthread.c:342)


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