======================================
| [ 2892.804208][ T66] #3: ffffffff93c07e88 (rtnl_mutex){+.+.}-{4:4}, at: default_device_exit_batch (net/core/dev.c:12103)
| [ 2892.804537][ T66] #4: ffffffff93bf6a70 ((netdev_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
| [ 2892.804869][   T66]
| [ 2892.804869][   T66] stack backtrace:
[ 2892.805361][   T66] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 2892.805567][   T66] Workqueue: netns cleanup_net
[ 2892.805741][   T66] Call Trace:
[ 2892.805867][   T66]  <TASK>
[ 2892.805956][ T66] dump_stack_lvl (lib/dump_stack.c:123)
[ 2892.806125][ T66] print_deadlock_bug (kernel/locking/lockdep.c:3040)
[ 2892.806298][ T66] validate_chain (kernel/locking/lockdep.c:3894)
[ 2892.806462][ T66] ? __pfx_validate_chain (kernel/locking/lockdep.c:3860)
[ 2892.806632][ 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)
[ 2892.806793][ T66] ? mark_lock (kernel/locking/lockdep.c:4727 (discriminator 3))
[ 2892.806922][ T66] __lock_acquire (kernel/locking/lockdep.c:5226)
[ 2892.807082][ T66] lock_acquire.part.0 (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5851)
[ 2892.807246][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2892.807450][ T66] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 2892.807605][ T66] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 2892.807769][ T66] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 2892.807930][ T66] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 2892.808099][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2892.808302][ 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)
[ 2892.808425][ T66] ? blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2892.808628][ T66] ? __pfx_down_read (kernel/locking/rwsem.c:1522)
[ 2892.808790][ T66] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4339 kernel/locking/lockdep.c:4406)
[ 2892.808995][ T66] blocking_notifier_call_chain (kernel/notifier.c:380 kernel/notifier.c:368)
[ 2892.809159][ 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))
[ 2892.809338][ T66] ? __pfx___netdev_upper_dev_unlink (net/core/dev.c:8151)
[ 2892.809541][ T66] ? find_held_lock (kernel/locking/lockdep.c:5339)
[ 2892.809706][ T66] ? del_nbp (net/bridge/br_if.c:215 net/bridge/br_if.c:359)
[ 2892.809835][ 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)
[ 2892.810000][ T66] netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 2892.810160][ T66] ? __pfx_netdev_upper_dev_unlink (net/core/dev.c:8190)
[ 2892.810364][ T66] ? 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)
[ 2892.810525][ T66] ? rtnl_is_locked (net/core/rtnetlink.c:164)
[ 2892.810705][ T66] del_nbp (net/bridge/br_if.c:365)
[ 2892.810835][ T66] br_del_if (net/bridge/br_if.c:741)
[ 2892.810962][ T66] br_device_event (net/bridge/br.c:141)
[ 2892.811127][ T66] ? __pfx_br_device_event (net/bridge/br.c:29)
[ 2892.811279][ T66] ? packet_notifier (net/packet/af_packet.c:4282)
[ 2892.811431][ T66] notifier_call_chain (kernel/notifier.c:85 (discriminator 2))
[ 2892.811584][ T66] blocking_notifier_call_chain (kernel/notifier.c:381 kernel/notifier.c:368)
[ 2892.811751][ T66] unregister_netdevice_many_notify (net/core/dev.c:11571)
[ 2892.811949][ T66] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11512)
[ 2892.812153][ T66] ? default_device_exit_batch (net/core/dev.c:12103)
[ 2892.812320][ T66] ? rtnl_is_locked (net/core/rtnetlink.c:164)
[ 2892.812479][ T66] ? unregister_netdevice_queue (net/core/dev.c:11497)
[ 2892.812676][ T66] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11496)
[ 2892.812885][ T66] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11496)
[ 2892.813097][ T66] default_device_exit_batch (net/core/dev.c:12117)
[ 2892.813261][ T66] ? __pfx_default_device_exit_batch (net/core/dev.c:12092)
[ 2892.813462][ T66] ? ops_exit_list (net/core/net_namespace.c:171 (discriminator 3))
[ 2892.813632][ T66] cleanup_net (net/core/net_namespace.c:647 (discriminator 3))
[ 2892.813793][ T66] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5814)
[ 2892.813958][ T66] ? __pfx_cleanup_net (net/core/net_namespace.c:592)
[ 2892.814121][ T66] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 37))
[ 2892.814284][ T66] ? lock_acquire (kernel/locking/lockdep.c:5822)
[ 2892.814447][ T66] ? process_one_work (kernel/workqueue.c:3205)
[ 2892.814609][ T66] process_one_work (kernel/workqueue.c:3229)
[ 2892.814769][ T66] ? __pfx___lock_release (kernel/locking/lockdep.c:5501)
[ 2892.814931][ T66] ? __pfx_process_one_work (kernel/workqueue.c:3131)
[ 2892.815096][ T66] ? assign_work (kernel/workqueue.c:1200)
[ 2892.815259][ T66] worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391)
[ 2892.815424][ T66] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4347 kernel/locking/lockdep.c:4406)
[ 2892.815625][ T66] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 2892.815789][ T66] ? __pfx_worker_thread (kernel/workqueue.c:3337)
[ 2892.815949][ T66] kthread (kernel/kthread.c:389)
[ 2892.816079][ T66] ? __pfx_kthread (kernel/kthread.c:342)
[ 2892.816241][ T66] ret_from_fork (arch/x86/kernel/process.c:147)
[ 2892.816409][ T66] ? __pfx_kthread (kernel/kthread.c:342)


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