======================================
| xx__-> echo $?
| 0
| xx__-> [   30.514395][    C3] BUG: spinlock bad magic on CPU#3, kworker/u16:0/11
| [ 30.514606][ C3] lock: noop_qdisc+0x240/0x300, .magic: 00000000, .owner: kworker/u16:0/11, .owner_cpu: 3 
[   30.515076][    C3] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   30.515368][    C3] Workqueue: netns cleanup_net
[   30.515507][    C3] Call Trace:
[   30.515611][    C3]  <IRQ>
[ 30.515692][ C3] dump_stack_lvl (lib/dump_stack.c:122) 
[ 30.515827][ C3] do_raw_spin_unlock (kernel/locking/spinlock_debug.c:100 kernel/locking/spinlock_debug.c:141) 
[ 30.515999][ C3] _raw_spin_unlock (./arch/x86/include/asm/preempt.h:94 ./include/linux/spinlock_api_smp.h:143 kernel/locking/spinlock.c:186) 
[ 30.516135][ C3] net_tx_action (./include/net/sch_generic.h:226 ./include/net/sch_generic.h:217 ./include/net/pkt_sched.h:128 ./include/net/pkt_sched.h:124 net/core/dev.c:5323) 
[ 30.516268][ C3] handle_softirqs (kernel/softirq.c:554) 
[ 30.516399][ C3] ? dev_deactivate_many (./include/linux/bottom_half.h:33 ./include/linux/netdevice.h:4448 net/sched/sch_generic.c:571 net/sched/sch_generic.c:1363) 
[ 30.516533][ C3] do_softirq (kernel/softirq.c:455 kernel/softirq.c:442) 
[   30.516632][    C3]  </IRQ>
[   30.516701][    C3]  <TASK>
[ 30.516766][ C3] __local_bh_enable_ip (kernel/softirq.c:382) 
[ 30.516894][ C3] dev_deactivate_many (net/sched/sch_generic.c:1356) 
[ 30.517022][ C3] __dev_close_many (net/core/dev.c:1547) 
[ 30.517150][ C3] ? __pfx___dev_close_many (net/core/dev.c:1522) 
[ 30.517283][ C3] ? __lock_acquire (kernel/locking/lockdep.c:5137) 
[ 30.517413][ C3] dev_close_many (net/core/dev.c:1585) 
[ 30.517542][ C3] ? lock_acquire.part.0 (kernel/locking/lockdep.c:466 kernel/locking/lockdep.c:5761) 
[ 30.517668][ C3] ? default_device_exit_batch (net/core/dev.c:11876) 
[ 30.517800][ C3] ? __pfx_dev_close_many (net/core/dev.c:1575) 
[ 30.517929][ C3] ? fou_exit_net (net/ipv4/fou_core.c:1215) 
[ 30.518058][ C3] ? __mutex_trylock_common (./arch/x86/include/asm/atomic64_64.h:101 ./include/linux/atomic/atomic-arch-fallback.h:4296 ./include/linux/atomic/atomic-long.h:1482 ./include/linux/atomic/atomic-instrumented.h:4458 kernel/locking/mutex.c:129) 
[ 30.518187][ C3] unregister_netdevice_many_notify (net/core/dev.c:11321) 
[ 30.518352][ C3] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 30.518482][ C3] ? trace_contention_end (./include/trace/events/lock.h:122 (discriminator 52)) 
[ 30.518612][ C3] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 30.518742][ C3] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11288) 
[ 30.518907][ C3] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 30.519035][ C3] ? find_held_lock (kernel/locking/lockdep.c:5249) 
[ 30.519166][ C3] ? 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.c:81 kernel/locking/mutex.c:91) 
[ 30.519297][ C3] ? rtnl_is_locked (net/core/rtnetlink.c:164) 
[ 30.519428][ C3] ? unregister_netdevice_queue (net/core/dev.c:11273) 
[ 30.519590][ C3] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11272) 
[ 30.519749][ C3] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11272) 
[ 30.519912][ C3] default_device_exit_batch (net/core/dev.c:11890) 
[ 30.520056][ C3] ? __pfx_default_device_exit_batch (net/core/dev.c:11865) 
[ 30.520216][ C3] ? ops_exit_list (net/core/net_namespace.c:172 (discriminator 3)) 
[ 30.520349][ C3] cleanup_net (net/core/net_namespace.c:625 (discriminator 3)) 
[ 30.520476][ C3] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5724) 
[ 30.520602][ C3] ? __pfx_cleanup_net (net/core/net_namespace.c:570) 
[ 30.520728][ C3] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 52)) 
[ 30.520856][ C3] ? process_one_work (kernel/workqueue.c:3207) 
[ 30.520982][ C3] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 30.521109][ C3] ? process_one_work (kernel/workqueue.c:3207) 
[ 30.521236][ C3] process_one_work (kernel/workqueue.c:3231) 
[ 30.521370][ C3] ? __pfx___lock_release (kernel/locking/lockdep.c:5411) 
[ 30.521501][ C3] ? __pfx_process_one_work (kernel/workqueue.c:3133) 
[ 30.521628][ C3] ? assign_work (kernel/workqueue.c:1202) 
[ 30.521756][ C3] worker_thread (kernel/workqueue.c:3306 kernel/workqueue.c:3390) 
[ 30.521889][ C3] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 30.522017][ C3] kthread (kernel/kthread.c:389) 
[ 30.522116][ C3] ? __pfx_kthread (kernel/kthread.c:342) 
[ 30.522244][ C3] ret_from_fork (arch/x86/kernel/process.c:147) 
[ 30.522379][ C3] ? __pfx_kthread (kernel/kthread.c:342) 


Finger prints:
do_raw_spin_unlock:_raw_spin_unlock:net_tx_action:handle_softirqs:do_softirq