======================================
| xx__-> echo $?
| 0
| xx__-> [   30.367668][    C2] BUG: spinlock bad magic on CPU#2, kworker/u16:0/11
| [ 30.367871][ C2] lock: noop_qdisc+0x240/0x300, .magic: 00000000, .owner: kworker/u16:0/11, .owner_cpu: 2 
[   30.368348][    C2] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   30.368640][    C2] Workqueue: netns cleanup_net
[   30.368780][    C2] Call Trace:
[   30.368884][    C2]  <IRQ>
[ 30.368954][ C2] dump_stack_lvl (lib/dump_stack.c:122) 
[ 30.369089][ C2] do_raw_spin_unlock (kernel/locking/spinlock_debug.c:100 kernel/locking/spinlock_debug.c:141) 
[ 30.369219][ C2] _raw_spin_unlock (./arch/x86/include/asm/preempt.h:94 ./include/linux/spinlock_api_smp.h:143 kernel/locking/spinlock.c:186) 
[ 30.369353][ C2] 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) 
[ 30.369483][ C2] handle_softirqs (kernel/softirq.c:554) 
[ 30.369615][ C2] ? 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) 
[ 30.369751][ C2] do_softirq (kernel/softirq.c:455 kernel/softirq.c:442) 
[   30.369849][    C2]  </IRQ>
[   30.369918][    C2]  <TASK>
[ 30.369984][ C2] __local_bh_enable_ip (kernel/softirq.c:382) 
[ 30.370115][ C2] dev_deactivate_many (net/sched/sch_generic.c:1356) 
[ 30.370244][ C2] __dev_close_many (net/core/dev.c:1547) 
[ 30.370371][ C2] ? __pfx___dev_close_many (net/core/dev.c:1522) 
[ 30.370500][ C2] ? __lock_acquire (kernel/locking/lockdep.c:5137) 
[ 30.370628][ C2] dev_close_many (net/core/dev.c:1585) 
[ 30.370756][ C2] ? lock_acquire.part.0 (kernel/locking/lockdep.c:466 kernel/locking/lockdep.c:5761) 
[ 30.370889][ C2] ? default_device_exit_batch (net/core/dev.c:11876) 
[ 30.371021][ C2] ? __pfx_dev_close_many (net/core/dev.c:1575) 
[ 30.371148][ C2] ? fou_exit_net (net/ipv4/fou_core.c:1215) 
[ 30.371282][ C2] ? __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) 
[ 30.371412][ C2] unregister_netdevice_many_notify (net/core/dev.c:11321) 
[ 30.371572][ C2] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 30.371701][ C2] ? trace_contention_end (./include/trace/events/lock.h:122 (discriminator 52)) 
[ 30.371830][ C2] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 30.371961][ C2] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11288) 
[ 30.372129][ C2] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 30.372257][ C2] ? find_held_lock (kernel/locking/lockdep.c:5249) 
[ 30.372385][ C2] ? 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) 
[ 30.372511][ C2] ? 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) 
[ 30.372637][ C2] ? rtnl_is_locked (net/core/rtnetlink.c:164) 
[ 30.372764][ C2] ? unregister_netdevice_queue (net/core/dev.c:11273) 
[ 30.372920][ C2] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11272) 
[ 30.373079][ C2] default_device_exit_batch (net/core/dev.c:11890) 
[ 30.373209][ C2] ? __pfx_default_device_exit_batch (net/core/dev.c:11865) 
[ 30.373366][ C2] ? ops_exit_list (net/core/net_namespace.c:172 (discriminator 3)) 
[ 30.373495][ C2] cleanup_net (net/core/net_namespace.c:625 (discriminator 3)) 
[ 30.373621][ C2] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5724) 
[ 30.373750][ C2] ? __pfx_cleanup_net (net/core/net_namespace.c:570) 
[ 30.373876][ C2] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 52)) 
[ 30.374004][ C2] ? process_one_work (kernel/workqueue.c:3207) 
[ 30.374133][ C2] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 30.374259][ C2] ? process_one_work (kernel/workqueue.c:3207) 
[ 30.374389][ C2] process_one_work (kernel/workqueue.c:3231) 
[ 30.374518][ C2] ? __pfx___lock_release (kernel/locking/lockdep.c:5411) 
[ 30.374643][ C2] ? __pfx_process_one_work (kernel/workqueue.c:3133) 
[ 30.374771][ C2] ? assign_work (kernel/workqueue.c:1202) 
[ 30.374900][ C2] worker_thread (kernel/workqueue.c:3306 kernel/workqueue.c:3390) 
[ 30.375028][ C2] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 30.375157][ C2] kthread (kernel/kthread.c:389) 
[ 30.375254][ C2] ? __pfx_kthread (kernel/kthread.c:342) 
[ 30.375383][ C2] ret_from_fork (arch/x86/kernel/process.c:147) 
[ 30.375511][ C2] ? __pfx_kthread (kernel/kthread.c:342) 


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