======================================
| xx__-> echo $?
| 0
| xx__-> [   29.115369][    C3] BUG: spinlock bad magic on CPU#3, kworker/u16:1/74
| [ 29.115578][ C3] lock: noop_qdisc+0x240/0x300, .magic: 00000000, .owner: kworker/u16:1/74, .owner_cpu: 3 
[   29.116031][    C3] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   29.116310][    C3] Workqueue: netns cleanup_net
[   29.116443][    C3] Call Trace:
[   29.116542][    C3]  <IRQ>
[ 29.116607][ C3] dump_stack_lvl (lib/dump_stack.c:122) 
[ 29.116743][ C3] do_raw_spin_unlock (kernel/locking/spinlock_debug.c:100 kernel/locking/spinlock_debug.c:141) 
[ 29.116878][ C3] _raw_spin_unlock (./arch/x86/include/asm/preempt.h:94 ./include/linux/spinlock_api_smp.h:143 kernel/locking/spinlock.c:186) 
[ 29.117005][ 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) 
[ 29.117135][ C3] handle_softirqs (kernel/softirq.c:554) 
[ 29.117264][ 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) 
[ 29.117396][ C3] do_softirq (kernel/softirq.c:455 kernel/softirq.c:442) 
[   29.117493][    C3]  </IRQ>
[   29.117557][    C3]  <TASK>
[ 29.117620][ C3] __local_bh_enable_ip (kernel/softirq.c:382) 
[ 29.117747][ C3] dev_deactivate_many (net/sched/sch_generic.c:1356) 
[ 29.117875][ C3] __dev_close_many (net/core/dev.c:1547) 
[ 29.118002][ C3] ? __pfx___dev_close_many (net/core/dev.c:1522) 
[ 29.118130][ C3] ? __lock_acquire (kernel/locking/lockdep.c:5137) 
[ 29.118259][ C3] dev_close_many (net/core/dev.c:1585) 
[ 29.118388][ C3] ? lock_acquire.part.0 (kernel/locking/lockdep.c:466 kernel/locking/lockdep.c:5761) 
[ 29.118516][ C3] ? default_device_exit_batch (net/core/dev.c:11876) 
[ 29.118645][ C3] ? __pfx_dev_close_many (net/core/dev.c:1575) 
[ 29.118773][ C3] ? fou_exit_net (net/ipv4/fou_core.c:1215) 
[ 29.118910][ 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) 
[ 29.119040][ C3] unregister_netdevice_many_notify (net/core/dev.c:11321) 
[ 29.119198][ C3] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 29.119325][ C3] ? trace_contention_end (./include/trace/events/lock.h:122 (discriminator 52)) 
[ 29.119455][ C3] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 29.119588][ C3] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11288) 
[ 29.119747][ C3] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 29.119884][ C3] ? find_held_lock (kernel/locking/lockdep.c:5249) 
[ 29.120017][ 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) 
[ 29.120147][ 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) 
[ 29.120275][ C3] ? rtnl_is_locked (net/core/rtnetlink.c:164) 
[ 29.120405][ C3] ? unregister_netdevice_queue (net/core/dev.c:11273) 
[ 29.120563][ C3] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11272) 
[ 29.120721][ C3] default_device_exit_batch (net/core/dev.c:11890) 
[ 29.120849][ C3] ? __pfx_default_device_exit_batch (net/core/dev.c:11865) 
[ 29.121009][ C3] ? ops_exit_list (net/core/net_namespace.c:172 (discriminator 3)) 
[ 29.121140][ C3] cleanup_net (net/core/net_namespace.c:625 (discriminator 3)) 
[ 29.121269][ C3] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5724) 
[ 29.121397][ C3] ? __pfx_cleanup_net (net/core/net_namespace.c:570) 
[ 29.121525][ C3] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 52)) 
[ 29.121654][ C3] ? process_one_work (kernel/workqueue.c:3207) 
[ 29.121781][ C3] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 29.121909][ C3] ? process_one_work (kernel/workqueue.c:3207) 
[ 29.122037][ C3] process_one_work (kernel/workqueue.c:3231) 
[ 29.122170][ C3] ? __pfx___lock_release (kernel/locking/lockdep.c:5411) 
[ 29.122298][ C3] ? __pfx_process_one_work (kernel/workqueue.c:3133) 
[ 29.122433][ C3] ? assign_work (kernel/workqueue.c:1202) 
[ 29.122561][ C3] worker_thread (kernel/workqueue.c:3306 kernel/workqueue.c:3390) 
[ 29.122693][ C3] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4299 kernel/locking/lockdep.c:4358) 
[ 29.122866][ C3] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 29.122993][ C3] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 29.123124][ C3] kthread (kernel/kthread.c:389) 
[ 29.123225][ C3] ? __pfx_kthread (kernel/kthread.c:342) 
[ 29.123351][ C3] ret_from_fork (arch/x86/kernel/process.c:147) 
[ 29.123487][ C3] ? __pfx_kthread (kernel/kthread.c:342) 


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