======================================
| xx__-> echo $?
| 0
| xx__-> [   87.008463][    C2] BUG: spinlock bad magic on CPU#2, kworker/u16:1/73
| [ 87.008660][ C2] lock: noop_qdisc+0x240/0x300, .magic: 00000000, .owner: kworker/u16:1/73, .owner_cpu: 2 
[   87.009134][    C2] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   87.009412][    C2] Workqueue: netns cleanup_net
[   87.009550][    C2] Call Trace:
[   87.009650][    C2]  <IRQ>
[ 87.009716][ C2] dump_stack_lvl (lib/dump_stack.c:122) 
[ 87.009845][ C2] do_raw_spin_unlock (kernel/locking/spinlock_debug.c:100 kernel/locking/spinlock_debug.c:141) 
[ 87.009972][ C2] _raw_spin_unlock (./arch/x86/include/asm/preempt.h:94 ./include/linux/spinlock_api_smp.h:143 kernel/locking/spinlock.c:186) 
[ 87.010100][ 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) 
[ 87.010228][ C2] handle_softirqs (kernel/softirq.c:554) 
[ 87.010354][ 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) 
[ 87.010486][ C2] do_softirq (kernel/softirq.c:455 kernel/softirq.c:442) 
[   87.010582][    C2]  </IRQ>
[   87.010647][    C2]  <TASK>
[ 87.010712][ C2] __local_bh_enable_ip (kernel/softirq.c:382) 
[ 87.010836][ C2] dev_deactivate_many (net/sched/sch_generic.c:1356) 
[ 87.010961][ C2] __dev_close_many (net/core/dev.c:1547) 
[ 87.011087][ C2] ? __pfx___dev_close_many (net/core/dev.c:1522) 
[ 87.011212][ C2] ? __lock_acquire (kernel/locking/lockdep.c:5137) 
[ 87.011338][ C2] dev_close_many (net/core/dev.c:1585) 
[ 87.011462][ C2] ? lock_acquire.part.0 (kernel/locking/lockdep.c:466 kernel/locking/lockdep.c:5761) 
[ 87.011586][ C2] ? default_device_exit_batch (net/core/dev.c:11876) 
[ 87.011712][ C2] ? __pfx_dev_close_many (net/core/dev.c:1575) 
[ 87.011836][ C2] ? fou_exit_net (net/ipv4/fou_core.c:1215) 
[ 87.011964][ 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) 
[ 87.012090][ C2] unregister_netdevice_many_notify (net/core/dev.c:11321) 
[ 87.012244][ C2] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 87.012369][ C2] ? trace_contention_end (./include/trace/events/lock.h:122 (discriminator 52)) 
[ 87.012492][ C2] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 87.012617][ C2] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11288) 
[ 87.012772][ C2] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 87.012897][ C2] ? find_held_lock (kernel/locking/lockdep.c:5249) 
[ 87.013023][ 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) 
[ 87.013148][ 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) 
[ 87.013272][ C2] ? rtnl_is_locked (net/core/rtnetlink.c:164) 
[ 87.013407][ C2] ? unregister_netdevice_queue (net/core/dev.c:11273) 
[ 87.013562][ C2] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11272) 
[ 87.013718][ C2] default_device_exit_batch (net/core/dev.c:11890) 
[ 87.013843][ C2] ? __pfx_default_device_exit_batch (net/core/dev.c:11865) 
[ 87.013996][ C2] ? ops_exit_list (net/core/net_namespace.c:172 (discriminator 3)) 
[ 87.014125][ C2] cleanup_net (net/core/net_namespace.c:625 (discriminator 3)) 
[ 87.014249][ C2] ? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5724) 
[ 87.014374][ C2] ? __pfx_cleanup_net (net/core/net_namespace.c:570) 
[ 87.014499][ C2] ? trace_lock_acquire (./include/trace/events/lock.h:24 (discriminator 52)) 
[ 87.014624][ C2] ? process_one_work (kernel/workqueue.c:3207) 
[ 87.014747][ C2] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 87.014869][ C2] ? process_one_work (kernel/workqueue.c:3207) 
[ 87.014993][ C2] process_one_work (kernel/workqueue.c:3231) 
[ 87.015118][ C2] ? __pfx___lock_release (kernel/locking/lockdep.c:5411) 
[ 87.015241][ C2] ? __pfx_process_one_work (kernel/workqueue.c:3133) 
[ 87.015367][ C2] ? assign_work (kernel/workqueue.c:1202) 
[ 87.015494][ C2] worker_thread (kernel/workqueue.c:3306 kernel/workqueue.c:3390) 
[ 87.015617][ C2] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4299 kernel/locking/lockdep.c:4358) 
[ 87.015772][ C2] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 87.015900][ C2] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 87.016023][ C2] kthread (kernel/kthread.c:389) 
[ 87.016117][ C2] ? __pfx_kthread (kernel/kthread.c:342) 
[ 87.016241][ C2] ret_from_fork (arch/x86/kernel/process.c:147) 
[ 87.016365][ C2] ? __pfx_kthread (kernel/kthread.c:342) 


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