======================================
| [   39.172672][  T327] ping6 (327) used greatest stack depth: 24120 bytes left
| [   39.885007][  T332] Initializing XFRM netlink socket
| [   49.273871][    C1] BUG: spinlock bad magic on CPU#1, kworker/u18:0/38
| [ 49.274102][ C1] lock: noop_qdisc+0x240/0x300, .magic: 00000000, .owner: kworker/u18:0/38, .owner_cpu: 1 
[   49.274659][    C1] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   49.274980][    C1] Workqueue: l2tp l2tp_session_del_work [l2tp_core]
[   49.275192][    C1] Call Trace:
[   49.275313][    C1]  <IRQ>
[ 49.275393][ C1] dump_stack_lvl (lib/dump_stack.c:122) 
[ 49.275544][ C1] do_raw_spin_unlock (kernel/locking/spinlock_debug.c:100 kernel/locking/spinlock_debug.c:141) 
[ 49.275693][ C1] _raw_spin_unlock (./arch/x86/include/asm/preempt.h:94 ./include/linux/spinlock_api_smp.h:143 kernel/locking/spinlock.c:186) 
[ 49.275843][ C1] 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) 
[ 49.275999][ C1] handle_softirqs (kernel/softirq.c:554) 
[ 49.276146][ C1] ? 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) 
[ 49.276303][ C1] do_softirq (kernel/softirq.c:455 kernel/softirq.c:442) 
[   49.276413][    C1]  </IRQ>
[   49.276488][    C1]  <TASK>
[ 49.276564][ C1] __local_bh_enable_ip (kernel/softirq.c:382) 
[ 49.276721][ C1] dev_deactivate_many (net/sched/sch_generic.c:1356) 
[ 49.276865][ C1] __dev_close_many (net/core/dev.c:1547) 
[ 49.277009][ C1] ? __pfx___dev_close_many (net/core/dev.c:1522) 
[ 49.277152][ C1] ? __pfx_validate_chain (kernel/locking/lockdep.c:3824) 
[ 49.277302][ C1] ? 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:227) 
[ 49.277453][ C1] dev_close_many (net/core/dev.c:1585) 
[ 49.277602][ C1] ? __lock_acquire (kernel/locking/lockdep.c:5142) 
[ 49.277747][ C1] ? __pfx_dev_close_many (net/core/dev.c:1575) 
[ 49.277896][ C1] unregister_netdevice_many_notify (net/core/dev.c:11321) 
[ 49.278083][ C1] ? __pfx_unregister_netdevice_many_notify (net/core/dev.c:11288) 
[ 49.278265][ C1] ? l2tp_eth_delete (net/l2tp/l2tp_eth.c:166 net/l2tp/l2tp_eth.c:157) l2tp_eth
[ 49.278410][ C1] ? lock_acquire (kernel/locking/lockdep.c:5732) 
[ 49.278556][ C1] ? trace_contention_end (./include/trace/events/lock.h:122 (discriminator 52)) 
[ 49.278701][ C1] ? __mutex_lock (./arch/x86/include/asm/preempt.h:94 kernel/locking/mutex.c:618 kernel/locking/mutex.c:752) 
[ 49.278849][ C1] ? __lock_release (kernel/locking/lockdep.c:5435) 
[ 49.278996][ C1] ? skb_dequeue (net/core/skbuff.c:3858) 
[ 49.279147][ C1] ? l2tp_eth_delete (net/l2tp/l2tp_eth.c:166 net/l2tp/l2tp_eth.c:157) l2tp_eth
[ 49.279308][ C1] unregister_netdevice_queue (net/core/dev.c:11283) 
[ 49.279456][ C1] ? __pfx_unregister_netdevice_queue (net/core/dev.c:11272) 
[ 49.279641][ C1] l2tp_eth_delete (net/l2tp/l2tp_eth.c:169 net/l2tp/l2tp_eth.c:157) l2tp_eth
[ 49.279790][ C1] l2tp_session_del_work (net/l2tp/l2tp_core.c:1656) l2tp_core
[ 49.279984][ C1] ? process_one_work (kernel/workqueue.c:3207) 
[ 49.280136][ C1] process_one_work (kernel/workqueue.c:3231) 
[ 49.280291][ C1] ? __pfx___lock_release (kernel/locking/lockdep.c:5411) 
[ 49.280442][ C1] ? __pfx_process_one_work (kernel/workqueue.c:3133) 
[ 49.280597][ C1] ? assign_work (kernel/workqueue.c:1202) 
[ 49.280753][ C1] worker_thread (kernel/workqueue.c:3306 kernel/workqueue.c:3390) 
[ 49.280909][ C1] ? __pfx_worker_thread (kernel/workqueue.c:3339) 
[ 49.281059][ C1] kthread (kernel/kthread.c:389) 
[ 49.281174][ C1] ? __pfx_kthread (kernel/kthread.c:342) 
[ 49.281327][ C1] ret_from_fork (arch/x86/kernel/process.c:147) 
[ 49.281482][ C1] ? __pfx_kthread (kernel/kthread.c:342) 


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