unregister_netdevice: waiting for lo to become free. Usage count = 3
unregister_netdevice: waiting for lo to become free. Usage count = 3
unregister_netdevice: waiting for lo to become free. Usage count = 3
unregister_netdevice: waiting for lo to become free. Usage count = 3
unregister_netdevice: waiting for lo to become free. Usage count = 3
INFO: task syz-executor3:15131 blocked for more than 120 seconds.
      Not tainted 4.9.93-gf6bec4e #4
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor3   D28760 15131      1 0x00000004
 ffff8801a0019800 0000000000000000 ffff8801bd60cfc0 ffff8801d9b73000
 ffff8801db321b98 ffff8801ae21fbc8 ffffffff838c4a7d ffff8801a001a0c0
 ffffed0034003417 ffff8801a0019800 00fffc0000000000 ffff8801db322468
Call Trace:
 [<ffffffff838c607f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3556
 [<ffffffff838c6a03>] schedule_preempt_disabled+0x13/0x20 kernel/sched/core.c:3589
 [<ffffffff838cae86>] __mutex_lock_common kernel/locking/mutex.c:582 [inline]
 [<ffffffff838cae86>] mutex_lock_nested+0x326/0x870 kernel/locking/mutex.c:621
 [<ffffffff82f39e15>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:387
 [<ffffffff8119e1bc>] create_new_namespaces+0x51c/0x730 kernel/nsproxy.c:106
 [<ffffffff8119e9b5>] unshare_nsproxy_namespaces+0xa5/0x1d0 kernel/nsproxy.c:205
 [<ffffffff81133289>] SYSC_unshare kernel/fork.c:2244 [inline]
 [<ffffffff81133289>] SyS_unshare+0x319/0x710 kernel/fork.c:2194
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff838d5953>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Showing all locks held in the system:
2 locks held by khungtaskd/515:
 #0:  (rcu_read_lock){......}, at: [<ffffffff813643dc>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff813643dc>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff81423b90>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3739:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff838d3b22>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+.+.}, at: [<ffffffff82007e22>] n_tty_read+0x202/0x16b0 drivers/tty/n_tty.c:2133
3 locks held by kworker/u4:5/7123:
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff8118ab9e>] work_static include/linux/workqueue.h:186 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff8118ab9e>] set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff8118ab9e>] set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff8118ab9e>] process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
 #1:  (net_cleanup_work){+.+.+.}, at: [<ffffffff8118abd8>] process_one_work+0x728/0x1500 kernel/workqueue.c:2089
 #2:  (net_mutex){+.+.+.}, at: [<ffffffff82f397cf>] cleanup_net+0x13f/0x630 net/core/net_namespace.c:420
1 lock held by syz-executor3/15131:
 #0:  (net_mutex){+.+.+.}, at: [<ffffffff82f39e15>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:387

=============================================

NMI backtrace for cpu 1
CPU: 1 PID: 515 Comm: khungtaskd Not tainted 4.9.93-gf6bec4e #4
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d85dfd08 ffffffff81d9c299 0000000000000000 0000000000000001
 0000000000000001 0000000000000001 ffffffff810b7d90 ffff8801d85dfd40
 ffffffff81da75d7 0000000000000001 0000000000000000 0000000000000002
Call Trace:
 [<ffffffff81d9c299>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d9c299>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81da75d7>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81da756a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810b7e94>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81364974>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81364974>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81364974>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81364974>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119ab4d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff838d5b1c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.93-gf6bec4e #4
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffffffff84429800 task.stack: ffffffff84400000
RIP: 0010:[<ffffffff8128848b>] 
c [<ffffffff8128848b>] __read_once_size include/linux/compiler.h:243 [inline]
RIP: 0010:[<ffffffff8128848b>] 
c [<ffffffff8128848b>] rcu_irq_enter+0x5b/0x120 kernel/rcu/tree.c:941
RSP: 0018:ffff8801db207fa8  EFLAGS: 00000802
RAX: dffffc0000000000 RBX: ffff8801db21a690 RCX: 1ffffffff084227c
RDX: 0000000000000000 RSI: ffffffff81e03c9b RDI: ffff8801db21a698
RBP: ffff8801db207fb8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: ffffffff84a39fc8 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f8cdd4080c0 CR3: 00000001cbd06000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 0000000000000000
c ffffffff84a2ae28
c ffff8801db207fd0
c ffffffff81148bfa
c
 ffffffff84a2ae28
c ffff8801db207fe8
c ffffffff838da07a
c ffffffff84a2ae28
c
 ffffffff84407dc0
c ffffffff838d7c50
c ffffffff84407d18
c <EOI> 
c 0000000000000000
c
Call Trace:
 <IRQ> 
d [<ffffffff81148bfa>] irq_enter+0xa/0xd0 kernel/softirq.c:338
 [<ffffffff838da07a>] smp_reschedule_interrupt+0xa/0x90 arch/x86/kernel/smp.c:267
 [<ffffffff838d7c50>] reschedule_interrupt+0xa0/0xb0 arch/x86/entry/entry_64.S:671
 <EOI> 
d [<ffffffff838d4586>] ? native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:53
 [<ffffffff838d3bc5>] arch_safe_halt arch/x86/include/asm/paravirt.h:104 [inline]
 [<ffffffff838d3bc5>] default_idle+0x55/0x360 arch/x86/kernel/process.c:295
 [<ffffffff81069fa0>] arch_cpu_idle+0x10/0x20 arch/x86/kernel/process.c:286
 [<ffffffff838d49e5>] default_idle_call+0x45/0x60 kernel/sched/idle.c:97
 [<ffffffff81220765>] cpuidle_idle_call kernel/sched/idle.c:155 [inline]
 [<ffffffff81220765>] cpu_idle_loop kernel/sched/idle.c:248 [inline]
 [<ffffffff81220765>] cpu_startup_entry+0x2b5/0x380 kernel/sched/idle.c:303
 [<ffffffff838c184c>] rest_init+0x183/0x189 init/main.c:409
 [<ffffffff84a73943>] start_kernel+0x67e/0x6b2 init/main.c:664
 [<ffffffff84a7229a>] x86_64_start_reservations+0x29/0x2b arch/x86/kernel/head64.c:196
 [<ffffffff84a723db>] x86_64_start_kernel+0x13f/0x162 arch/x86/kernel/head64.c:177
Code: 
c80 
c3c 
c11 
c00 
c0f 
c85 
cbb 
c00 
c00 
c00 
c48 
c03 
c1c 
cc5 
ce0 
c13 
c21 
c84 
c48 
cb8 
c00 
c00 
c00 
c00 
c00 
cfc 
cff 
cdf 
c48 
c8d 
c7b 
c08 
c48 
c89 
cfa 
c48 
cc1 
cea 
c03 
c0f 
cb6 
c14 
c02 
c<48> 
c89 
cf8 
c83 
ce0 
c07 
c83 
cc0 
c03 
c38 
cd0 
c7c 
c04 
c84 
cd2 
c75 
c7f 
c8b 
c43 
c08 
c85 
c