unregister_netdevice: waiting for lo to become free. Usage count = 4
INFO: task syz-executor0:8199 blocked for more than 120 seconds.
      Not tainted 4.9.67-gf26d3c7 #106
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor0   D29392  8199      1 0x00000004
 ffff8801a8293000 0000000000000000 ffff8801aa754a80 ffff8801da29b000
 ffff8801db321418 ffff8801d5f1fc18 ffffffff838981cb 0000000000000000
 0000000000000007 00ff8801a8293000 ffff8801db321ce8 ffff8801db321d10
Call Trace:
 [<ffffffff8389976f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3550
 [<ffffffff8389a0f3>] schedule_preempt_disabled+0x13/0x20 kernel/sched/core.c:3583
 [<ffffffff8389f702>] __mutex_lock_common kernel/locking/mutex.c:582 [inline]
 [<ffffffff8389f702>] mutex_lock_nested+0x312/0x870 kernel/locking/mutex.c:621
unregister_netdevice: waiting for lo to become free. Usage count = 4
 [<ffffffff82f14b05>] copy_net_ns+0x155/0x280 net/core/net_namespace.c:387
 [<ffffffff8119bb1f>] create_new_namespaces+0x37f/0x730 kernel/nsproxy.c:106
 [<ffffffff8119c4ae>] unshare_nsproxy_namespaces+0xae/0x1e0 kernel/nsproxy.c:205
 [<ffffffff8112e33d>] SYSC_unshare kernel/fork.c:2238 [inline]
 [<ffffffff8112e33d>] SyS_unshare+0x3dd/0x6f0 kernel/fork.c:2188
 [<ffffffff838a9985>] entry_SYSCALL_64_fastpath+0x23/0xc6

Showing all locks held in the system:
2 locks held by khungtaskd/514:
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136fe05>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136fe05>] watchdog+0x125/0xa70 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff812355d0>] debug_show_all_locks+0x70/0x280 kernel/locking/lockdep.c:4336
2 locks held by getty/3237:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff838a7b92>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+.+.}, at: [<ffffffff82001d24>] n_tty_read+0x1f4/0x16c0 drivers/tty/n_tty.c:2133
3 locks held by kworker/u4:9/6142:
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811876e9>] __write_once_size include/linux/compiler.h:272 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811876e9>] atomic64_set arch/x86/include/asm/atomic64_64.h:33 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811876e9>] atomic_long_set include/asm-generic/atomic-long.h:56 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811876e9>] set_work_data kernel/workqueue.c:616 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811876e9>] set_work_pool_and_clear_pending kernel/workqueue.c:643 [inline]
 #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811876e9>] process_one_work+0x6a9/0x15f0 kernel/workqueue.c:2083
 #1:  (net_cleanup_work){+.+.+.}, at: [<ffffffff8118771b>] process_one_work+0x6db/0x15f0 kernel/workqueue.c:2087
 #2:  (net_mutex){+.+.+.}, at: [<ffffffff82f144df>] cleanup_net+0x13f/0x610 net/core/net_namespace.c:420
1 lock held by syz-executor0/8199:
 #0:  (net_mutex){+.+.+.}, at: [<ffffffff82f14b05>] copy_net_ns+0x155/0x280 net/core/net_namespace.c:387

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

NMI backtrace for cpu 0
CPU: 0 PID: 514 Comm: khungtaskd Not tainted 4.9.67-gf26d3c7 #106
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d897fd00 ffffffff81d906e9 0000000000000000 0000000000000000
 0000000000000000 0000000000000001 ffffffff810ba170 ffff8801d897fd38
 ffffffff81d9b80d 0000000000000000 0000000000000000 ffff8801a8293418
Call Trace:
 [<ffffffff81d906e9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d906e9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81d9b80d>] nmi_cpu_backtrace+0xfd/0x120 lib/nmi_backtrace.c:99
 [<ffffffff81d9b947>] nmi_trigger_cpumask_backtrace+0x117/0x190 lib/nmi_backtrace.c:60
 [<ffffffff810ba264>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff813703d0>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff813703d0>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff813703d0>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff813703d0>] watchdog+0x6f0/0xa70 kernel/hung_task.c:239
 [<ffffffff811985ad>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff838a9c2a>] ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:433
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 6142 Comm: kworker/u4:9 Not tainted 4.9.67-gf26d3c7 #106
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: netns cleanup_net
c
task: ffff8801ce60b000 task.stack: ffff8801d85a0000
RIP: 0010:[<ffffffff81239cb9>] 
c [<ffffffff81239cb9>] validate_chain kernel/locking/lockdep.c:2235 [inline]
RIP: 0010:[<ffffffff81239cb9>] 
c [<ffffffff81239cb9>] __lock_acquire+0x849/0x3640 kernel/locking/lockdep.c:3345
RSP: 0018:ffff8801d85a7010  EFLAGS: 00000046
RAX: dffffc0000000000 RBX: ffff8801ce60b978 RCX: 1ffff10039cc1733
RDX: 1ffffffff09a7f63 RSI: 000000008372c5cd RDI: ffffffff84d3fb18
RBP: ffff8801d85a71d0 R08: 1ffff10039cc172e R09: 0000000000000001
R10: 0000000000000000 R11: ffff8801ce60b000 R12: ffffffff84d3fb08
R13: 00000000bfb9ea4c R14: 689ca9db8372c5cd R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f5c46a099b8 CR3: 00000001cf364000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d85a7080
c ffff8801d85a7058
c ffff8801da163da0
c ffff8801da163e40
c
 0000000041b58ab3
c ffffffff841889dd
c ffffffff811fc4c0
c ffff8801d85a7088
c
 0000000000004117
c 0000000000002571
c 0000000000002571
c 0000000000000367
c
Call Trace:
 [<ffffffff8123d4ee>] lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
 [<ffffffff838a97ee>] __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:112 [inline]
 [<ffffffff838a97ee>] _raw_spin_lock_irqsave+0x4e/0x70 kernel/locking/spinlock.c:159
 [<ffffffff812010d2>] load_balance+0x6d2/0x2e90 kernel/sched/fair.c:9421
 [<ffffffff81203fa1>] idle_balance kernel/sched/fair.c:9689 [inline]
 [<ffffffff81203fa1>] pick_next_task_fair+0x711/0x1fe0 kernel/sched/fair.c:7506
 [<ffffffff83897dd8>] pick_next_task kernel/sched/core.c:3355 [inline]
 [<ffffffff83897dd8>] __schedule+0x288/0x1ba0 kernel/sched/core.c:3477
 [<ffffffff8389976f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3550
 [<ffffffff838a59a3>] schedule_timeout+0x643/0x1220 kernel/time/timer.c:1770
 [<ffffffff8389c8ae>] do_wait_for_common kernel/sched/completion.c:75 [inline]
 [<ffffffff8389c8ae>] __wait_for_common kernel/sched/completion.c:93 [inline]
 [<ffffffff8389c8ae>] wait_for_common kernel/sched/completion.c:101 [inline]
 [<ffffffff8389c8ae>] wait_for_completion+0x20e/0x2e0 kernel/sched/completion.c:122
 [<ffffffff8128c179>] _rcu_barrier+0x249/0x330 kernel/rcu/tree.c:3701
 [<ffffffff8128c2b0>] rcu_barrier+0x10/0x20 kernel/rcu/tree_plugin.h:692
 [<ffffffff82f483e5>] netdev_wait_allrefs net/core/dev.c:7423 [inline]
 [<ffffffff82f483e5>] netdev_run_todo+0x2a5/0x6b0 net/core/dev.c:7511
 [<ffffffff82f6d44e>] rtnl_unlock+0xe/0x10 net/core/rtnetlink.c:104
 [<ffffffff82f2aaf8>] default_device_exit_batch+0x358/0x410 net/core/dev.c:8311
 [<ffffffff82f118f0>] ops_exit_list.isra.4+0x100/0x150 net/core/net_namespace.c:139
 [<ffffffff82f146bd>] cleanup_net+0x31d/0x610 net/core/net_namespace.c:454
 [<ffffffff811877cf>] process_one_work+0x78f/0x15f0 kernel/workqueue.c:2090
 [<ffffffff81188710>] worker_thread+0xe0/0x10d0 kernel/workqueue.c:2224
 [<ffffffff811985ad>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff838a9c2a>] ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:433
Code: 
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c