unregister_netdevice: waiting for lo to become free. Usage count = 6
unregister_netdevice: waiting for lo to become free. Usage count = 6
unregister_netdevice: waiting for lo to become free. Usage count = 6
unregister_netdevice: waiting for lo to become free. Usage count = 6
unregister_netdevice: waiting for lo to become free. Usage count = 6
INFO: task syz-executor3:15854 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   D30240 15854   3835 0x00000004
 ffff8801d0541800 ffff8801b7a72f40 ffff8801cbefb480 ffff8801b85eb000
 ffff8801db221b98 ffff8801c0e3fbc8 ffffffff838c4a7d ffff8801d05420c0
 ffffed003a0a8417 ffff8801d0541800 00fffc0000000000 ffff8801db222468
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
1 lock held by rsyslogd/3652:
 #0:  (&f->f_pos_lock){+.+.+.}, at: [<ffffffff815d560c>] __fdget_pos+0xac/0xd0 fs/file.c:781
2 locks held by getty/3748:
 #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/u4:5/9270:
 #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-executor3/15854:
 #0:  (net_mutex){+.+.+.}, at: [<ffffffff82f39e15>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:387
1 lock held by syz-executor3/15886:
 #0:  (net_mutex){+.+.+.}, at: [<ffffffff82f39e15>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:387
1 lock held by syz-executor6/15908:
 #0:  (net_mutex){+.+.+.}, at: [<ffffffff82f39e15>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:387

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

NMI backtrace for cpu 0
CPU: 0 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
 ffff8801d852fd08 ffffffff81d9c299 0000000000000000 0000000000000000
 0000000000000000 0000000000000001 ffffffff810b7d90 ffff8801d852fd40
 ffffffff81da75d7 0000000000000000 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 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 3649 Comm: rs:main Q:Reg Not tainted 4.9.93-gf6bec4e #4
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801b57db000 task.stack: ffff8801b9960000
RIP: 0010:[<ffffffff8135c6e0>] 
c [<ffffffff8135c6e0>] __sanitizer_cov_trace_pc+0x0/0x50
RSP: 0018:ffff8801b9967338  EFLAGS: 00000293
RAX: ffff8801b57db000 RBX: 0000000000000001 RCX: 0000000000000002
RDX: 0000000000000000 RSI: ffffffff81e03c96 RDI: ffffffff83c19de0
RBP: ffff8801b9967370 R08: ffff8801b57db980 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000001 R12: ffffffff83c19de0
R13: ffffffff83c19da0 R14: ffff8801b5f0063c R15: 0000000000000019
FS:  00007fb422a4f700(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c4250ed000 CR3: 00000001c6a78000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffffffff81e03c9b
c ffff8801b57db8b8
c ffffffff846512c0
c dffffc0000000000
c
 0000000000000019
c ffff8801b5f0063c
c 0000000000000019
c ffff8801b9967380
c
 ffffffff81e03dec
c ffff8801b99673a0
c ffffffff8127d0c5
c ffff8801b5f0063c
c
Call Trace:
 [<ffffffff81e03dec>] debug_smp_processor_id+0x1c/0x20 lib/smp_processor_id.c:56
 [<ffffffff8127d0c5>] __rcu_is_watching kernel/rcu/tree.c:1053 [inline]
 [<ffffffff8127d0c5>] rcu_is_watching+0x15/0xa0 kernel/rcu/tree.c:1067
 [<ffffffff81279b8d>] rcu_read_lock_sched_held+0x8d/0x120 kernel/rcu/update.c:107
 [<ffffffff81836a9c>] trace_ext4_es_lookup_extent_exit include/trace/events/ext4.h:2364 [inline]
 [<ffffffff81836a9c>] ext4_es_lookup_extent+0x8ec/0xaf0 fs/ext4/extents_status.c:835
 [<ffffffff81732b41>] ext4_da_map_blocks fs/ext4/inode.c:1753 [inline]
 [<ffffffff81732b41>] ext4_da_get_block_prep+0x221/0x1150 fs/ext4/inode.c:1888
 [<ffffffff816273e1>] __block_write_begin_int+0x391/0x1040 fs/buffer.c:1987
 [<ffffffff816280c0>] __block_write_begin+0x30/0x40 fs/buffer.c:2038
 [<ffffffff8174be5d>] ext4_da_write_begin+0x2ed/0x1020 fs/ext4/inode.c:3004
 [<ffffffff81433bc1>] generic_perform_write+0x301/0x510 mm/filemap.c:2741
 [<ffffffff8143b642>] __generic_file_write_iter+0x352/0x540 mm/filemap.c:2866
 [<ffffffff817222b3>] ext4_file_write_iter+0x463/0xd90 fs/ext4/file.c:165
 [<ffffffff8156c430>] new_sync_write fs/read_write.c:496 [inline]
 [<ffffffff8156c430>] __vfs_write+0x3e0/0x580 fs/read_write.c:509
 [<ffffffff81570227>] vfs_write+0x187/0x530 fs/read_write.c:557
 [<ffffffff81574059>] SYSC_write fs/read_write.c:604 [inline]
 [<ffffffff81574059>] SyS_write+0xd9/0x1c0 fs/read_write.c:596
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff838d5953>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: 
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c