INFO: task kworker/u4:3:1851 blocked for more than 140 seconds.
      Not tainted 4.14.153+ #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:3    D26832  1851      2 0x80000000
Workqueue: netns cleanup_net
Call Trace:
 schedule+0x92/0x1c0 kernel/sched/core.c:3499
 exp_funnel_lock kernel/rcu/tree_exp.h:295 [inline]
 _synchronize_rcu_expedited+0x68c/0x840 kernel/rcu/tree_exp.h:596
 synchronize_net+0x2b/0x40 net/core/dev.c:8222
 nf_unregister_net_hooks+0x356/0x410 net/netfilter/core.c:439
 ip6t_unregister_table+0x49/0x60 net/ipv6/netfilter/ip6_tables.c:1815
 ip6table_mangle_net_exit+0x4d/0x90 net/ipv6/netfilter/ip6table_mangle.c:112
 ops_exit_list.isra.0+0xa8/0x150 net/core/net_namespace.c:142
 cleanup_net+0x3ce/0x870 net/core/net_namespace.c:484
 process_one_work+0x7f1/0x1580 kernel/workqueue.c:2134
 worker_thread+0xdd/0xdf0 kernel/workqueue.c:2271
 kthread+0x31f/0x430 kernel/kthread.c:232
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:404

Showing all locks held in the system:
1 lock held by khungtaskd/23:
 #0:  (tasklist_lock){.+.+}, at: [<00000000a35b01af>] debug_show_all_locks+0x7c/0x21a kernel/locking/lockdep.c:4544
1 lock held by rsyslogd/1626:
 #0:  (&f->f_pos_lock){+.+.}, at: [<00000000cebdb78d>] __fdget_pos+0xb5/0xd0 fs/file.c:769
2 locks held by getty/1754:
 #0:  (&tty->ldisc_sem){++++}, at: [<000000008cb9591c>] tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:284
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c4fcaa34>] n_tty_read+0x1f7/0x1700 drivers/tty/n_tty.c:2156
3 locks held by kworker/u4:3/1851:
 #0:  ("%s""netns"){+.+.}, at: [<0000000098b61dc2>] process_one_work+0x6ff/0x1580 kernel/workqueue.c:2105
 #1:  (net_cleanup_work){+.+.}, at: [<00000000ce045505>] process_one_work+0x735/0x1580 kernel/workqueue.c:2109
 #2:  (net_mutex){+.+.}, at: [<000000003b4ae4d1>] cleanup_net+0x136/0x870 net/core/net_namespace.c:450
1 lock held by syz-executor.1/8739:
 #0:  (&pipe->mutex/1){+.+.}, at: [<00000000ede8f577>] pipe_lock_nested fs/pipe.c:67 [inline]
 #0:  (&pipe->mutex/1){+.+.}, at: [<00000000ede8f577>] pipe_lock+0x58/0x70 fs/pipe.c:75
1 lock held by syz-executor.0/11190:
 #0:  (&pipe->mutex/1){+.+.}, at: [<00000000ede8f577>] pipe_lock_nested fs/pipe.c:67 [inline]
 #0:  (&pipe->mutex/1){+.+.}, at: [<00000000ede8f577>] pipe_lock+0x58/0x70 fs/pipe.c:75
1 lock held by syz-executor.3/14816:
 #0:  (net_mutex){+.+.}, at: [<0000000089279cfe>] copy_net_ns+0x150/0x480 net/core/net_namespace.c:410
1 lock held by syz-executor.5/14822:
 #0:  (net_mutex){+.+.}, at: [<0000000089279cfe>] copy_net_ns+0x150/0x480 net/core/net_namespace.c:410

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

NMI backtrace for cpu 1
CPU: 1 PID: 23 Comm: khungtaskd Not tainted 4.14.153+ #0
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xca/0x134 lib/dump_stack.c:53
 nmi_cpu_backtrace.cold+0x47/0x86 lib/nmi_backtrace.c:101
 nmi_trigger_cpumask_backtrace+0x119/0x147 lib/nmi_backtrace.c:62
 trigger_all_cpu_backtrace include/linux/nmi.h:140 [inline]
 check_hung_uninterruptible_tasks kernel/hung_task.c:195 [inline]
 watchdog+0x629/0xbe0 kernel/hung_task.c:274
 kthread+0x31f/0x430 kernel/kthread.c:232
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:404
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at pc 0xffffffffa1aa5373