BUG: workqueue lockup - pool[  572.580773] INFO: task kworker/u4:1:64 blocked for more than 140 seconds.
      Not tainted 4.9.128+ #41
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:1    D
24536    64      2 0x00000000
Workqueue: netns cleanup_net
 ffff8801d78597c0
 ffff8801c9b54d00 ffff8801c9b56300 ffff8801d6290000 ffff8801db721018
 ffff8801d79a7500 ffffffff8277ee02 ffff8801d785a158 ffffffff83a928a0
 ffff8801d79a74d8 0000000000003d92 ffff8801db7218f0Call Trace:
 [<ffffffff8278032f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3553
 [<ffffffff8278a955>] schedule_timeout+0x735/0xe20 kernel/time/timer.c:1771
 [<ffffffff82781d59>] do_wait_for_common kernel/sched/completion.c:75 [inline]
 [<ffffffff82781d59>] __wait_for_common kernel/sched/completion.c:93 [inline]
 [<ffffffff82781d59>] wait_for_common+0x2f9/0x400 kernel/sched/completion.c:101
 [<ffffffff82781e78>] wait_for_completion+0x18/0x20 kernel/sched/completion.c:122
 [<ffffffff81128a44>] flush_work+0x3a4/0x6c0 kernel/workqueue.c:2846
 [<ffffffff82286140>] flush_all_backlogs net/core/dev.c:4405 [inline]
 [<ffffffff82286140>] rollback_registered_many+0x430/0x920 net/core/dev.c:6818
 [<ffffffff8228fbeb>] unregister_netdevice_many.part.27+0x1b/0x110 net/core/dev.c:7879
 [<ffffffff8228fd19>] unregister_netdevice_many+0x39/0x50 net/core/dev.c:7878
 [<ffffffff82726f0d>] ip6_tnl_destroy_tunnels net/ipv6/ip6_tunnel.c:2183 [inline]
 [<ffffffff82726f0d>] ip6_tnl_exit_net+0x3dd/0x5b0 net/ipv6/ip6_tunnel.c:2230
 [<ffffffff82268540>] ops_exit_list.isra.0+0xb0/0x160 net/core/net_namespace.c:136
 [<ffffffff8226ac71>] cleanup_net+0x321/0x630 net/core/net_namespace.c:473
 [<ffffffff8112cb71>] process_one_work+0x791/0x1470 kernel/workqueue.c:2092
 [<ffffffff8112d926>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8113e00d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff8278e0dc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373

Showing all locks held in the system:
3 locks held by kworker/1:1/22:
 #0: 
 ("%s"("ipv6_addrconf")){.+.+..}, at: [<ffffffff8112ca80>] process_one_work+0x6a0/0x1470 kernel/workqueue.c:2085
 ((addr_chk_work).work){+.+...}, at: [<ffffffff8112cab8>] process_one_work+0x6d8/0x1470 kernel/workqueue.c:2089
 (rtnl_mutex){+.+.+.}, at: [<ffffffff822c2fc7>] rtnl_lock+0x17/0x20 net/core/rtnetlink.c:70
 #0: 
 (rcu_read_lock){......}, at: [<ffffffff8130a0ac>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 (rcu_read_lock){......}, at: [<ffffffff8130a0ac>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 (tasklist_lock){.+.+..}, at: [<ffffffff813e71ec>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
 #0: 
 ("%s""netns"){.+.+.+}, at: [<ffffffff8112ca80>] process_one_work+0x6a0/0x1470 kernel/workqueue.c:2085
 (net_cleanup_work){+.+.+.}, at: [<ffffffff8112cab8>] process_one_work+0x6d8/0x1470 kernel/workqueue.c:2089
 (net_mutex){+.+.+.}, at: [<ffffffff8226aa8f>] cleanup_net+0x13f/0x630 net/core/net_namespace.c:439
 (rtnl_mutex){+.+.+.}, at: [<ffffffff822c2fc7>] rtnl_lock+0x17/0x20 net/core/rtnetlink.c:70
 (cpu_hotplug.dep_map){++++++}, at: [<ffffffff810df182>] get_online_cpus+0x32/0x90 kernel/cpu.c:253
 #0: 
 (&f->f_pos_lock){+.+.+.}, at: [<ffffffff8154c6fc>] __fdget_pos+0xac/0xd0 fs/file.c:781
 #0: 
 (&tty->ldisc_sem){++++++}, at: [<ffffffff8278bfd2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff81cdee72>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2142
 #0: 
 (net_mutex){+.+.+.}, at: [<ffffffff8226b0d5>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:406
 #0: 
 (net_mutex){+.+.+.}, at: [<ffffffff8226b0d5>] copy_net_ns+0x155/0x290 net/core/net_namespace.c:406
=============================================

NMI backtrace for cpu 1
CPU: 1 PID: 24 Comm: khungtaskd Not tainted 4.9.128+ #41
 ffff8801d9907d08
 ffffffff81af2469 0000000000000000 0000000000000001 0000000000000001
 0000000000000001 ffffffff810967d0 ffff8801d9907d40 ffffffff81afd1c9
 0000000000000001 0000000000000000 0000000000000003Call Trace:
 [<ffffffff81af2469>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81af2469>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81afd1c9>] nmi_cpu_backtrace.cold.0+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81afd15c>] nmi_trigger_cpumask_backtrace+0x12c/0x151 lib/nmi_backtrace.c:60
 [<ffffffff810968d4>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff8130a63d>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff8130a63d>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff8130a63d>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff8130a63d>] watchdog+0x6ad/0xa20 kernel/hung_task.c:239
 [<ffffffff8113e00d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff8278e0dc>] 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: 26002 Comm: syz-executor0 Not tainted 4.9.128+ #41
task: ffff8801a40edf00 task.stack: ffff8801d8620000
RIP: 0010:[<ffffffff81d09b9b>] 
c [<ffffffff81d09b9b>] inb arch/x86/include/asm/io.h:316 [inline]
RIP: 0010:[<ffffffff81d09b9b>] 
c [<ffffffff81d09b9b>] io_serial_in+0x6b/0x90 drivers/tty/serial/8250/8250_port.c:413
RSP: 0000:ffff8801db607790  EFLAGS: 00000002
RAX: dffffc0000000000 RBX: 00000000000003fd RCX: 0000000000000000
RDX: 00000000000003fd RSI: ffffffff81d09b41 RDI: ffffffff84959a18
RBP: ffff8801db6077a0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffffffff849599e0
R13: 0000000000000020 R14: fffffbfff092b383 R15: fffffbfff092b345
FS:  00007f0fcc13b700(0000) GS:ffff8801db600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fb99b131000 CR3: 00000001c943e000 CR4: 00000000001606b0
DR0: 0000000020000000 DR1: 0000000020000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
Stack:
 ffffffff849599e0
c 000000000000270d
c ffff8801db6