INFO: task kworker/u4:3:1699 blocked for more than 120 seconds.
      Not tainted 4.9.109-g7cecc75 #2
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:3    D28424  1699      2 0x00000000
Workqueue: events_unbound flush_to_ldisc
 ffff8801cf579800 0000000000000000 ffff8801cd622f40 ffffffff84629800
 ffff8801db221c18 ffff8801cf7a7908 ffffffff839e8a3d ffff8801cf57a0c8
 ffffed0039eaf418 ffff8801cf579800 00fffc0000000000 ffff8801db2224e8
Call Trace:
 [<ffffffff839ea03f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3557
 [<ffffffff839ea9c3>] schedule_preempt_disabled+0x13/0x20 kernel/sched/core.c:3590
 [<ffffffff839eee46>] __mutex_lock_common kernel/locking/mutex.c:582 [inline]
 [<ffffffff839eee46>] mutex_lock_nested+0x326/0x870 kernel/locking/mutex.c:621
 [<ffffffff8212558d>] flush_echoes drivers/tty/n_tty.c:799 [inline]
 [<ffffffff8212558d>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 [<ffffffff8212558d>] n_tty_receive_buf_common+0xcfd/0x2300 drivers/tty/n_tty.c:1711
 [<ffffffff82126bc3>] n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1746
 [<ffffffff8212c6bf>] tty_ldisc_receive_buf+0xaf/0x190 drivers/tty/tty_buffer.c:455
 [<ffffffff8212d6c3>] receive_buf drivers/tty/tty_buffer.c:474 [inline]
 [<ffffffff8212d6c3>] flush_to_ldisc+0x253/0x370 drivers/tty/tty_buffer.c:533
 [<ffffffff8118d121>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
 [<ffffffff8118df16>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8119d04d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f9adc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373

Showing all locks held in the system:
2 locks held by khungtaskd/519:
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136662c>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8136662c>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff81425cb7>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
6 locks held by kworker/u4:3/1699:
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d02e>] work_static include/linux/workqueue.h:186 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d02e>] set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d02e>] set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d02e>] process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
 #1:  ((&buf->work)){+.+...}, at: [<ffffffff8118d068>] process_one_work+0x728/0x1500 kernel/workqueue.c:2089
 #2:  (&tty->ldisc_sem){++++++}, at: [<ffffffff8212ae90>] tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:296
 #3:  (&port->buf.lock/1){+.+...}, at: [<ffffffff8212d4f6>] flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
 #4:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff8212491b>] n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff8212558d>] flush_echoes drivers/tty/n_tty.c:799 [inline]
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff8212558d>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff8212558d>] n_tty_receive_buf_common+0xcfd/0x2300 drivers/tty/n_tty.c:1711
2 locks held by getty/3771:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f7ae2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211fce2>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2133

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

NMI backtrace for cpu 0
CPU: 0 PID: 519 Comm: khungtaskd Not tainted 4.9.109-g7cecc75 #2
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d8f7fd08 ffffffff81eb3e29 0000000000000000 0000000000000000
 0000000000000000 0000000000000001 ffffffff810b9580 ffff8801d8f7fd40
 ffffffff81ebf127 0000000000000000 0000000000000000 0000000000000002
Call Trace:
 [<ffffffff81eb3e29>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb3e29>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81ebf127>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81ebf0ba>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810b9684>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81366bc4>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81366bc4>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81366bc4>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81366bc4>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119d04d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f9adc>] 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: 5426 Comm: syz-executor908 Not tainted 4.9.109-g7cecc75 #2
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cd25b000 task.stack: ffff8801cd168000
RIP: 0010:[<ffffffff8211bbb1>] 
c [<ffffffff8211bbb1>] echo_buf drivers/tty/n_tty.c:146 [inline]
RIP: 0010:[<ffffffff8211bbb1>] 
c [<ffffffff8211bbb1>] __process_echoes+0x5f1/0x780 drivers/tty/n_tty.c:734
RSP: 0018:ffff8801cd16f980  EFLAGS: 00000202
RAX: 000000000000017a RBX: ffffc900018b1000 RCX: 0000000000000002
RDX: 0000000000000000 RSI: ffffffff8211bb8f RDI: ffffc900018b23da
RBP: ffff8801cd16f9e8 R08: ffff8801cd25b938 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000007a4ee117a
R13: 0000000000003b4b R14: dffffc0000000000 R15: 00000007a4ee117a
FS:  00007f4585fbe700(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fca47846000 CR3: 00000001d8dc4000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801cd16f9c0
c 0000000000000000
c ffffc900018b3278
c ffffed0039a55f55
c
 ffff8801cd2afaac
c ffffc900018b1020
c 0000000000000900
c ffff8801cd2af700
c
 ffffc900018b3300
c ffffc900018b1000
c dffffc0000000000
c ffffc900018b1020
c
Call Trace:
 [<ffffffff821255c4>] flush_echoes drivers/tty/n_tty.c:801 [inline]
 [<ffffffff821255c4>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 [<ffffffff821255c4>] n_tty_receive_buf_common+0xd34/0x2300 drivers/tty/n_tty.c:1711
 [<ffffffff82126c00>] n_tty_receive_buf+0x30/0x40 drivers/tty/n_tty.c:1740
 [<ffffffff8211701f>] tiocsti drivers/tty/tty_io.c:2314 [inline]
 [<ffffffff8211701f>] tty_ioctl+0xc8f/0x2270 drivers/tty/tty_io.c:2905
 [<ffffffff815b2b9c>] vfs_ioctl fs/ioctl.c:43 [inline]
 [<ffffffff815b2b9c>] file_ioctl fs/ioctl.c:493 [inline]
 [<ffffffff815b2b9c>] do_vfs_ioctl+0x1ac/0x11a0 fs/ioctl.c:677
 [<ffffffff815b3c1f>] SYSC_ioctl fs/ioctl.c:694 [inline]
 [<ffffffff815b3c1f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff839f9913>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: 
ced 
cfb 
cff 
cff 
ce8 
cb1 
c2d 
c24 
cff 
c4c 
c89 
ce0 
c25 
cff 
c0f 
c00 
c00 
c48 
c8d 
cbc 
c03 
c60 
c12 
c00 
c00 
c48 
c89 
cfa 
c48 
c89 
cf9 
c48 
cc1 
cea 
c03 
c83 
ce1 
c07 
c42 
c0f 
cb6 
c14 
c32 
c<38> 
cca 
c7f 
c08 
c84 
cd2 
c0f 
c85 
cb0 
c00 
c00 
c00 
c80 
cbc 
c03 
c60 
c12 
c00 
c00 
cff 
c4d 
c