INFO: task kworker/u4:6:7560 blocked for more than 140 seconds.
      Not tainted 4.9.111-g03c70fe #6
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:6    D25736  7560      2 0x00000000
Workqueue: events_unbound flush_to_ldisc
 ffff8801af519800 ffff88019eb8f480 ffff88019ebc5f80 ffff8801d33a6000
 ffff8801db221c18 ffff8801b6d478c8 ffffffff839e7ded ffff8801af51a0c8
 ffffed0035ea3418 ffff8801af519800 00fffc0000000000 ffff8801db2224e8
Call Trace:
 [<ffffffff839e93ef>] schedule+0x7f/0x1b0 kernel/sched/core.c:3557
 [<ffffffff839e9d73>] schedule_preempt_disabled+0x13/0x20 kernel/sched/core.c:3590
 [<ffffffff839ee1f6>] __mutex_lock_common kernel/locking/mutex.c:582 [inline]
 [<ffffffff839ee1f6>] mutex_lock_nested+0x326/0x870 kernel/locking/mutex.c:621
 [<ffffffff8211bdd0>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
 [<ffffffff82124a28>] n_tty_receive_char_fast drivers/tty/n_tty.c:1418 [inline]
 [<ffffffff82124a28>] n_tty_receive_buf_fast drivers/tty/n_tty.c:1578 [inline]
 [<ffffffff82124a28>] __receive_buf drivers/tty/n_tty.c:1613 [inline]
 [<ffffffff82124a28>] n_tty_receive_buf_common+0x1878/0x2300 drivers/tty/n_tty.c:1711
 [<ffffffff821254e3>] n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1746
 [<ffffffff8212afdf>] tty_ldisc_receive_buf+0xaf/0x190 drivers/tty/tty_buffer.c:455
 [<ffffffff8212bfe3>] receive_buf drivers/tty/tty_buffer.c:474 [inline]
 [<ffffffff8212bfe3>] flush_to_ldisc+0x253/0x370 drivers/tty/tty_buffer.c:533
 [<ffffffff8118d131>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
 [<ffffffff8118df26>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8119d05d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f8e9c>] 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: [<ffffffff813666fc>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff813666fc>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff81425d87>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3779:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f6e92>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211e602>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2133
6 locks held by kworker/u4:6/7560:
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d03e>] work_static include/linux/workqueue.h:186 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d03e>] set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d03e>] set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118d03e>] process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
 #1:  ((&buf->work)){+.+...}, at: [<ffffffff8118d078>] process_one_work+0x728/0x1500 kernel/workqueue.c:2089
 #2:  (&tty->ldisc_sem){++++++}, at: [<ffffffff821297b0>] tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:296
 #3:  (&port->buf.lock/1){+.+...}, at: [<ffffffff8212be16>] flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
 #4:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff8212323b>] n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff8211bdd0>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764

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

NMI backtrace for cpu 1
CPU: 1 PID: 519 Comm: khungtaskd Not tainted 4.9.111-g03c70fe #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d84bfd08 ffffffff81eb2729 0000000000000000 0000000000000001
 0000000000000001 0000000000000001 ffffffff810b9af0 ffff8801d84bfd40
 ffffffff81ebda27 0000000000000001 0000000000000000 0000000000000002
Call Trace:
 [<ffffffff81eb2729>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb2729>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81ebda27>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81ebd9ba>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810b9bf4>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81366c94>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81366c94>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81366c94>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81366c94>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119d05d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f8e9c>] 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: 9945 Comm: syz-executor5 Not tainted 4.9.111-g03c70fe #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801a0813000 task.stack: ffff8801cd9a8000
RIP: 0010:[<ffffffff8211a498>] 
c [<ffffffff8211a498>] __process_echoes+0x5b8/0x780 drivers/tty/n_tty.c:733
RSP: 0018:ffff8801cd9af980  EFLAGS: 00000246
RAX: 0000000000040000 RBX: ffffc900094b8000 RCX: ffffc90000b06000
RDX: 0000000000040000 RSI: ffffffff8211a498 RDI: ffffc900094b97db
RBP: ffff8801cd9af9e8 R08: ffff8801a0813938 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000009bbac557c
R13: 00000000000001e1 R14: dffffc0000000000 R15: 00000009bbac557c
FS:  00007f56dfd2c700(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000001c15000 CR3: 00000001d93cc000 CR4: 00000000001606f0
DR0: ffffffffffffffff DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
Stack:
 ffff8801cd9af9c0
c 0000000000000000
c ffffc900094ba278
c ffffed0033cb9185
c
 ffff88019e5c8c2c
c ffffc900094b8020
c 0000000000001f00
c ffff88019e5c8880
c
 ffffc900094ba300
c ffffc900094b8000
c dffffc0000000000
c ffffc900094b8020
c
Call Trace:
 [<ffffffff82123ee4>] flush_echoes drivers/tty/n_tty.c:801 [inline]
 [<ffffffff82123ee4>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 [<ffffffff82123ee4>] n_tty_receive_buf_common+0xd34/0x2300 drivers/tty/n_tty.c:1711
 [<ffffffff82125520>] n_tty_receive_buf+0x30/0x40 drivers/tty/n_tty.c:1740
 [<ffffffff8211593f>] tiocsti drivers/tty/tty_io.c:2314 [inline]
 [<ffffffff8211593f>] tty_ioctl+0xc8f/0x2270 drivers/tty/tty_io.c:2905
 [<ffffffff815b2c6c>] vfs_ioctl fs/ioctl.c:43 [inline]
 [<ffffffff815b2c6c>] file_ioctl fs/ioctl.c:493 [inline]
 [<ffffffff815b2c6c>] do_vfs_ioctl+0x1ac/0x11a0 fs/ioctl.c:677
 [<ffffffff815b3cef>] SYSC_ioctl fs/ioctl.c:694 [inline]
 [<ffffffff815b3cef>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff839f8cd3>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: 
cc1 
cea 
c03 
c80 
c3c 
c02 
c00 
c0f 
c85 
c47 
c01 
c00 
c00 
c8b 
c4d 
ccc 
c4c 
c8b 
c6b 
c20 
c29 
c4d 
cc8 
c49 
cbe 
c00 
c00 
c00 
c00 
c00 
cfc 
cff 
cdf 
ceb 
c03 
c4d 
c89 
cfc 
ce8 
c78 
c45 
c24 
cff 
c<4c> 
c89 
ce8 
c4c 
c29 
ce0 
c48 
c3d 
cdf 
c0e 
c00 
c00 
c0f 
c86 
ced 
cfb 
cff 
cff 
ce8 
c61 
c45 
c