INFO: task kworker/u4:1:862 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/u4:1    D28568   862      2 0x00000000
Workqueue: events_unbound flush_to_ldisc
 ffff8801abfe0000 ffff8801b93864c0 ffff8801d07a4540 ffff8801b6866000
 ffff8801db321c18 ffff8801cd187908 ffffffff839e7ded ffff8801abfe08c8
 ffffed00357fc118 ffff8801abfe0000 00fffc0000000000 ffff8801db3224e8
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
 [<ffffffff82123ead>] flush_echoes drivers/tty/n_tty.c:799 [inline]
 [<ffffffff82123ead>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 [<ffffffff82123ead>] n_tty_receive_buf_common+0xcfd/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/3769:
 #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/u4:1/862:
 #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: [<ffffffff82123ead>] flush_echoes drivers/tty/n_tty.c:799 [inline]
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff82123ead>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff82123ead>] n_tty_receive_buf_common+0xcfd/0x2300 drivers/tty/n_tty.c:1711

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

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
 ffff8801d85c7d08 ffffffff81eb2729 0000000000000000 0000000000000001
 0000000000000001 0000000000000001 ffffffff810b9af0 ffff8801d85c7d40
 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: 1313 Comm: syz-executor2 Not tainted 4.9.111-g03c70fe #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801a0bc6000 task.stack: ffff8801ab948000
RIP: 0010:[<ffffffff8211a4af>] 
c [<ffffffff8211a4af>] echo_buf drivers/tty/n_tty.c:146 [inline]
RIP: 0010:[<ffffffff8211a4af>] 
c [<ffffffff8211a4af>] __process_echoes+0x5cf/0x780 drivers/tty/n_tty.c:734
RSP: 0018:ffff8801ab94f980  EFLAGS: 00000246
RAX: 0000000000040000 RBX: ffffc900139fe000 RCX: ffffc90006d57000
RDX: 0000000000040000 RSI: ffffffff8211a4af RDI: ffffc900139ff772
RBP: ffff8801ab94f9e8 R08: ffff8801a0bc6938 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000988de4513
R13: 00000000000009e1 R14: dffffc0000000000 R15: 0000000988de4513
FS:  00007f1bf23be700(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc6f5181000 CR3: 00000001cb661000 CR4: 00000000001606f0
DR0: 0000000020000000 DR1: 0000000020000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
Stack:
 ffff880100000000
c 0000000100000000
c ffffc90013a00278
c ffffed0033c42295
c
 ffff88019e2114ac
c ffffc900139fe020
c 0000000000001f00
c ffff88019e211100
c
 ffffc90013a00300
c ffffc900139fe000
c dffffc0000000000
c ffffc900139fe020
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: 
c49 
cbe 
c00 
c00 
c00 
c00 
c00 
cfc 
cff 
cdf 
ceb 
c03 
c4d 
c89 
cfc 
ce8 
c78 
c45 
c24 
cff 
c4c 
c89 
ce8 
c4c 
c29 
ce0 
c48 
c3d 
cdf 
c0e 
c00 
c00 
c0f 
c86 
ced 
cfb 
cff 
cff 
ce8 
c61 
c45 
c24 
cff 
c<4c> 
c89 
ce0 
c25 
cff 
c0f 
c00 
c00 
c48 
c8d 
cbc 
c03 
c60 
c12 
c00 
c00 
c48 
c89 
cfa 
c48 
c89 
c