INFO: task kworker/u4:2:3780 blocked for more than 120 seconds.
      Not tainted 4.9.96-g71fce1e #10
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:2    D28600  3780      2 0x00000000
Workqueue: events_unbound flush_to_ldisc
 ffff8801d8139800 0000000000000000 ffff8801cac21f80 ffff8801d00b4800
 ffff8801db221b98 ffff8801d56af8c8 ffffffff839e240d ffff8801d813a0c8
 ffffed003b027418 ffff8801d8139800 00fffc0000000000 ffff8801db222468
Call Trace:
 [<ffffffff839e3a0f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3557
 [<ffffffff839e4393>] schedule_preempt_disabled+0x13/0x20 kernel/sched/core.c:3590
 [<ffffffff839e8816>] __mutex_lock_common kernel/locking/mutex.c:582 [inline]
 [<ffffffff839e8816>] mutex_lock_nested+0x326/0x870 kernel/locking/mutex.c:621
 [<ffffffff82119f60>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
 [<ffffffff82122bb8>] n_tty_receive_char_fast drivers/tty/n_tty.c:1418 [inline]
 [<ffffffff82122bb8>] n_tty_receive_buf_fast drivers/tty/n_tty.c:1578 [inline]
 [<ffffffff82122bb8>] __receive_buf drivers/tty/n_tty.c:1613 [inline]
 [<ffffffff82122bb8>] n_tty_receive_buf_common+0x1878/0x2300 drivers/tty/n_tty.c:1711
 [<ffffffff82123673>] n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1746
 [<ffffffff8212917f>] tty_ldisc_receive_buf+0xaf/0x190 drivers/tty/tty_buffer.c:455
 [<ffffffff8212a183>] receive_buf drivers/tty/tty_buffer.c:474 [inline]
 [<ffffffff8212a183>] flush_to_ldisc+0x253/0x370 drivers/tty/tty_buffer.c:533
 [<ffffffff8118ae31>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
 [<ffffffff8118bc26>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
 [<ffffffff8119ad5d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f34dc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373

Showing all locks held in the system:
2 locks held by khungtaskd/518:
 #0:  (rcu_read_lock){......}, at: [<ffffffff813646ec>] check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff813646ec>] watchdog+0x11c/0xa20 kernel/hung_task.c:239
 #1:  (tasklist_lock){.+.+..}, at: [<ffffffff81423ce0>] debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3631:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211c792>] n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2133
6 locks held by kworker/u4:2/3780:
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] work_static include/linux/workqueue.h:186 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
 #1:  ((&buf->work)){+.+...}, at: [<ffffffff8118ad78>] process_one_work+0x728/0x1500 kernel/workqueue.c:2089
 #2:  (&tty->ldisc_sem){++++++}, at: [<ffffffff82127940>] tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:297
 #3:  (&port->buf.lock/1){+.+...}, at: [<ffffffff82129fb6>] flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
 #4:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff821213cb>] n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff82119f60>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
6 locks held by kworker/u4:6/5106:
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] work_static include/linux/workqueue.h:186 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
 #1:  ((&buf->work)){+.+...}, at: [<ffffffff8118ad78>] process_one_work+0x728/0x1500 kernel/workqueue.c:2089
 #2:  (&tty->ldisc_sem){++++++}, at: [<ffffffff82127940>] tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:297
 #3:  (&port->buf.lock/1){+.+...}, at: [<ffffffff82129fb6>] flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
 #4:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff821213cb>] n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff82119f60>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
6 locks held by kworker/u4:7/5390:
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] work_static include/linux/workqueue.h:186 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
 #1:  ((&buf->work)){+.+...}, at: [<ffffffff8118ad78>] process_one_work+0x728/0x1500 kernel/workqueue.c:2089
 #2:  (&tty->ldisc_sem){++++++}, at: [<ffffffff82127940>] tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:297
 #3:  (&port->buf.lock/1){+.+...}, at: [<ffffffff82129fb6>] flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
 #4:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff821213cb>] n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
 #5:  (&ldata->output_lock){+.+...}, at: [<ffffffff82119f60>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
2 locks held by syz-executor226/32325:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8210a8a1>] tty_write_lock+0x21/0x60 drivers/tty/tty_io.c:1107
2 locks held by syz-executor226/32324:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8210a8a1>] tty_write_lock+0x21/0x60 drivers/tty/tty_io.c:1107
2 locks held by syz-executor226/32330:
 #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>] ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
 #1:  (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8210a8a1>] tty_write_lock+0x21/0x60 drivers/tty/tty_io.c:1107

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

NMI backtrace for cpu 0
CPU: 0 PID: 518 Comm: khungtaskd Not tainted 4.9.96-g71fce1e #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d860fd08 ffffffff81eb0b69 0000000000000000 0000000000000000
 0000000000000000 0000000000000001 ffffffff810b7d60 ffff8801d860fd40
 ffffffff81ebbe97 0000000000000000 0000000000000000 0000000000000003
Call Trace:
 [<ffffffff81eb0b69>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb0b69>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81ebbe97>] nmi_cpu_backtrace.cold.2+0x48/0x87 lib/nmi_backtrace.c:99
 [<ffffffff81ebbe2a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f lib/nmi_backtrace.c:60
 [<ffffffff810b7e64>] arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:37
 [<ffffffff81364c84>] trigger_all_cpu_backtrace include/linux/nmi.h:58 [inline]
 [<ffffffff81364c84>] check_hung_task kernel/hung_task.c:125 [inline]
 [<ffffffff81364c84>] check_hung_uninterruptible_tasks kernel/hung_task.c:182 [inline]
 [<ffffffff81364c84>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
 [<ffffffff8119ad5d>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff839f34dc>] 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: 30775 Comm: syz-executor226 Not tainted 4.9.96-g71fce1e #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801c42f6000 task.stack: ffff8801c1f60000
RIP: 0010:[<ffffffff82118620>] 
c [<ffffffff82118620>] echo_buf drivers/tty/n_tty.c:146 [inline]
RIP: 0010:[<ffffffff82118620>] 
c [<ffffffff82118620>] __process_echoes+0x5b0/0x780 drivers/tty/n_tty.c:734
RSP: 0018:ffff8801c1f67980  EFLAGS: 00000293
RAX: ffff8801c42f6000 RBX: ffffc900003bb000 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffffff821186be RDI: ffffc900003bd24a
RBP: ffff8801c1f679e8 R08: ffff8801c42f6938 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000188395fe9
R13: 0000000000001f7a R14: dffffc0000000000 R15: 0000000188395feb
FS:  00007fdc03009700(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004c26e4 CR3: 00000001da38d000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801c1f679c0
c 0000000000000000
c ffffc900003bd278
c ffffed003868ec25
c
 ffff8801c347612c
c ffffc900003bb020
c 0000000000001700
c ffff8801c3475d80
c
 ffffc900003bd300
c ffffc900003bb000
c dffffc0000000000
c ffffc900003bb020
c
Call Trace:
 [<ffffffff82122074>] flush_echoes drivers/tty/n_tty.c:801 [inline]
 [<ffffffff82122074>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
 [<ffffffff82122074>] n_tty_receive_buf_common+0xd34/0x2300 drivers/tty/n_tty.c:1711
 [<ffffffff821236b0>] n_tty_receive_buf+0x30/0x40 drivers/tty/n_tty.c:1740
 [<ffffffff82113aef>] tiocsti drivers/tty/tty_io.c:2314 [inline]
 [<ffffffff82113aef>] tty_ioctl+0xc8f/0x2270 drivers/tty/tty_io.c:2905
 [<ffffffff815b051c>] vfs_ioctl fs/ioctl.c:43 [inline]
 [<ffffffff815b051c>] file_ioctl fs/ioctl.c:493 [inline]
 [<ffffffff815b051c>] do_vfs_ioctl+0x1ac/0x11a0 fs/ioctl.c:677
 [<ffffffff815b159f>] SYSC_ioctl fs/ioctl.c:694 [inline]
 [<ffffffff815b159f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff839f3313>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: 
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cbe 
c00 
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c<4d> 
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c