INFO: rcu detected stall in n_tty_receive_char_special

[syzbot <syzbot+18df353d7540aa6b5467@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on upstream commit
3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91 (Sat Mar 31 01:52:36 2018 +0000)
kernel.h: Retain constant expression output for max()/min()
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=18df353d7540aa6b5467

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5836679554269184
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-1647968177339044852
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

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Reported-by: syzbot+18df353d7540aa6b5467@syzkaller.appspotmail.com
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details.
If you forward the report, please keep this part and the footer.

INFO: rcu_sched detected stalls on CPUs/tasks:
	(detected by 1, t=125007 jiffies, g=42488, c=42487, q=11)
All QSes seen, last rcu_sched kthread activity 125014  
(4295022441-4294897427), jiffies_till_next_fqs=3, root ->qsmask 0x0
kworker/u4:5    R  running task    15272  8806      2 0x80000008
Workqueue: events_unbound flush_to_ldisc
Call Trace:
  <IRQ>
  sched_show_task.cold.87+0x27a/0x301 kernel/sched/core.c:5325
  print_other_cpu_stall.cold.79+0x92f/0x9d2 kernel/rcu/tree.c:1481
  check_cpu_stall.isra.61+0x706/0xf50 kernel/rcu/tree.c:1599
  __rcu_pending kernel/rcu/tree.c:3356 [inline]
  rcu_pending kernel/rcu/tree.c:3401 [inline]
  rcu_check_callbacks+0x21b/0xad0 kernel/rcu/tree.c:2763
  update_process_times+0x2d/0x70 kernel/time/timer.c:1636
  tick_sched_handle+0xa0/0x180 kernel/time/tick-sched.c:171
  tick_sched_timer+0x42/0x130 kernel/time/tick-sched.c:1179
  __run_hrtimer kernel/time/hrtimer.c:1337 [inline]
  __hrtimer_run_queues+0x3e3/0x10a0 kernel/time/hrtimer.c:1399
  hrtimer_interrupt+0x286/0x650 kernel/time/hrtimer.c:1457
  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1025 [inline]
  smp_apic_timer_interrupt+0x15d/0x710 arch/x86/kernel/apic/apic.c:1050
  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:862
  </IRQ>
RIP: 0010:echo_char+0xae/0x2e0 drivers/tty/n_tty.c:915
RSP: 0018:ffff8801d33e71e0 EFLAGS: 00000a07 ORIG_RAX: ffffffffffffff13
RAX: dffffc0000000000 RBX: ffffc90013158000 RCX: ffffffff8375b1b7
RDX: 1ffff1003ad87636 RSI: ffffffff8375b1c6 RDI: ffff8801d6c3b1b4
RBP: ffff8801d33e7210 R08: ffff8801cf482540 R09: fffff5200262b460
R10: fffff5200262b460 R11: ffffc9001315a307 R12: 00000000000000cb
R13: ffff8801d6c3ae00 R14: 00000000c240f0bb R15: 00000000000000bb
  n_tty_receive_char_special+0x13b3/0x31c0 drivers/tty/n_tty.c:1306
  n_tty_receive_buf_fast drivers/tty/n_tty.c:1577 [inline]
  __receive_buf drivers/tty/n_tty.c:1611 [inline]
  n_tty_receive_buf_common+0x20ca/0x2c50 drivers/tty/n_tty.c:1709
  n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1744
  tty_ldisc_receive_buf+0xb0/0x190 drivers/tty/tty_buffer.c:456
  tty_port_default_receive_buf+0x110/0x170 drivers/tty/tty_port.c:38
  receive_buf drivers/tty/tty_buffer.c:475 [inline]
  flush_to_ldisc+0x3e9/0x560 drivers/tty/tty_buffer.c:524
  process_one_work+0xc1e/0x1b50 kernel/workqueue.c:2145
  worker_thread+0x1cc/0x1440 kernel/workqueue.c:2279
  kthread+0x345/0x410 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:411
rcu_sched kthread starved for 125626 jiffies! g42488 c42487 f0x2  
RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=0
RCU grace-period kthread stack dump:
rcu_sched       R  running task    23592     9      2 0x80000000
Call Trace:
  context_switch kernel/sched/core.c:2848 [inline]
  __schedule+0x807/0x1e40 kernel/sched/core.c:3490
  schedule+0xef/0x430 kernel/sched/core.c:3549
  schedule_timeout+0x138/0x240 kernel/time/timer.c:1801
  rcu_gp_kthread+0x6b5/0x1940 kernel/rcu/tree.c:2231
  kthread+0x345/0x410 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:411


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