syzbot

INFO: rcu_sched self-detected stall on CPU
	1-....: (124998 ticks this GP) idle=7ba/1/4611686018427387906 softirq=9341/9341 fqs=31242 
	 (t=125000 jiffies g=4322 c=4321 q=3)
NMI backtrace for cpu 1
CPU: 1 PID: 4465 Comm: kworker/u4:30 Not tainted 4.16.0+ #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_unbound flush_to_ldisc
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x24d lib/dump_stack.c:53
 nmi_cpu_backtrace+0x1d2/0x210 lib/nmi_backtrace.c:103
 nmi_trigger_cpumask_backtrace+0x123/0x180 lib/nmi_backtrace.c:62
 arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
 trigger_single_cpu_backtrace include/linux/nmi.h:156 [inline]
 rcu_dump_cpu_stacks+0x186/0x1de kernel/rcu/tree.c:1375
 print_cpu_stall kernel/rcu/tree.c:1524 [inline]
 check_cpu_stall.isra.61+0xbb8/0x15b0 kernel/rcu/tree.c:1592
 __rcu_pending kernel/rcu/tree.c:3361 [inline]
 rcu_pending kernel/rcu/tree.c:3423 [inline]
 rcu_check_callbacks+0x238/0xd20 kernel/rcu/tree.c:2763
 update_process_times+0x30/0x60 kernel/time/timer.c:1636
 tick_sched_handle+0x85/0x160 kernel/time/tick-sched.c:162
 tick_sched_timer+0x42/0x120 kernel/time/tick-sched.c:1194
 __run_hrtimer kernel/time/hrtimer.c:1349 [inline]
 __hrtimer_run_queues+0x39c/0xec0 kernel/time/hrtimer.c:1411
 hrtimer_interrupt+0x2a5/0x6f0 kernel/time/hrtimer.c:1469
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1025 [inline]
 smp_apic_timer_interrupt+0x14a/0x700 arch/x86/kernel/apic/apic.c:1050
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:857
 </IRQ>
RIP: 0010:echo_buf drivers/tty/n_tty.c:144 [inline]
RIP: 0010:__process_echoes+0x62c/0x770 drivers/tty/n_tty.c:733
RSP: 0018:ffff8801b6b9f078 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff12
RAX: 0000000000000000 RBX: ffffc900028b4000 RCX: ffffffff8352d4c2
RDX: 0000000000000002 RSI: 1ffff10038575a04 RDI: ffffc900028b5712
RBP: ffff8801b6b9f0e0 R08: 1ffff10036d73db5 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000006734944b1
R13: dffffc0000000000 R14: 00000000000004b2 R15: 0000000000001d05
 commit_echoes+0x147/0x1b0 drivers/tty/n_tty.c:764
 n_tty_receive_char_fast drivers/tty/n_tty.c:1416 [inline]
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1576 [inline]
 __receive_buf drivers/tty/n_tty.c:1611 [inline]
 n_tty_receive_buf_common+0x1156/0x2520 drivers/tty/n_tty.c:1709
 n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1744
 tty_ldisc_receive_buf+0xa7/0x180 drivers/tty/tty_buffer.c:456
 tty_port_default_receive_buf+0x106/0x160 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:475 [inline]
 flush_to_ldisc+0x3c4/0x590 drivers/tty/tty_buffer.c:524
 process_one_work+0xc47/0x1bb0 kernel/workqueue.c:2113
 worker_thread+0x223/0x1990 kernel/workqueue.c:2247
 kthread+0x33c/0x400 kernel/kthread.c:238
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406