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SubjectINFO: rcu detected stall in __process_echoes

syzbot hit the following crash on upstream commit
3eb2ce825ea1ad89d20f7a3b5780df850e4be274 (Sun Mar 25 22:44:30 2018 +0000)
Linux 4.16-rc7
syzbot dashboard link:

So far this crash happened 10 times on upstream.
C reproducer:
syzkaller reproducer:
Raw console output:
Kernel config:
compiler: gcc (GCC) 7.1.1 20170620

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INFO: rcu_sched self-detected stall on CPU
0-....: (124999 ticks this GP) idle=6aa/1/4611686018427387906
softirq=9528/9528 fqs=31245
(t=125000 jiffies g=4714 c=4713 q=8)
NMI backtrace for cpu 0
CPU: 0 PID: 51 Comm: kworker/u4:2 Not tainted 4.16.0-rc7+ #3
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: events_unbound flush_to_ldisc
Call Trace:
__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
RIP: 0010:__write_once_size include/linux/compiler.h:215 [inline]
RIP: 0010:__sanitizer_cov_trace_pc+0x4b/0x50 kernel/kcov.c:109
RSP: 0018:ffff8801d96170a8 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff12
RAX: ffff8801d960a5c0 RBX: ffffc90001fb8000 RCX: ffffffff82f38349
RDX: 0000000000000000 RSI: 1ffff1003b2c15e4 RDI: ffffc90001fb9963
RBP: ffff8801d96170a8 R08: 1ffff1003b2c2dbd R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 000000057efa5704
R13: dffffc0000000000 R14: 000000057efa5704 R15: 0000000000003321
echo_buf drivers/tty/n_tty.c:144 [inline]
__process_echoes+0x5c9/0x770 drivers/tty/n_tty.c:732
flush_echoes drivers/tty/n_tty.c:799 [inline]
__receive_buf drivers/tty/n_tty.c:1615 [inline]
n_tty_receive_buf_common+0x1380/0x2520 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

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