INFO: rcu detected stall in n_tty_receive_buf_common

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
0b412605ef5f5c64b31f19e2910b1d5eba9929c3 (Thu Mar 29 01:07:23 2018 +0000)
Merge tag 'drm-fixes-for-v4.16-rc8' of  
git://people.freedesktop.org/~airlied/linux
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=b7d0f37ffd431e2d698c

So far this crash happened 2 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5308434815647744
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=5176078016446464
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5940022372990976
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-8440362230543204781
compiler: gcc (GCC) 7.1.1 20170620

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Reported-by: syzbot+b7d0f37ffd431e2d698c@xxxxxxxxxxxxxxxxxxxxxxxxx
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

INFO: rcu_sched self-detected stall on CPU
	0-....: (124999 ticks this GP) idle=6ea/1/4611686018427387906  
softirq=9748/9748 fqs=31240
	 (t=125000 jiffies g=4823 c=4822 q=10)
NMI backtrace for cpu 0
CPU: 0 PID: 4469 Comm: syzkaller058368 Not tainted 4.16.0-rc7+ #5
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
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+0x5d9/0x770 drivers/tty/n_tty.c:732
RSP: 0018:ffff8801af12f7f0 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff12
RAX: 0000000000000594 RBX: ffffc90001946000 RCX: ffffffff82f38469
RDX: 0000000000000000 RSI: 1ffff1003543c565 RDI: ffffc900019477f4
RBP: ffff8801af12f858 R08: 1ffff10035e25ea4 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000005e6b39594
R13: dffffc0000000000 R14: 00000005e6b39594 R15: 0000000000003803
 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_buf+0x30/0x40 drivers/tty/n_tty.c:1738
 tiocsti drivers/tty/tty_io.c:2171 [inline]
 tty_ioctl+0x9c5/0x1610 drivers/tty/tty_io.c:2557
 vfs_ioctl fs/ioctl.c:46 [inline]
 do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x446979
RSP: 002b:00007fb18094bd18 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006e29fc RCX: 0000000000446979
RDX: 00000000200000c0 RSI: 0000000000005412 RDI: 0000000000000004
RBP: 00000000006e29f8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 6d74702f7665642f
R13: 00007ffc95813d0f R14: 00007fb18094c9c0 R15: 0000000000000001


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