Date:   Sat, 31 Mar 2018 17:56:18 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzbot <syzbot+b7d0f37ffd431e2d698c@...kaller.appspotmail.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: INFO: rcu detected stall in n_tty_receive_buf_common

On Sat, Mar 31, 2018 at 5:55 PM, syzbot
<syzbot+b7d0f37ffd431e2d698c@...kaller.appspotmail.com> wrote:
> 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

Looks like:

#syz dup: INFO: rcu detected stall in __process_echoes


> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+b7d0f37ffd431e2d698c@...kaller.appspotmail.com
> 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
>
>
> ---
> This bug is generated by a dumb bot. It may contain errors.
> See https://goo.gl/tpsmEJ for details.
> Direct all questions to syzkaller@...glegroups.com.
>
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