Message-ID: <CACT4Y+ZUNMdO7qEH7hymGTdx5uKqoEF6pWXfMgfkyK2BCgmj8A@mail.gmail.com>
Date:   Sat, 26 May 2018 17:38:01 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzbot <syzbot+deec965c578bb9b81613@...kaller.appspotmail.com>
Cc:     David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>, linux-sctp@...r.kernel.org,
        netdev <netdev@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Vladislav Yasevich <vyasevich@...il.com>
Subject: Re: INFO: rcu detected stall in kmem_cache_alloc_node_trace

On Mon, Apr 30, 2018 at 8:05 PM, syzbot
<syzbot+deec965c578bb9b81613@...kaller.appspotmail.com> wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    17dec0a94915 Merge branch 'userns-linus' of
> git://git.kerne...
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?id=6093051722203136
> kernel config:
> https://syzkaller.appspot.com/x/.config?id=-2735707888269579554
> dashboard link: https://syzkaller.appspot.com/bug?extid=deec965c578bb9b81613
> compiler:       gcc (GCC) 8.0.1 20180301 (experimental)
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+deec965c578bb9b81613@...kaller.appspotmail.com
>
> sctp: [Deprecated]: syz-executor3 (pid 10218) Use of int in max_burst socket
> option.
> Use struct sctp_assoc_value instead
> sctp: [Deprecated]: syz-executor3 (pid 10218) Use of int in max_burst socket
> option.
> Use struct sctp_assoc_value instead
> random: crng init done
> INFO: rcu_sched self-detected stall on CPU
>         0-....: (120712 ticks this GP) idle=ac6/1/4611686018427387908
> softirq=31693/31693 fqs=31173
>          (t=125001 jiffies g=17039 c=17038 q=303419)
> NMI backtrace for cpu 0
> CPU: 0 PID: 10218 Comm: syz-executor3 Not tainted 4.16.0+ #1
> 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+0x1b9/0x29f lib/dump_stack.c:53
>  nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
>  nmi_trigger_cpumask_backtrace+0x151/0x192 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+0x175/0x1c2 kernel/rcu/tree.c:1376
>  print_cpu_stall kernel/rcu/tree.c:1525 [inline]
>  check_cpu_stall.isra.61.cold.80+0x36c/0x59a kernel/rcu/tree.c:1593
>  __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:162
>  tick_sched_timer+0x42/0x130 kernel/time/tick-sched.c:1170
>  __run_hrtimer kernel/time/hrtimer.c:1349 [inline]
>  __hrtimer_run_queues+0x3e3/0x10a0 kernel/time/hrtimer.c:1411
>  hrtimer_interrupt+0x2f3/0x750 kernel/time/hrtimer.c:1469
>  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
> RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783
> [inline]
> RIP: 0010:lock_is_held_type+0x18b/0x210 kernel/locking/lockdep.c:3960
> RSP: 0018:ffff8801db006400 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff12
> RAX: dffffc0000000000 RBX: 0000000000000282 RCX: 0000000000000000
> RDX: 1ffffffff1162e55 RSI: ffffffff88b90c60 RDI: 0000000000000282
> RBP: ffff8801db006420 R08: ffffed003b6046c3 R09: ffffed003b6046c2
> R10: ffffed003b6046c2 R11: ffff8801db023613 R12: ffff8801b2f623c0
> R13: 0000000000000000 R14: ffff88009932bb00 R15: 00000000ffffffff
>  lock_is_held include/linux/lockdep.h:344 [inline]
>  rcu_read_lock_sched_held+0x108/0x120 kernel/rcu/update.c:117
>  trace_kmalloc_node include/trace/events/kmem.h:100 [inline]
>  kmem_cache_alloc_node_trace+0x34e/0x770 mm/slab.c:3652
>  __do_kmalloc_node mm/slab.c:3669 [inline]
>  __kmalloc_node_track_caller+0x33/0x70 mm/slab.c:3684
>  __kmalloc_reserve.isra.38+0x3a/0xe0 net/core/skbuff.c:137
>  __alloc_skb+0x14d/0x780 net/core/skbuff.c:205
>  alloc_skb include/linux/skbuff.h:987 [inline]
>  sctp_packet_transmit+0x45e/0x3ba0 net/sctp/output.c:585
>  sctp_outq_flush+0x1373/0x4370 net/sctp/outqueue.c:1197
>  sctp_outq_uncork+0x6a/0x80 net/sctp/outqueue.c:776
>  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1820 [inline]
>  sctp_side_effects net/sctp/sm_sideeffect.c:1220 [inline]
>  sctp_do_sm+0x596/0x7160 net/sctp/sm_sideeffect.c:1191
>  sctp_generate_heartbeat_event+0x218/0x450 net/sctp/sm_sideeffect.c:406

#syz fix: sctp: not allow to set rto_min with a value below 200 msecs


>  call_timer_fn+0x230/0x940 kernel/time/timer.c:1326
>  expire_timers kernel/time/timer.c:1363 [inline]
>  __run_timers+0x79e/0xc50 kernel/time/timer.c:1666
>  run_timer_softirq+0x4c/0x70 kernel/time/timer.c:1692
>  __do_softirq+0x2e0/0xaf5 kernel/softirq.c:285
>  invoke_softirq kernel/softirq.c:365 [inline]
>  irq_exit+0x1d1/0x200 kernel/softirq.c:405
>  exiting_irq arch/x86/include/asm/apic.h:525 [inline]
>  smp_apic_timer_interrupt+0x17e/0x710 arch/x86/kernel/apic/apic.c:1052
>  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:862
>  </IRQ>
> RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783
> [inline]
> RIP: 0010:console_unlock+0xcdf/0x1100 kernel/printk/printk.c:2403
> RSP: 0018:ffff8801946eec00 EFLAGS: 00000212 ORIG_RAX: ffffffffffffff12
> RAX: 0000000000040000 RBX: 0000000000000200 RCX: ffffc90002ee8000
> RDX: 0000000000004461 RSI: ffffffff815f3446 RDI: 0000000000000212
> RBP: ffff8801946eed68 R08: ffff8801b2f62c38 R09: 0000000000000006
> R10: ffff8801b2f623c0 R11: 0000000000000000 R12: 0000000000000000
> R13: ffffffff84b84430 R14: 0000000000000001 R15: dffffc0000000000
>  vprintk_emit+0x6ad/0xdd0 kernel/printk/printk.c:1907
>  vprintk_default+0x28/0x30 kernel/printk/printk.c:1947
>  vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:379
>  printk+0x9e/0xba kernel/printk/printk.c:1980
>  sctp_getsockopt_maxburst net/sctp/socket.c:6265 [inline]
>  sctp_getsockopt.cold.34+0x11d/0x14c net/sctp/socket.c:7240
>  sock_common_getsockopt+0x9a/0xe0 net/core/sock.c:2998
>  __sys_getsockopt+0x1a5/0x370 net/socket.c:1940
>  SYSC_getsockopt net/socket.c:1951 [inline]
>  SyS_getsockopt+0x34/0x50 net/socket.c:1948
>  do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
> RIP: 0033:0x455279
> RSP: 002b:00007f5c0c0f2c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
> RAX: ffffffffffffffda RBX: 00007f5c0c0f36d4 RCX: 0000000000455279
> RDX: 0000000000000014 RSI: 0000000000000084 RDI: 0000000000000014
> RBP: 000000000072bea0 R08: 0000000020000240 R09: 0000000000000000
> R10: 0000000020000140 R11: 0000000000000246 R12: 00000000ffffffff
> R13: 000000000000012b R14: 00000000006f4ca8 R15: 0000000000000000
> INFO: rcu_sched detected stalls on CPUs/tasks:
>         0-....: (120712 ticks this GP) idle=ac6/1/4611686018427387908
> softirq=31693/31693 fqs=31173
>         (detected by 1, t=125002 jiffies, g=17039, c=17038, q=303419)
> Sending NMI from CPU 1 to CPUs 0:
> NMI backtrace for cpu 0
> CPU: 0 PID: 10218 Comm: syz-executor3 Not tainted 4.16.0+ #1
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: 0010:__lock_acquire+0xa78/0x5130 kernel/locking/lockdep.c:3434
> RSP: 0018:ffff8801db005b40 EFLAGS: 00000046
> RAX: dffffc0000000000 RBX: 00000000858813a6 RCX: 0000000000000001
> RDX: 1ffff100365ec585 RSI: ffff8801b2f62c38 RDI: ffff8801b2f62cf9
> RBP: ffff8801db005ed0 R08: 0000000000000008 R09: 0000000000000004
> R10: ffff8801b2f62cd8 R11: ffff8801b2f623c0 R12: 00000000be6c7baf
> R13: 0000000000000000 R14: 2ca977e0cccfd81f R15: 0000000000000000
> FS:  00007f5c0c0f3700(0000) GS:ffff8801db000000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffe57df8fa8 CR3: 00000001d7124000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <IRQ>
>  lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
>  rcu_lock_acquire include/linux/rcupdate.h:246 [inline]
>  rcu_read_lock include/linux/rcupdate.h:632 [inline]
>  is_bpf_text_address+0x3b/0x170 kernel/bpf/core.c:478
>  kernel_text_address+0x79/0xf0 kernel/extable.c:152
>  __kernel_text_address+0xd/0x40 kernel/extable.c:107
>  unwind_get_return_address+0x61/0xa0 arch/x86/kernel/unwind_frame.c:18
>  __save_stack_trace+0x7e/0xd0 arch/x86/kernel/stacktrace.c:45
>  save_stack_trace+0x1a/0x20 arch/x86/kernel/stacktrace.c:60
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>  set_track mm/kasan/kasan.c:459 [inline]
>  __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:520
>  kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:527
>  __cache_free mm/slab.c:3486 [inline]
>  kmem_cache_free+0x86/0x2d0 mm/slab.c:3744
>  kfree_skbmem+0x13c/0x210 net/core/skbuff.c:582
>  __kfree_skb net/core/skbuff.c:642 [inline]
>  consume_skb+0x193/0x550 net/core/skbuff.c:701
>  sctp_chunk_destroy net/sctp/sm_make_chunk.c:1477 [inline]
>  sctp_chunk_put+0x2c0/0x440 net/sctp/sm_make_chunk.c:1504
>  sctp_chunk_free+0x53/0x60 net/sctp/sm_make_chunk.c:1491
>  sctp_packet_pack net/sctp/output.c:489 [inline]
>  sctp_packet_transmit+0x142e/0x3ba0 net/sctp/output.c:610
>  sctp_outq_flush+0x1373/0x4370 net/sctp/outqueue.c:1197
>  sctp_outq_uncork+0x6a/0x80 net/sctp/outqueue.c:776
>  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1820 [inline]
>  sctp_side_effects net/sctp/sm_sideeffect.c:1220 [inline]
>  sctp_do_sm+0x596/0x7160 net/sctp/sm_sideeffect.c:1191
>  sctp_generate_heartbeat_event+0x218/0x450 net/sctp/sm_sideeffect.c:406
>  call_timer_fn+0x230/0x940 kernel/time/timer.c:1326
>  expire_timers kernel/time/timer.c:1363 [inline]
>  __run_timers+0x79e/0xc50 kernel/time/timer.c:1666
>  run_timer_softirq+0x4c/0x70 kernel/time/timer.c:1692
>  __do_softirq+0x2e0/0xaf5 kernel/softirq.c:285
>  invoke_softirq kernel/softirq.c:365 [inline]
>  irq_exit+0x1d1/0x200 kernel/softirq.c:405
>  exiting_irq arch/x86/include/asm/apic.h:525 [inline]
>  smp_apic_timer_interrupt+0x17e/0x710 arch/x86/kernel/apic/apic.c:1052
>  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:862
>  </IRQ>
> RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783
> [inline]
> RIP: 0010:console_unlock+0xcdf/0x1100 kernel/printk/printk.c:2403
> RSP: 0018:ffff8801946eec00 EFLAGS: 00000212 ORIG_RAX: ffffffffffffff12
> RAX: 0000000000040000 RBX: 0000000000000200 RCX: ffffc90002ee8000
> RDX: 0000000000004461 RSI: ffffffff815f3446 RDI: 0000000000000212
> RBP: ffff8801946eed68 R08: ffff8801b2f62c38 R09: 0000000000000006
> R10: ffff8801b2f623c0 R11: 0000000000000000 R12: 0000000000000000
> R13: ffffffff84b84430 R14: 0000000000000001 R15: dffffc0000000000
>  vprintk_emit+0x6ad/0xdd0 kernel/printk/printk.c:1907
>  vprintk_default+0x28/0x30 kernel/printk/printk.c:1947
>  vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:379
>  printk+0x9e/0xba kernel/printk/printk.c:1980
>  sctp_getsockopt_maxburst net/sctp/socket.c:6265 [inline]
>  sctp_getsockopt.cold.34+0x11d/0x14c net/sctp/socket.c:7240
>  sock_common_getsockopt+0x9a/0xe0 net/core/sock.c:2998
>  __sys_getsockopt+0x1a5/0x370 net/socket.c:1940
>  SYSC_getsockopt net/socket.c:1951 [inline]
>  SyS_getsockopt+0x34/0x50 net/socket.c:1948
>  do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
> RIP: 0033:0x455279
> RSP: 002b:00007f5c0c0f2c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
> RAX: ffffffffffffffda RBX: 00007f5c0c0f36d4 RCX: 0000000000455279
> RDX: 0000000000000014 RSI: 0000000000000084 RDI: 0000000000000014
> RBP: 000000000072bea0 R08: 0000000020000240 R09: 0000000000000000
> R10: 0000000020000140 R11: 0000000000000246 R12: 00000000ffffffff
> R13: 000000000000012b R14: 00000000006f4ca8 R15: 0000000000000000
> Code: 0f 85 dc 32 00 00 8b 0d b7 9e 8b 07 85 c9 0f 84 62 f7 ff ff 48 b8 00
> 00 00 00 00 fc ff df 48 8b 54 24 78 48 c1 ea 03 80 3c 02 00 <0f> 85 0b 31 00
> 00 48 8b 94 24 88 00 00 00 4d 89 b3 68 08 00 00
> INFO: NMI handler (nmi_cpu_backtrace_handler) took too long to run: 3.007
> msecs
> INFO: task kworker/u4:4:688 blocked for more than 120 seconds.
>       Not tainted 4.16.0+ #1
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> kworker/u4:4    D19560   688      2 0x80000000
> Workqueue: events_unbound fsnotify_mark_destroy_workfn
> 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+0x1b5/0x240 kernel/time/timer.c:1777
>  do_wait_for_common kernel/sched/completion.c:83 [inline]
>  __wait_for_common kernel/sched/completion.c:104 [inline]
>  wait_for_common kernel/sched/completion.c:115 [inline]
>  wait_for_completion+0x3e7/0x870 kernel/sched/completion.c:136
>  __synchronize_srcu+0x189/0x240 kernel/rcu/srcutree.c:924
>  synchronize_srcu+0x408/0x54f kernel/rcu/srcutree.c:1002
>  fsnotify_mark_destroy_workfn+0x1aa/0x530 fs/notify/mark.c:759
>  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
>
> Showing all locks held in the system:
> 2 locks held by kworker/u4:4/688:
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> __write_once_size include/linux/compiler.h:215 [inline]
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> atomic64_set include/asm-generic/atomic-instrumented.h:40 [inline]
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> atomic_long_set include/asm-generic/atomic-long.h:57 [inline]
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> set_work_data kernel/workqueue.c:617 [inline]
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
>  #0: 000000007b6e92d0 ((wq_completion)"events_unbound"){+.+.}, at:
> process_one_work+0xaef/0x1b50 kernel/workqueue.c:2116
>  #1: 000000004c7e11cf ((reaper_work).work){+.+.}, at:
> process_one_work+0xb46/0x1b50 kernel/workqueue.c:2120
> 2 locks held by khungtaskd/878:
>  #0: 000000001cc267e2 (rcu_read_lock){....}, at:
> check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
>  #0: 000000001cc267e2 (rcu_read_lock){....}, at: watchdog+0x1ff/0xf60
> kernel/hung_task.c:249
>  #1: 000000002f71223f (tasklist_lock){.+.+}, at:
> debug_show_all_locks+0xde/0x34a kernel/locking/lockdep.c:4470
> 2 locks held by kworker/1:2/1980:
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at: __write_once_size
> include/linux/compiler.h:215 [inline]
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at: arch_atomic64_set
> arch/x86/include/asm/atomic64_64.h:34 [inline]
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at: atomic64_set
> include/asm-generic/atomic-instrumented.h:40 [inline]
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at: atomic_long_set
> include/asm-generic/atomic-long.h:57 [inline]
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at: set_work_data
> kernel/workqueue.c:617 [inline]
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at:
> set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
>  #0: 00000000dc6681fd ((wq_completion)"events"){+.+.}, at:
> process_one_work+0xaef/0x1b50 kernel/workqueue.c:2116
>  #1: 000000008e69c2e7 (xfrm_state_gc_work){+.+.}, at:
> process_one_work+0xb46/0x1b50 kernel/workqueue.c:2120
> 1 lock held by rsyslogd/4365:
>  #0: 00000000ef321630 (&f->f_pos_lock){+.+.}, at: __fdget_pos+0x1a9/0x1e0
> fs/file.c:766
> 2 locks held by getty/4456:
>  #0: 0000000044e43f49 (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 0000000093b079e0 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
> 2 locks held by getty/4457:
>  #0: 000000005b25b55f (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 0000000099955ee5 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
> 2 locks held by getty/4458:
>  #0: 0000000050f5738d (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 00000000cccc0402 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
> 2 locks held by getty/4459:
>  #0: 00000000ddecfb5c (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 000000003c071f3a (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
> 2 locks held by getty/4460:
>  #0: 000000003bec706e (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 00000000dd28453c (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
> 2 locks held by getty/4461:
>  #0: 00000000313fc55a (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 00000000e9766156 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
> 2 locks held by getty/4462:
>  #0: 000000003212bb13 (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> drivers/tty/tty_ldsem.c:365
>  #1: 0000000065fa2f73 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
>
> =============================================
>
> NMI backtrace for cpu 1
> CPU: 1 PID: 878 Comm: khungtaskd Not tainted 4.16.0+ #1
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x1b9/0x29f lib/dump_stack.c:53
>  nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
>  nmi_trigger_cpumask_backtrace+0x151/0x192 lib/nmi_backtrace.c:62
>  arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
>  trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
>  check_hung_task kernel/hung_task.c:132 [inline]
>  check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
>  watchdog+0xc10/0xf60 kernel/hung_task.c:249
>  kthread+0x345/0x410 kernel/kthread.c:238
>  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:411
> Sending NMI from CPU 1 to CPUs 0:
> NMI backtrace for cpu 0
> CPU: 0 PID: 10218 Comm: syz-executor3 Not tainted 4.16.0+ #1
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: 0010:debug_lockdep_rcu_enabled.part.1+0xb/0x60 kernel/rcu/update.c:298
> RSP: 0018:ffff8801db0062e8 EFLAGS: 00000202
> RAX: dffffc0000000000 RBX: 1ffff1003b600c61 RCX: ffffffff86583d15
> RDX: 0000000000000004 RSI: ffffffff86583d65 RDI: ffffffff88e6cc40
> RBP: ffff8801db0062f8 R08: ffff8801b2f623c0 R09: ffffed003b6046c2
> R10: ffffed003b6046c2 R11: ffff8801db023613 R12: ffff8801c3d740c0
> R13: 0000000000000001 R14: 0000000000010000 R15: ffff88019cc0a900
> FS:  00007f5c0c0f3700(0000) GS:ffff8801db000000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffe57df8fa8 CR3: 00000001d7124000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <IRQ>
>  rcu_read_unlock include/linux/rcupdate.h:684 [inline]
>  ip6_mtu+0x36a/0x590 net/ipv6/route.c:2420
>  dst_mtu include/net/dst.h:210 [inline]
>  ip6_xmit+0xb42/0x23f0 net/ipv6/ip6_output.c:262
>  sctp_v6_xmit+0x4a5/0x6b0 net/sctp/ipv6.c:225
>  sctp_packet_transmit+0x26f6/0x3ba0 net/sctp/output.c:642
>  sctp_outq_flush+0x1373/0x4370 net/sctp/outqueue.c:1197
>  sctp_outq_uncork+0x6a/0x80 net/sctp/outqueue.c:776
>  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1820 [inline]
>  sctp_side_effects net/sctp/sm_sideeffect.c:1220 [inline]
>  sctp_do_sm+0x596/0x7160 net/sctp/sm_sideeffect.c:1191
>  sctp_generate_heartbeat_event+0x218/0x450 net/sctp/sm_sideeffect.c:406
>  call_timer_fn+0x230/0x940 kernel/time/timer.c:1326
>  expire_timers kernel/time/timer.c:1363 [inline]
>  __run_timers+0x79e/0xc50 kernel/time/timer.c:1666
>  run_timer_softirq+0x4c/0x70 kernel/time/timer.c:1692
>  __do_softirq+0x2e0/0xaf5 kernel/softirq.c:285
>  invoke_softirq kernel/softirq.c:365 [inline]
>  irq_exit+0x1d1/0x200 kernel/softirq.c:405
>  exiting_irq arch/x86/include/asm/apic.h:525 [inline]
>  smp_apic_timer_interrupt+0x17e/0x710 arch/x86/kernel/apic/apic.c:1052
>  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:862
>  </IRQ>
> RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783
> [inline]
> RIP: 0010:console_unlock+0xcdf/0x1100 kernel/printk/printk.c:2403
> RSP: 0018:ffff8801946eec00 EFLAGS: 00000212 ORIG_RAX: ffffffffffffff12
> RAX: 0000000000040000 RBX: 0000000000000200 RCX: ffffc90002ee8000
> RDX: 0000000000004461 RSI: ffffffff815f3446 RDI: 0000000000000212
> RBP: ffff8801946eed68 R08: ffff8801b2f62c38 R09: 0000000000000006
> R10: ffff8801b2f623c0 R11: 0000000000000000 R12: 0000000000000000
> R13: ffffffff84b84430 R14: 0000000000000001 R15: dffffc0000000000
>  vprintk_emit+0x6ad/0xdd0 kernel/printk/printk.c:1907
>  vprintk_default+0x28/0x30 kernel/printk/printk.c:1947
>  vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:379
>  printk+0x9e/0xba kernel/printk/printk.c:1980
>  sctp_getsockopt_maxburst net/sctp/socket.c:6265 [inline]
>  sctp_getsockopt.cold.34+0x11d/0x14c net/sctp/socket.c:7240
>  sock_common_getsockopt+0x9a/0xe0 net/core/sock.c:2998
>  __sys_getsockopt+0x1a5/0x370 net/socket.c:1940
>  SYSC_getsockopt net/socket.c:1951 [inline]
>  SyS_getsockopt+0x34/0x50 net/socket.c:1948
>  do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
> RIP: 0033:0x455279
> RSP: 002b:00007f5c0c0f2c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
> RAX: ffffffffffffffda RBX: 00007f5c0c0f36d4 RCX: 0000000000455279
> RDX: 0000000000000014 RSI: 0000000000000084 RDI: 0000000000000014
> RBP: 000000000072bea0 R08: 0000000020000240 R09: 0000000000000000
> R10: 0000000020000140 R11: 0000000000000246 R12: 00000000ffffffff
> R13: 000000000000012b R14: 00000000006f4ca8 R15: 0000000000000000
> Code: e9 c3 fd ff ff 48 8b 7d c8 e8 52 6c 50 00 e9 9c fd ff ff 0f 1f 00 66
> 2e 0f 1f 84 00 00 00 00 00 48 b8 00 00 00 00 00 fc ff df 55 <48> 89 e5 53 65
> 48 8b 1c 25 c0 ed 01 00 48 8d bb 74 08 00 00 48
>
>
> ---
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