INFO: task hung in synchronize_rcu

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: a36b5017 Merge 4.9.205 into android-4.9-q
git tree: android-4.9
console output: https://syzkaller.appspot.com/x/log.txt?x=136caabce00000
kernel config: https://syzkaller.appspot.com/x/.config?x=9637f720fe234f2d
dashboard link: https://syzkaller.appspot.com/bug?extid=56201aea72c90e338575
compiler: gcc (GCC) 9.0.0 20181231 (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+56201a...@syzkaller.appspotmail.com

INFO: task kworker/u4:11:20658 blocked for more than 140 seconds.
Not tainted 4.9.205-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:11 D26824 20658 2 0x80000000
Workqueue: netns cleanup_net
0000000000000083 ffff8801a04d4740 ffff8801866b8580 ffff8801db71ffc0
ffff880189d20000 ffff8801db71ffd8 ffff880183e8f760 ffffffff8280a6ae
ffff880183e8f6b0 ffffffff8120d305 0000000000000292 ffff8801db7208b0
Call Trace:
[<000000005058936b>] schedule+0x92/0x1c0 kernel/sched/core.c:3546
[<000000000bb85f4e>] exp_funnel_lock kernel/rcu/tree_exp.h:279 [inline]
[<000000000bb85f4e>] _synchronize_rcu_expedited+0x68a/0x850
kernel/rcu/tree_exp.h:569
[<00000000550b5df0>] synchronize_rcu_expedited kernel/rcu/tree_exp.h:687
[inline]
[<00000000550b5df0>] synchronize_rcu_expedited+0x22/0x30
kernel/rcu/tree_exp.h:681
[<000000005afa5e0c>] synchronize_net+0x2f/0x50 net/core/dev.c:7867
[<000000003329db58>] nf_unregister_net_hook+0x1f8/0x380
net/netfilter/core.c:161
[<00000000c27125ad>] nf_unregister_net_hooks+0x39/0x60
net/netfilter/core.c:193
[<000000008d69bdff>] ip6t_unregister_table+0x51/0x70
net/ipv6/netfilter/ip6_tables.c:1842
[<0000000012a39049>] ip6table_mangle_net_exit+0x52/0xa0
net/ipv6/netfilter/ip6table_mangle.c:112
[<000000002464a50b>] ops_exit_list.isra.0+0xb0/0x160
net/core/net_namespace.c:136
[<00000000a69b4895>] cleanup_net+0x3d6/0x8a0 net/core/net_namespace.c:474
[<0000000077171719>] process_one_work+0x88b/0x1600 kernel/workqueue.c:2114
[<00000000d0679d18>] worker_thread+0x5df/0x11d0 kernel/workqueue.c:2251
[<0000000045571e08>] kthread+0x278/0x310 kernel/kthread.c:211
[<000000009094c0be>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:375

Showing all locks held in the system:
2 locks held by khungtaskd/24:
#0: (rcu_read_lock){......}, at: [<000000005795c399>]
check_hung_uninterruptible_tasks kernel/hung_task.c:169 [inline]
#0: (rcu_read_lock){......}, at: [<000000005795c399>]
watchdog+0x14b/0xaf0 kernel/hung_task.c:263
#1: (tasklist_lock){.+.+..}, at: [<00000000f8204ab2>]
debug_show_all_locks+0x7f/0x21f kernel/locking/lockdep.c:4339
1 lock held by rsyslogd/1896:
#0: (&f->f_pos_lock){+.+.+.}, at: [<00000000313c55e9>]
__fdget_pos+0xa8/0xd0 fs/file.c:782
2 locks held by getty/2024:
#0: (&tty->ldisc_sem){++++++}, at: [<00000000a7b12d5e>]
ldsem_down_read+0x33/0x40 drivers/tty/tty_ldsem.c:376
#1: (&ldata->atomic_read_lock){+.+.+.}, at: [<00000000719d5a78>]
n_tty_read+0x1fe/0x1820 drivers/tty/n_tty.c:2156
3 locks held by kworker/u4:11/20658:
#0: ("%s""netns"){.+.+.+}, at: [<000000009e9c13b0>]
process_one_work+0x790/0x1600 kernel/workqueue.c:2107
#1: (net_cleanup_work){+.+.+.}, at: [<000000009091d0f8>]
process_one_work+0x7ce/0x1600 kernel/workqueue.c:2111
#2: (net_mutex){+.+.+.}, at: [<0000000016cc41e0>] cleanup_net+0x131/0x8a0
net/core/net_namespace.c:440
1 lock held by syz-executor.3/2677:
#0: (net_mutex){+.+.+.}, at: [<00000000207be3b3>] copy_net_ns+0x15b/0x340
net/core/net_namespace.c:407

=============================================

NMI backtrace for cpu 1
CPU: 1 PID: 24 Comm: khungtaskd Not tainted 4.9.205-syzkaller #0
ffff8801d98d7cc8 ffffffff81b55e6b 0000000000000001 0000000000000000
0000000000000001 ffffffff8109a001 dffffc0000000000 ffff8801d98d7d00
ffffffff81b6110c 0000000000000001 0000000000000000 0000000000000001
Call Trace:
[<000000003be5ad9d>] __dump_stack lib/dump_stack.c:15 [inline]
[<000000003be5ad9d>] dump_stack+0xcb/0x130 lib/dump_stack.c:56
[<00000000a8df3a68>] nmi_cpu_backtrace.cold+0x47/0x87
lib/nmi_backtrace.c:99
[<000000008e17dd44>] nmi_trigger_cpumask_backtrace+0x124/0x155
lib/nmi_backtrace.c:60
[<00000000855e9ae9>] arch_trigger_cpumask_backtrace+0x14/0x20
arch/x86/kernel/apic/hw_nmi.c:37
[<00000000a7c31567>] trigger_all_cpu_backtrace include/linux/nmi.h:58
[inline]
[<00000000a7c31567>] check_hung_task kernel/hung_task.c:126 [inline]
[<00000000a7c31567>] check_hung_uninterruptible_tasks
kernel/hung_task.c:183 [inline]
[<00000000a7c31567>] watchdog+0x670/0xaf0 kernel/hung_task.c:263
[<0000000045571e08>] kthread+0x278/0x310 kernel/kthread.c:211
[<000000009094c0be>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:375
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at pc 0xffffffff82819801


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.