INFO: task hung in stop_sync_thread (2)

[syzbot]

Hello,

syzbot hit the following crash on net-next commit
5d22d47b9ed96eddb35821dc2cc4f629f45827f7 (Tue Mar 27 17:33:21 2018 +0000)
Merge branch 'sfc-filter-locking'
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=5fe074c01b2032ce9618

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=6119456711376896
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=4372867303600475372
compiler: gcc (GCC) 7.1.1 20170620

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+5fe074c01b2032ce9618@xxxxxxxxxxxxxxxxxxxxxxxxx
It will help syzbot understand when the bug is fixed. See footer for  
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If you forward the report, please keep this part and the footer.

IPVS: sync thread started: state = BACKUP, mcast_ifn = syz_tun, syncid = 4,  
id = 0
IPVS: stopping backup sync thread 25415 ...
INFO: task syz-executor7:25421 blocked for more than 120 seconds.
      Not tainted 4.16.0-rc6+ #284
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor7   D23688 25421   4408 0x00000004
Call Trace:
 context_switch kernel/sched/core.c:2862 [inline]
 __schedule+0x8fb/0x1ec0 kernel/sched/core.c:3440
 schedule+0xf5/0x430 kernel/sched/core.c:3499
 schedule_timeout+0x1a3/0x230 kernel/time/timer.c:1777
 do_wait_for_common kernel/sched/completion.c:86 [inline]
 __wait_for_common kernel/sched/completion.c:107 [inline]
 wait_for_common kernel/sched/completion.c:118 [inline]
 wait_for_completion+0x415/0x770 kernel/sched/completion.c:139
 kthread_stop+0x14a/0x7a0 kernel/kthread.c:530
 stop_sync_thread+0x3d9/0x740 net/netfilter/ipvs/ip_vs_sync.c:1996
 do_ip_vs_set_ctl+0x2b1/0x1cc0 net/netfilter/ipvs/ip_vs_ctl.c:2394
 nf_sockopt net/netfilter/nf_sockopt.c:106 [inline]
 nf_setsockopt+0x67/0xc0 net/netfilter/nf_sockopt.c:115
 ip_setsockopt+0x97/0xa0 net/ipv4/ip_sockglue.c:1253
 sctp_setsockopt+0x2ca/0x63e0 net/sctp/socket.c:4154
 sock_common_setsockopt+0x95/0xd0 net/core/sock.c:3039
 SYSC_setsockopt net/socket.c:1850 [inline]
 SyS_setsockopt+0x189/0x360 net/socket.c:1829
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x454889
RSP: 002b:00007fc927626c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 00007fc9276276d4 RCX: 0000000000454889
RDX: 000000000000048c RSI: 0000000000000000 RDI: 0000000000000017
RBP: 000000000072bf58 R08: 0000000000000018 R09: 0000000000000000
R10: 0000000020000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 000000000000051c R14: 00000000006f9b40 R15: 0000000000000001

Showing all locks held in the system:
2 locks held by khungtaskd/868:
 #0:  (rcu_read_lock){....}, at: [<00000000a1a8f002>]  
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
 #0:  (rcu_read_lock){....}, at: [<00000000a1a8f002>] watchdog+0x1c5/0xd60  
kernel/hung_task.c:249
 #1:  (tasklist_lock){.+.+}, at: [<0000000037c2f8f9>]  
debug_show_all_locks+0xd3/0x3d0 kernel/locking/lockdep.c:4470
1 lock held by rsyslogd/4247:
 #0:  (&f->f_pos_lock){+.+.}, at: [<000000000d8d6983>]  
__fdget_pos+0x12b/0x190 fs/file.c:765
2 locks held by getty/4338:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4339:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4340:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4341:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4342:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4343:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4344:
 #0:  (&tty->ldisc_sem){++++}, at: [<00000000bee98654>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
 #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c1d180aa>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
3 locks held by kworker/0:5/6494:
 #0:  ((wq_completion)"%s"("ipv6_addrconf")){+.+.}, at:  
[<00000000a062b18e>] work_static include/linux/workqueue.h:198 [inline]
 #0:  ((wq_completion)"%s"("ipv6_addrconf")){+.+.}, at:  
[<00000000a062b18e>] set_work_data kernel/workqueue.c:619 [inline]
 #0:  ((wq_completion)"%s"("ipv6_addrconf")){+.+.}, at:  
[<00000000a062b18e>] set_work_pool_and_clear_pending kernel/workqueue.c:646  
[inline]
 #0:  ((wq_completion)"%s"("ipv6_addrconf")){+.+.}, at:  
[<00000000a062b18e>] process_one_work+0xb12/0x1bb0 kernel/workqueue.c:2084
 #1:  ((addr_chk_work).work){+.+.}, at: [<00000000278427d5>]  
process_one_work+0xb89/0x1bb0 kernel/workqueue.c:2088
 #2:  (rtnl_mutex){+.+.}, at: [<00000000066e35ac>] rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74
1 lock held by syz-executor7/25421:
 #0:  (ipvs->sync_mutex){+.+.}, at: [<00000000d414a689>]  
do_ip_vs_set_ctl+0x277/0x1cc0 net/netfilter/ipvs/ip_vs_ctl.c:2393
2 locks held by syz-executor7/25427:
 #0:  (rtnl_mutex){+.+.}, at: [<00000000066e35ac>] rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74
 #1:  (ipvs->sync_mutex){+.+.}, at: [<00000000e6d48489>]  
do_ip_vs_set_ctl+0x10f8/0x1cc0 net/netfilter/ipvs/ip_vs_ctl.c:2388
1 lock held by syz-executor7/25435:
 #0:  (rtnl_mutex){+.+.}, at: [<00000000066e35ac>] rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74
1 lock held by ipvs-b:2:0/25415:
 #0:  (rtnl_mutex){+.+.}, at: [<00000000066e35ac>] rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74

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

NMI backtrace for cpu 1
CPU: 1 PID: 868 Comm: khungtaskd Not tainted 4.16.0-rc6+ #284
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+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_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+0x90c/0xd60 kernel/hung_task.c:249
 kthread+0x33c/0x400 kernel/kthread.c:238
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0x6/0x10  
arch/x86/include/asm/irqflags.h:54


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