WARNING: possible recursive locking detected

[syzbot <syzbot+3c43eecd7745a5ce1640@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on upstream commit
b284d4d5a6785f8cd07eda2646a95782373cd01e (Tue Apr 10 19:25:30 2018 +0000)
Merge tag 'ceph-for-4.17-rc1' of git://github.com/ceph/ceph-client
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=3c43eecd7745a5ce1640

So far this crash happened 3 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5103706542440448
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=5641659786199040
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5099510896263168
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-1223000601505858474
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

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Reported-by: syzbot+3c43eecd7745a5ce1640@syzkaller.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.

IPVS: sync thread started: state = BACKUP, mcast_ifn = lo, syncid = 0, id =  
0
IPVS: stopping backup sync thread 4546 ...

============================================
IPVS: stopping backup sync thread 4559 ...
WARNING: possible recursive locking detected
4.16.0+ #19 Not tainted
--------------------------------------------
syzkaller046099/4543 is trying to acquire lock:
000000008d06d497 (rtnl_mutex){+.+.}, at: rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74

but task is already holding lock:
IPVS: stopping backup sync thread 4557 ...
000000008d06d497 (rtnl_mutex){+.+.}, at: rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74

other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(rtnl_mutex);
   lock(rtnl_mutex);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

2 locks held by syzkaller046099/4543:
  #0: 000000008d06d497 (rtnl_mutex){+.+.}, at: rtnl_lock+0x17/0x20  
net/core/rtnetlink.c:74
  #1: 000000008326bc5c (ipvs->sync_mutex){+.+.}, at:  
do_ip_vs_set_ctl+0x562/0x1d30 net/netfilter/ipvs/ip_vs_ctl.c:2388

stack backtrace:
CPU: 1 PID: 4543 Comm: syzkaller046099 Not tainted 4.16.0+ #19
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x1b9/0x294 lib/dump_stack.c:113
  print_deadlock_bug kernel/locking/lockdep.c:1761 [inline]
  check_deadlock kernel/locking/lockdep.c:1805 [inline]
  validate_chain kernel/locking/lockdep.c:2401 [inline]
  __lock_acquire.cold.62+0x18c/0x55b kernel/locking/lockdep.c:3431
  lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
  __mutex_lock_common kernel/locking/mutex.c:756 [inline]
  __mutex_lock+0x16d/0x17f0 kernel/locking/mutex.c:893
  mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
  rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
  ip_mc_drop_socket+0x8f/0x270 net/ipv4/igmp.c:2643
  inet_release+0x4e/0x1f0 net/ipv4/af_inet.c:413
  sock_release+0x96/0x1b0 net/socket.c:594
  start_sync_thread+0xdc3/0x2d40 net/netfilter/ipvs/ip_vs_sync.c:1924
  do_ip_vs_set_ctl+0x59c/0x1d30 net/netfilter/ipvs/ip_vs_ctl.c:2389
  nf_sockopt net/netfilter/nf_sockopt.c:106 [inline]
  nf_setsockopt+0x7d/0xd0 net/netfilter/nf_sockopt.c:115
  ip_setsockopt+0xd8/0xf0 net/ipv4/ip_sockglue.c:1253
  udp_setsockopt+0x62/0xa0 net/ipv4/udp.c:2413
  ipv6_setsockopt+0x149/0x170 net/ipv6/ipv6_sockglue.c:917
  udpv6_setsockopt+0x62/0xa0 net/ipv6/udp.c:1424
  sock_common_setsockopt+0x9a/0xe0 net/core/sock.c:3039
  __sys_setsockopt+0x1bd/0x390 net/socket.c:1903
  SYSC_setsockopt net/socket.c:1914 [inline]
  SyS_setsockopt+0x34/0x50 net/socket.c:1911
  do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x447c19
RSP: 002b:00007fb627a93db8 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000000000700024 RCX: 0000000000447c19
RDX: 000000000000048b RSI: 0000000000000000 RDI: 0000000000000004
RBP: 0000000000700020 R08: 0000000000000018 R09: 0000000000000000
R10: 0000000020000100 R11: 0000000000000246 R12: 0000000000000000
R13: 000000000080fe4f R14: 00007fb627a949c0 R15: 0000000000002710


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