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+3c43eecd7745a5ce1...@syzkaller.appspotmail.com
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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:


 *** 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
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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