IPVS: sync thread started: state = BACKUP, mcast_ifn = sit0, syncid = 0, id = 0
IPVS: sync thread started: state = BACKUP, mcast_ifn = sit0, syncid = 0, id = 0
IPVS: sync thread started: state = BACKUP, mcast_ifn = sit0, syncid = 0, id = 0

============================================
WARNING: possible recursive locking detected
4.16.0+ #288 Not tainted
--------------------------------------------
syz-executor6/4638 is trying to acquire lock:
000000005d1b27dc (rtnl_mutex){+.+.}, at: rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74

but task is already holding lock:
000000005d1b27dc (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 syz-executor6/4638:
 #0: 000000005d1b27dc (rtnl_mutex){+.+.}, at: rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
 #1: 00000000368a05eb (ipvs->sync_mutex){+.+.}, at: do_ip_vs_set_ctl+0x10f8/0x1cc0 net/netfilter/ipvs/ip_vs_ctl.c:2388

stack backtrace:
CPU: 0 PID: 4638 Comm: syz-executor6 Not tainted 4.16.0+ #288
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+0x1a7/0x27d lib/dump_stack.c:53
 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+0xf03/0x3c30 kernel/locking/lockdep.c:3431
 lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
 __mutex_lock_common kernel/locking/mutex.c:756 [inline]
 __mutex_lock+0x16d/0x1810 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/0x240 net/ipv4/igmp.c:2643
 inet_release+0x4e/0x1c0 net/ipv4/af_inet.c:413
 sock_release+0x8d/0x1e0 net/socket.c:595
 start_sync_thread+0x2213/0x2b70 net/netfilter/ipvs/ip_vs_sync.c:1924
 do_ip_vs_set_ctl+0x1139/0x1cc0 net/netfilter/ipvs/ip_vs_ctl.c:2389
 compat_nf_sockopt net/netfilter/nf_sockopt.c:146 [inline]
 compat_nf_setsockopt+0xfa/0x130 net/netfilter/nf_sockopt.c:156
 compat_ip_setsockopt+0x8b/0xd0 net/ipv4/ip_sockglue.c:1287
 compat_udp_setsockopt+0x45/0x80 net/ipv4/udp.c:2416
 compat_sock_common_setsockopt+0xb2/0x140 net/core/sock.c:2986
 __compat_sys_setsockopt+0x183/0x710 net/compat.c:403
 C_SYSC_setsockopt net/compat.c:416 [inline]
 compat_SyS_setsockopt+0x34/0x50 net/compat.c:413
 do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
 do_fast_syscall_32+0x3ec/0xf9f arch/x86/entry/common.c:392
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7fcdc99
RSP: 002b:00000000f7fc90ac EFLAGS: 00000282 ORIG_RAX: 000000000000016e
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000000000000
RDX: 000000000000048b RSI: 0000000020000040 RDI: 0000000000000018
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
IPVS: stopping backup sync thread 4660 ...
IPVS: stopping backup sync thread 4662 ...
IPVS: stopping backup sync thread 4657 ...