Syzkaller reported this on a slightly older kernel but it's still
applicable to the current kernel -

======================================================
WARNING: possible circular locking dependency detected
4.18.0-next-20180823+ #46 Not tainted
------------------------------------------------------
syz-executor4/26841 is trying to acquire lock:
00000000dd41ef48 ((wq_completion)bond_dev->name){+.+.}, at: flush_workqueue+0x2db/0x1e10 kernel/workqueue.c:2652

but task is already holding lock:
00000000768ab431 (rtnl_mutex){+.+.}, at: rtnl_lock net/core/rtnetlink.c:77 [inline]
00000000768ab431 (rtnl_mutex){+.+.}, at: rtnetlink_rcv_msg+0x412/0xc30 net/core/rtnetlink.c:4708

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (rtnl_mutex){+.+.}:
       __mutex_lock_common kernel/locking/mutex.c:925 [inline]
       __mutex_lock+0x171/0x1700 kernel/locking/mutex.c:1073
       mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1088
       rtnl_lock+0x17/0x20 net/core/rtnetlink.c:77
       bond_netdev_notify drivers/net/bonding/bond_main.c:1310 [inline]
       bond_netdev_notify_work+0x44/0xd0 drivers/net/bonding/bond_main.c:1320
       process_one_work+0xc73/0x1aa0 kernel/workqueue.c:2153
       worker_thread+0x189/0x13c0 kernel/workqueue.c:2296
       kthread+0x35a/0x420 kernel/kthread.c:246
       ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:415

-> #1 ((work_completion)(&(&nnw->work)->work)){+.+.}:
       process_one_work+0xc0b/0x1aa0 kernel/workqueue.c:2129
       worker_thread+0x189/0x13c0 kernel/workqueue.c:2296
       kthread+0x35a/0x420 kernel/kthread.c:246
       ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:415

-> #0 ((wq_completion)bond_dev->name){+.+.}:
       lock_acquire+0x1e4/0x4f0 kernel/locking/lockdep.c:3901
       flush_workqueue+0x30a/0x1e10 kernel/workqueue.c:2655
       drain_workqueue+0x2a9/0x640 kernel/workqueue.c:2820
       destroy_workqueue+0xc6/0x9d0 kernel/workqueue.c:4155
       __alloc_workqueue_key+0xef9/0x1190 kernel/workqueue.c:4138
       bond_init+0x269/0x940 drivers/net/bonding/bond_main.c:4734
       register_netdevice+0x337/0x1100 net/core/dev.c:8410
       bond_newlink+0x49/0xa0 drivers/net/bonding/bond_netlink.c:453
       rtnl_newlink+0xef4/0x1d50 net/core/rtnetlink.c:3099
       rtnetlink_rcv_msg+0x46e/0xc30 net/core/rtnetlink.c:4711
       netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2454
       rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4729
       netlink_unicast_kernel net/netlink/af_netlink.c:1317 [inline]
       netlink_unicast+0x5a0/0x760 net/netlink/af_netlink.c:1343
       netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1908
       sock_sendmsg_nosec net/socket.c:622 [inline]
       sock_sendmsg+0xd5/0x120 net/socket.c:632
       ___sys_sendmsg+0x7fd/0x930 net/socket.c:2115
       __sys_sendmsg+0x11d/0x290 net/socket.c:2153
       __do_sys_sendmsg net/socket.c:2162 [inline]
       __se_sys_sendmsg net/socket.c:2160 [inline]
       __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2160
       do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
  (wq_completion)bond_dev->name --> (work_completion)(&(&nnw->work)->work) --> rtnl_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(rtnl_mutex);
                               lock((work_completion)(&(&nnw->work)->work));
                               lock(rtnl_mutex);
  lock((wq_completion)bond_dev->name);

 *** DEADLOCK ***

1 lock held by syz-executor4/26841:

stack backtrace:
CPU: 1 PID: 26841 Comm: syz-executor4 Not tainted 4.18.0-next-20180823+ #46
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+0x1c9/0x2b4 lib/dump_stack.c:113
 print_circular_bug.isra.34.cold.55+0x1bd/0x27d kernel/locking/lockdep.c:1222
 check_prev_add kernel/locking/lockdep.c:1862 [inline]
 check_prevs_add kernel/locking/lockdep.c:1975 [inline]
 validate_chain kernel/locking/lockdep.c:2416 [inline]
 __lock_acquire+0x3449/0x5020 kernel/locking/lockdep.c:3412
 lock_acquire+0x1e4/0x4f0 kernel/locking/lockdep.c:3901
 flush_workqueue+0x30a/0x1e10 kernel/workqueue.c:2655
 drain_workqueue+0x2a9/0x640 kernel/workqueue.c:2820
 destroy_workqueue+0xc6/0x9d0 kernel/workqueue.c:4155
 __alloc_workqueue_key+0xef9/0x1190 kernel/workqueue.c:4138
 bond_init+0x269/0x940 drivers/net/bonding/bond_main.c:4734
 register_netdevice+0x337/0x1100 net/core/dev.c:8410
 bond_newlink+0x49/0xa0 drivers/net/bonding/bond_netlink.c:453
 rtnl_newlink+0xef4/0x1d50 net/core/rtnetlink.c:3099
 rtnetlink_rcv_msg+0x46e/0xc30 net/core/rtnetlink.c:4711
 netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2454
 rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4729
 netlink_unicast_kernel net/netlink/af_netlink.c:1317 [inline]
 netlink_unicast+0x5a0/0x760 net/netlink/af_netlink.c:1343
 netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1908
 sock_sendmsg_nosec net/socket.c:622 [inline]
 sock_sendmsg+0xd5/0x120 net/socket.c:632
 ___sys_sendmsg+0x7fd/0x930 net/socket.c:2115
 __sys_sendmsg+0x11d/0x290 net/socket.c:2153
 __do_sys_sendmsg net/socket.c:2162 [inline]
 __se_sys_sendmsg net/socket.c:2160 [inline]
 __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2160
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457089
Code: fd b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 cb b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f2df20a5c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f2df20a66d4 RCX: 0000000000457089
RDX: 0000000000000000 RSI: 0000000020000180 RDI: 0000000000000003
RBP: 0000000000930140 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000004d40b8 R14: 00000000004c8ad8 R15: 0000000000000001

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>