bisecting fixing commit since 3d40d7117e353b84627c1e8c5ed9ae0b1237ef5c
building syzkaller on 438e1227121e9d4bbca7b12b6e1b715524d512c2
testing commit 3d40d7117e353b84627c1e8c5ed9ae0b1237ef5c with gcc (GCC) 8.1.0
kernel signature: 9e1b2ffbe668a2cd8ce9da88f41860a668dbc3dbbb38440cbe9918f4c95fb882
all runs: crashed: possible deadlock in rtnl_lock
testing current HEAD dce0f88600e49746b4bda873965b671a23ff4313
testing commit dce0f88600e49746b4bda873965b671a23ff4313 with gcc (GCC) 8.1.0
kernel signature: b50db0d0cde1f687781d6184507e1e93c46c348fac4ae265cf71c21645eefd77
all runs: crashed: possible deadlock in rtnl_lock
revisions tested: 2, total time: 24m54.7822442s (build: 17m44.463225084s, test: 6m7.709468953s)
the crash still happens on HEAD
commit msg: Linux 4.19.132
crash: possible deadlock in rtnl_lock
netlink: 20 bytes leftover after parsing attributes in process `syz-executor.2'.
netlink: 20 bytes leftover after parsing attributes in process `syz-executor.1'.
netlink: 20 bytes leftover after parsing attributes in process `syz-executor.3'.
============================================
WARNING: possible recursive locking detected
4.19.132-syzkaller #0 Not tainted
--------------------------------------------
syz-executor.2/7652 is trying to acquire lock:
000000009e76cc38 (rtnl_mutex){+.+.}, at: rtnl_lock+0x12/0x20 net/core/rtnetlink.c:77

but task is already holding lock:
000000009e76cc38 (rtnl_mutex){+.+.}, at: rtnl_lock net/core/rtnetlink.c:77 [inline]
000000009e76cc38 (rtnl_mutex){+.+.}, at: rtnetlink_rcv_msg+0x301/0x8f0 net/core/rtnetlink.c:4774

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

1 lock held by syz-executor.2/7652:
 #0: 000000009e76cc38 (rtnl_mutex){+.+.}, at: rtnl_lock net/core/rtnetlink.c:77 [inline]
 #0: 000000009e76cc38 (rtnl_mutex){+.+.}, at: rtnetlink_rcv_msg+0x301/0x8f0 net/core/rtnetlink.c:4774

stack backtrace:
CPU: 1 PID: 7652 Comm: syz-executor.2 Not tainted 4.19.132-syzkaller #0
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+0x123/0x177 lib/dump_stack.c:118
 print_deadlock_bug kernel/locking/lockdep.c:1763 [inline]
 check_deadlock kernel/locking/lockdep.c:1807 [inline]
 validate_chain kernel/locking/lockdep.c:2403 [inline]
 __lock_acquire.cold.62+0x259/0x57a kernel/locking/lockdep.c:3415
 lock_acquire+0x173/0x3d0 kernel/locking/lockdep.c:3907
 __mutex_lock_common kernel/locking/mutex.c:925 [inline]
 __mutex_lock+0xf5/0x1210 kernel/locking/mutex.c:1072
 mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
 rtnl_lock+0x12/0x20 net/core/rtnetlink.c:77
 hsr_dev_destroy+0x1a/0xb0 net/hsr/hsr_device.c:373
 register_netdevice+0xa43/0xfb0 net/core/dev.c:8777
 hsr_dev_finalize+0x4d2/0x720 net/hsr/hsr_device.c:491
 hsr_newlink+0x22f/0x3c0 net/hsr/hsr_netlink.c:78
 rtnl_newlink+0xc7d/0x1320 net/core/rtnetlink.c:3141
 rtnetlink_rcv_msg+0x34f/0x8f0 net/core/rtnetlink.c:4777
 netlink_rcv_skb+0x13e/0x3d0 net/netlink/af_netlink.c:2455
 rtnetlink_rcv+0x10/0x20 net/core/rtnetlink.c:4795
 netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
 netlink_unicast+0x443/0x650 net/netlink/af_netlink.c:1344
 netlink_sendmsg+0x765/0xc40 net/netlink/af_netlink.c:1909
 sock_sendmsg_nosec net/socket.c:622 [inline]
 sock_sendmsg+0xb5/0xf0 net/socket.c:632
 ___sys_sendmsg+0x647/0x950 net/socket.c:2115
 __sys_sendmsg+0xd9/0x180 net/socket.c:2153
 __do_sys_sendmsg net/socket.c:2162 [inline]
 __se_sys_sendmsg net/socket.c:2160 [inline]
 __x64_sys_sendmsg+0x73/0xb0 net/socket.c:2160
 do_syscall_64+0xd0/0x4e0 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45afc9
Code: 2d b6 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 fb b5 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f25a0ab3c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045afc9
RDX: 0000000000000000 RSI: 0000000020000080 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f25a0ab46d4
R13: 00000000004ca891 R14: 00000000004e3a38 R15: 00000000ffffffff