WARNING: refcount bug in kfree

device gretap0 left promiscuous mode
bridge0: port 3(gretap0) entered disabled state
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 4711 at lib/refcount.c:190 refcount_sub_and_test_checked lib/refcount.c:190 [inline]
WARNING: CPU: 1 PID: 4711 at lib/refcount.c:190 refcount_sub_and_test_checked+0x1d0/0x200 lib/refcount.c:180
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 4711 Comm: kworker/u4:11 Not tainted 5.2.0-rc3+ #21
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: netns cleanup_net
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 panic+0x2cb/0x744 kernel/panic.c:219
 __warn.cold+0x20/0x4d kernel/panic.c:576
 report_bug+0x263/0x2b0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:986
RIP: 0010:refcount_sub_and_test_checked lib/refcount.c:190 [inline]
RIP: 0010:refcount_sub_and_test_checked+0x1d0/0x200 lib/refcount.c:180
Code: 1d 7d 9d 48 06 31 ff 89 de e8 1c 5c 3c fe 84 db 75 94 e8 d3 5a 3c fe 48 c7 c7 a0 a0 a4 87 c6 05 5d 9d 48 06 01 e8 ce 9c 0e fe <0f> 0b e9 75 ff ff ff e8 b4 5a 3c fe e9 6e ff ff ff 48 89 df e8 f7
RSP: 0018:ffff888088c578b0 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815ac936 RDI: ffffed101118af08
RBP: ffff888088c57948 R08: ffff888092658700 R09: ffffed1015d26019
R10: ffffed1015d26018 R11: ffff8880ae9300c7 R12: 00000000ffffffff
R13: 0000000000000001 R14: ffff888088c57920 R15: 0000000000000000
 refcount_dec_and_test_checked+0x1b/0x20 lib/refcount.c:220
 skb_unref include/linux/skbuff.h:1018 [inline]
 skb_unref include/linux/skbuff.h:1012 [inline]
 kfree_skb+0x2c2/0x390 net/core/skbuff.c:691
 __skb_queue_purge include/linux/skbuff.h:2719 [inline]
 gro_cells_destroy net/core/gro_cells.c:103 [inline]
 gro_cells_destroy+0x1c9/0x360 net/core/gro_cells.c:92
 ip_tunnel_dev_free+0x19/0x60 net/ipv4/ip_tunnel.c:1013
 netdev_run_todo+0x48b/0x7c0 net/core/dev.c:8964
 rtnl_unlock+0xe/0x10 net/core/rtnetlink.c:112
 ip_tunnel_delete_nets+0x423/0x5f0 net/ipv4/ip_tunnel.c:1127
 ipgre_exit_batch_net+0x23/0x30 net/ipv4/ip_gre.c:993
 ops_exit_list.isra.0+0xfc/0x150 net/core/net_namespace.c:157
 cleanup_net+0x3fb/0x960 net/core/net_namespace.c:553
 process_one_work+0x989/0x1790 kernel/workqueue.c:2269
 worker_thread+0x98/0xe40 kernel/workqueue.c:2415
 kthread+0x354/0x420 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..