Comment 91 for bug 1403152

seeing this on xenial after this sequence of operations:

lxc stop run2
lxc delete run2
lxc copy run1 run2
lxc start run2 <- hangs here, with later dmesg

[337766.146479] unregister_netdevice: waiting for lo to become free. Usage count = 1
[337772.435786] INFO: task lxd:20665 blocked for more than 120 seconds.
[337772.435856] Tainted: P OE 4.4.0-47-generic #68-Ubuntu
[337772.435922] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[337772.436002] lxd D ffff88006b6cbcb8 0 20665 1 0x00000004
[337772.436006] ffff88006b6cbcb8 ffffffff821d0560 ffff880235aa8000 ffff8801b57d2580
[337772.436009] ffff88006b6cc000 ffffffff81ef5f24 ffff8801b57d2580 00000000ffffffff
[337772.436010] ffffffff81ef5f28 ffff88006b6cbcd0 ffffffff81830f15 ffffffff81ef5f20
[337772.436012] Call Trace:
[337772.436020] [<ffffffff81830f15>] schedule+0x35/0x80
[337772.436022] [<ffffffff818311be>] schedule_preempt_disabled+0xe/0x10
[337772.436024] [<ffffffff81832df9>] __mutex_lock_slowpath+0xb9/0x130
[337772.436026] [<ffffffff81832e8f>] mutex_lock+0x1f/0x30
[337772.436029] [<ffffffff8171f98e>] copy_net_ns+0x6e/0x120
[337772.436033] [<ffffffff810a148b>] create_new_namespaces+0x11b/0x1d0
[337772.436035] [<ffffffff810a15ad>] copy_namespaces+0x6d/0xa0
[337772.436038] [<ffffffff8107f135>] copy_process+0x905/0x1b70
[337772.436040] [<ffffffff81080530>] _do_fork+0x80/0x360
[337772.436044] [<ffffffff8120bf28>] ? SyS_access+0x1e8/0x230
[337772.436046] [<ffffffff810808b9>] SyS_clone+0x19/0x20
[337772.436048] [<ffffffff81834ff2>] entry_SYSCALL_64_fastpath+0x16/0x71

fwiw, at the time of stop, run2 had an additional IP address on dev lo, and at the time of copy, run1 also was running and had an additional ip address on dev lo