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

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https://bugs.launchpad.net/bugs/1403152

Title:
  unregister_netdevice: waiting for lo to become free. Usage count

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