Comment 22 for bug 1711407

Since xenial we have this very annoying behavior (unregister_netdevice: waiting for lo to become free. Usage count = 1) as well, which in turn makes a container reboot taking several minutes instead of seconds e.g. on vivid (lxc-ls -f also hangs for that time). E.g.:

 [ +10.244888] unregister_netdevice: waiting for lo to become free. Usage count = 1
[Oct 2 17:16] INFO: task lxc-autostart:5321 blocked for more than 120 seconds.
[ +0.007147] Tainted: P OE 4.4.0-96-generic #119-Ubuntu
[ +0.006796] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ +0.007913] lxc-autostart D ffff883fb99fbcb8 0 5321 1 0x00000000
[ +0.000007] ffff883fb99fbcb8 ffffffffffffffff ffff883ff2a3aa00 ffff881fed2fe200
[ +0.000004] ffff883fb99fc000 ffffffff81ef79a4 ffff881fed2fe200 00000000ffffffff
[ +0.000004] ffffffff81ef79a8 ffff883fb99fbcd0 ffffffff8183f165 ffffffff81ef79a0
[ +0.000004] Call Trace:
[ +0.000014] [<ffffffff8183f165>] schedule+0x35/0x80
[ +0.000005] [<ffffffff8183f40e>] schedule_preempt_disabled+0xe/0x10
[ +0.000016] [<ffffffff81841049>] __mutex_lock_slowpath+0xb9/0x130
[ +0.000014] [<ffffffff818410df>] mutex_lock+0x1f/0x30
[ +0.000009] [<ffffffff8172da4e>] copy_net_ns+0x6e/0x120
[ +0.000009] [<ffffffff810a179b>] create_new_namespaces+0x11b/0x1d0
[ +0.000003] [<ffffffff810a18bd>] copy_namespaces+0x6d/0xa0
[ +0.000007] [<ffffffff8107f1d2>] copy_process+0x8e2/0x1b30
[ +0.000011] [<ffffffff810805b0>] _do_fork+0x80/0x360
[ +0.000015] [<ffffffff81003226>] ? exit_to_usermode_loop+0xa6/0xd0
[ +0.000004] [<ffffffff81080939>] SyS_clone+0x19/0x20
[ +0.000009] [<ffffffff81843272>] entry_SYSCALL_64_fastpath+0x16/0x71
[ +4.882232] unregister_netdevice: waiting for lo to become free. Usage count = 1
...