random thoughts on optmizing network namespace exit.

[ebiederm@xmission.com (Eric W. Biederman)]

Fundamentally if we want things to get better we have to remove
unnecessary serialization.  It is entirely too easy to sleep when
cleaning up a networking subsystem and create long hold times on
net_mutex for no particular reasons.

What probably makes sense to do is to add the concept of a
non-serialized pernet_operation.  And then work through the networking
stack converting all of the pernet_operations.  That should allow
network namespace exits to overlap while they clean up, and it should
allow the net_mutex to be dropped at the same point we drop rtnl_lock
in cleanup_net.

It might be a touch tricky during the transition period to take
advantage of an early drop of net_mutex, but that is where I would
start.

Once net_mutex is no longer used to serialize initialization/cleanup
methods for a network namespace.  We can look at other bottlenecks.

Eric