[RFC][PATCH 0/3] cgroups: support for module-loadable subsystems

[Ben Blum]

This patch series implements simple support for building and loading
subsystems as modules, both within and outside the kernel source tree.
Module unloading is as yet unimplemented - it will require more advanced
reference counting in the mount/unmount code, and I plan to work on this
soon.

Patch #1 sets up the subsys[] array so its contents can be dynamic as
modules appear and (eventually) disappear. I introduce an rwsem called
subsys_mutex to protect against concurrent loads/unloads/reads, and
modify iterations over the array to handle when subsystems are absent.

Patch #2 implements an interface for modules to load subsystems, called
cgroup_load_subsys, similar to cgroup_init_subsys, and adds a module
pointer in struct cgroup_subsys.

Patch #3 modifies the net_cls subsystem, which already had some module
declarations, to be configurable as a module, which also serves as a
simple proof-of-concept.

-- bblum