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Subject[RFC] [PATCH 0/5] cgroups: support for module-loadable subsystems
[This is a revision of ]

This patch series implements support for building, loading, and
unloading subsystems as modules, both within and outside the kernel
source tree. It provides an interface cgroup_load_subsys() and
cgroup_unload_subsys() which modular subsystems can use to register and
depart during runtime. The net_cls classifier subsystem serves as the
example for a subsystem which can be converted into a module using these
changes. Additionally, support for subsystem dependencies is implemented
with the new modular subsystem scenario in mind.

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.

Patch #4 adds a mechanism for unloading modular subsystems, which
includes a more advanced rework of the rudimentary reference counting
introduced in patch 2. It also modifies the exitcall of net_cls to use
this interface.

Patch #5 implements support for subsystems to specify dependencies on
other subsystems. It requires the subsystem to give a list of its
dependencies in the form of a NULL-terminated array of strings, each
being the name of a depended-upon subsystem, for reasons explained in
the patch itself.

Part of implementing patches 2 and 4 involved updating css pointers in
each css_set when the module appears or leaves. In doing this, it was
discovered that css_sets always remain linked to the dummy cgroup,
regardless of whether or not any subsystems are actually bound to it
(i.e., not mounted on an actual hierarchy). The subsystem loading and
unloading code therefore should keep in mind the special cases where the
added subsystem is the only one in the dummy cgroup (and therefore all
css_sets need to be linked back into it) and where the removed subsys
was the only one in the dummy cgroup (and therefore all css_sets should
be unlinked from it) - however, as all css_sets always stay attached to
the dummy cgroup anyway, these cases are ignored. Any fix that addresses
this issue should also make sure these cases are addressed in the
subsystem loading and unloading code.

-- bblum

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