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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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