Subsystems

A subsystem is the top level in the hierarchy
Correspond to directories in /sys

Subsystems are really just ksets
...with a semaphore tossed in

struct subsystem {
        struct kset               kset;
        struct rw_semaphore     rwsem;
};

Every kset must belong to a subsystem
Child list operations need that semaphore
Reached through kset->subsys