Subsystems

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

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
...in 2.6.11 in prior kernels
Reached through kset->subsys