file arch/x86...current.h, definition of per_cpu objects

As I understand, the macro current in x86 expands to a function returning the value in current_task, which is defind for each cpu (is it?), and the segment register %fs points to a descriptor with a base address of the offset per-cpu, for (this? all?) per-cpu objects.. So, basically we are reading the object current_task + the offset contained in the base of the percpu (27th in the GDT) segment.. Could somebody elaborate? thanks