help? usage of indirect per-cpu variables

From: Chris Friesen
Date: Tue Sep 27 2016 - 15:41:47 EST


I'm trying to wrap my head around indirect percpu variables, and I'm hoping someone can school me on how they work.

For example, in mm/slub.c we have "struct kmem_cache *s". s->cpu_slab is a per-cpu variable, so we access it with something like:

c = raw_cpu_ptr(s->cpu_slab);

and then a bit later on we do

object = c->freelist;

So far so good. Essentially the raw_cpu_ptr() macro applies a unique per-CPU offset to s->cpu_slab to generate "c" which is a real pointer so we can dereference it to get c->freelist.

What confuses me is how we can do something like this:

object, tid,
next_object, next_tid(tid))

If s->cpu_slab is a special pointer that needs to be accessed only via the per-CPU macros, then how is it valid to specify "s->cpu_slab->freelist"? Since s->cpuslab isn't actually a valid address, how can we dereference it?