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Subjecthelp? usage of indirect per-cpu variables

Hi,

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:

this_cpu_cmpxchg_double(s->cpu_slab->freelist,
s->cpu_slab->tid,
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?

Thanks,
Chris

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