[rfc] SLOB memory ordering issue

[Nick Piggin]

I think I see a possible memory ordering problem with SLOB:
In slab caches with constructors, the constructor is run
before returning the object to caller, with no memory barrier
afterwards.

Now there is nothing that indicates the _exact_ behaviour
required here. Is it at all reasonable to expect ->ctor() to
be visible to all CPUs and not just the allocating CPU?

SLAB and SLUB don't appear to have this problem. Of course,
they have per-CPU fastpath queues, so _can_ have effectively
exactly the same ordering issue if the object was brought
back into the "initialized" state before being freed, rather
than by ->ctor(). However in that case, it is at least
kind of visible to the caller.

Anyone care or think it is a problem? Should we just document
that ->ctor doesn't imply any barriers? Better ideas?

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