[PATCH] Document Linux's memory barriers [try #7]

The attached patch documents the Linux kernel's memory barriers.

I've updated it from the comments I've been given.

The per-arch notes sections are gone because it's clear that there are so many
exceptions, that it's not worth having them.

I've added a list of references to other documents.

I've tried to get rid of the concept of memory accesses appearing on the bus;
what matters is apparent behaviour with respect to other observers in the
system.

Interrupts barrier effects are now considered to be non-existent. They may be
there, but you may not rely on them.

I've added a couple of definition sections at the top of the document: one to
specify the minimum execution model that may be assumed, the other to specify
what this document refers to by the term "memory".

I've made greater mention of the use of mmiowb().

I've adjusted the way in which caches are described, and described the fun
that can be had with cache coherence maintenance being unordered and data
dependency not being necessarily implicit.

I've described (smp_)read_barrier_depends().

I've rearranged the order of the sections, so that memory barriers are
discussed in abstract first, and then described the memory barrier facilities
available on Linux, before going on to more real-world discussions and examples.

I've added information about the lack of memory barriering effects with atomic
ops and bitops.

I've added information about control dependencies.

I've added more diagrams to illustrate caching interactions between CPUs.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>