This atomic_cmpxchg, unlike a "regular" cmpxchg, has the advantage
that the memory altered should always be going through the atomic_
accessors, and thus should be implementable with spinlocks.

See for example, arch/sparc/lib/atomic32.c

At least, that's what I'm hoping for.

Ok, as long as the rule is that all accesses have to go
through accessor macros, it would work.  This is not true
for existing uses of cmpxchg() btw, userland accesses shared
locks with the kernel would using any kind of accessors we
can control.

This means that your atomic_cmpxchg() cannot be used for locking
objects shared with userland, as DRM wants, since the hashed spinlock
trick does not work in such a case.
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