[RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return_* variants

[Peter Zijlstra]

Sorry for being tardy, I had a wee spell of feeling horrible and then I
procrastinated longer than I should have.

On Fri, Sep 11, 2015 at 01:45:07PM +0100, Will Deacon wrote:

> Peter, any thoughts? I'm not au fait with the x86 memory model, but what
> Paul's saying is worrying.

Right, so Paul is right -- and I completely forgot (I used to know about
that).

So all the TSO archs (SPARC-TSO, x86 (!OOSTORE) and s390) can do
smp_load_acquire()/smp_store_release() with just barrier(), and while:

	smp_store_release(&x);
	smp_load_acquire(&x);

will provide full order by means of the address dependency,

	smp_store_release(&x);
	smp_load_acquire(&y);

will not. Because the one reorder TSO allows is exactly that one.

> Peter -- if the above reordering can happen on x86, then moving away
> from RCpc is going to be less popular than I hoped...

Sadly yes.. We could of course try and split LOCK from ACQUIRE again,
but I'm not sure that's going to help anything except confusion.