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

[Peter Zijlstra]

On Mon, Sep 14, 2015 at 02:01:53PM +0200, Peter Zijlstra wrote:
> The scenario is:
> 
> 	CPU0			CPU1
> 
> 				unlock(x)
> 				  smp_store_release(&x->lock, 0);
> 
> 	unlock(y)
> 	  smp_store_release(&next->lock, 1); /* next == &y */
> 
> 				lock(y)
> 				  while (!(smp_load_acquire(&y->lock))
> 					cpu_relax();
> 
> 
> Where the lock does _NOT_ issue a store to acquire the lock at all. Now
> I don't think any of our current primitives manage this, so we should be
> good, but it might just be possible.

So with a bit more through this seems fundamentally impossible, you
always needs some stores in a lock() implementation, the above for
instance needs to queue itself, otherwise CPU0 will not be able to find
it etc..