Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

[Will Deacon <will.deacon@arm.com>]

On Tue, Aug 01, 2017 at 11:01:21AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 31, 2017 at 10:43:45AM -0700, Paul E. McKenney wrote:
> 
> > Why wouldn't the following have ACQUIRE semantics?
> > 
> > 	atomic_inc(&var);
> > 	smp_mb__after_atomic();
> > 
> > Is the issue that there is no actual value returned or some such?
> 
> Yes, so that the inc is a load-store, and thus there is a load, we loose
> the value.
> 
> But I see your point I think. Irrespective of still having the value,
> the ordering is preserved and nothing should pass across that.
> 
> > So if I have something like this, the assertion really can trigger?
> > 
> > 	WRITE_ONCE(x, 1);		atomic_inc(&y);
> > 	r0 = xchg_release(&y, 5);	smp_mb__after_atomic();
> > 					r1 = READ_ONCE(x);
> > 
> > 
> > 	WARN_ON(r0 == 0 && r1 == 0);
> > 
> > I must confess that I am not seeing why we would want to allow this
> > outcome.
> 
> No you are indeed quite right. I just wasn't creative enough. Thanks for
> the inspiration.

Just to close this out, we agree that an smp_rmb() instead of
smp_mb__after_atomic() would *not* forbid this outcome, right?

Will