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

[Peter Zijlstra]

On Tue, Aug 01, 2017 at 11:19:00AM +0100, Will Deacon wrote:
> 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?

So that really hurts my brain. Per the normal rules that smp_rmb() would
order the read of @x against the last ll of @y and per ll/sc ordering
you then still don't get to make the WARN happen.

On IRC you explained that your 8.1 LSE instructions are not in fact
ordered by a smp_rmb, only by smp_wmb, which is 'surprising' since you
really need to load the old value to compute the new value.

Not happy... :-(