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

From: Peter Zijlstra
Date: Tue Aug 01 2017 - 05:01:39 EST

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.