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.
