Question about PB rule of LKMM

[Kenneth-Lee-2012]

Hi, there,

I'm new to LKMM. May I ask a may-be-stupid question?

In the LKMM document, it said the pb link:

   E ->coe W ->cumul-fence* X ->rfe? Y ->strong-fence Z ->hb* F

can make sure E execute before F. But the cat file define pb as follow:

  let pb = prop ; strong-fence ; hb* ; [Marked]
  acyclic pb as propagation

So the acyclic rule is only on pb relationshit itself. So it won't
forbid F -rfe-> E, will it? It only forgit F -pb-> E. So how can
propagation rule ensure E execute before F?
  
Can anyone explain this? Thank you in advance.

- Kenneth Lee