Flow of packets in a classful queue

         classful qdisc
          +--------
          |- qdisc1
          |        
write()-> |- qdisc2
          |        
          |- qdisc3
          +--------
         classful qdisc
          +--------
          |- qdisc1
          |        
write()-> |- qdisc2
          |        
          |- qdisc3
          +--------
      filter()     
     determines    
where to enqueue()
         classful qdisc
          +--------
          |- qdisc1
          |        
write()-> |- qdisc2
          |        
          |* qdisc3
          +--------
      filter()     
     determines    
where to enqueue()
         classful qdisc
          +----------+ 
          |- qdisc1  |
          |          |
write()-> |- qdisc2  | ->dequeue()
          |          |
          |* qdisc3  |
          +----------+         
      filter()    
     determines   
where to enqueue()
         classful qdisc
          +----------+ 
          |- qdisc1  |
          |          |
write()-> |- qdisc2  | ->dequeue()
          |          |
          |* qdisc3  |
          +----------+         
      filter()     tries dequeue()..
     determines      
where to enqueue()
         classful qdisc
          +----------+ 
          |- qdisc1 ?|
          |          |
write()-> |- qdisc2  | ->dequeue()
          |          |
          |* qdisc3  |
          +----------+         
      filter()     tries dequeue()..
     determines      
where to enqueue()
         classful qdisc
          +----------+ 
          |- qdisc1 ?|
          |          |
write()-> |- qdisc2 ?| ->dequeue()
          |          |
          |* qdisc3  |
          +----------+         
      filter()     tries dequeue()..
     determines      
where to enqueue()
         classful qdisc
          +----------+ 
          |- qdisc1 ?|
          |          |
write()-> |- qdisc2 ?| ->dequeue()
          |          |
          |* qdisc3 !|
          +----------+         
      filter()     tries dequeue()..
     determines      until success!
where to enqueue()

This is the 'PRIO' qdisc.