Re: [PATCH 1/2] NET: Multiple queue network device support

* Waskiewicz Jr, Peter P [off-list ref] 2007-03-09 15:27

That's the entire point of this extra locking.  enqueue() is going to
put an skb into a band somewhere that maps to some queue, and there is
no way to guarantee the skb I retrieve from dequeue() is headed for the
same queue.  Therefore, I need to unlock the queue after I finish
enqueuing, since having that lock makes little sense to dequeue().
dequeue() will then grab *a* lock on a queue; it may be the same one we
had during enqueue(), but it may not be.  And the placement of the
unlock of that queue is exactly where it happens in non-multiqueue,
which is right before the hard_start_xmit().

The lock is already unlocked after dequeue, from your prio_dequeue():

       if (netif_is_multiqueue(sch->dev)) {
               queue = q->band2queue[prio];
               if (spin_trylock(&sch->dev->egress_subqueue[queue].queue_lock)) {
                       qdisc = q->queues[prio];
                       skb = qdisc->dequeue(qdisc);
                       if (skb) {
                               sch->q.qlen--;
                               skb->priority = prio;
                               spin_unlock(&sch->dev->egress_subqueue[queue].queue_lock);
                               return skb;
                       }
                       spin_unlock(&sch->dev->egress_subqueue[queue].queue_lock);
       }