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

Waskiewicz Jr, Peter P
2007-03-10T20:37:30Z

> -----Original Message-----
> From: Thomas Graf [mailto:tgraf@suug.ch]
> Sent: Friday, March 09, 2007 6:35 PM
> To: Waskiewicz Jr, Peter P
> Cc: Kok, Auke-jan H; David Miller; Garzik, Jeff;
> netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
> Brandeburg, Jesse; Kok, Auke; Ronciak, John
> Subject: Re: [PATCH 1/2] NET: Multiple queue network device support
>
> * Waskiewicz Jr, Peter P <peter.p.waskiewicz.jr@intel.com>
> 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);
> }

Ok, now I see what's wrong. Taking Dave M.'s recommendation to store
the queue mapping in the skb will let me unlock the queue when dequeue()
returns. I'll fix this locking issue; thanks for the feedback and
persistent drilling into my thick head.

-PJ Waskiewicz
peter.p.waskiewicz.jr@intel.com