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    SubjectRE: [PATCH 1/2] NET: Multiple queue network device support
    Date
    From
    > -----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
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