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

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Date:	Sat, 10 Mar 2007 03:34:32 +0100
From:	Thomas Graf <tgraf@...g.ch>
To:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
Cc:	"Kok, Auke-jan H" <auke-jan.h.kok@...el.com>,
	David Miller <davem@...emloft.net>,
	"Garzik, Jeff" <jgarzik@...ox.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
	"Kok, Auke" <auke@...-projects.org>,
	"Ronciak, John" <john.ronciak@...el.com>
Subject: Re: [PATCH 1/2] NET: Multiple queue network device support

* Waskiewicz Jr, Peter P <peter.p.waskiewicz.jr@...el.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);
       }
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