On Mon, Aug 11, 2008 at 06:12:35PM -0700, David Miller wrote:

quoted text
> From: Jarek Poplawski

> Date: Mon, 11 Aug 2008 22:53:57 +0200

>

> > pkt_sched: Destroy gen estimators under rtnl_lock().

> >

> > gen_kill_estimator() requires rtnl_lock() protection, and since it is

> > called in qdisc ->destroy() too, this has to go back from RCU callback

> > to qdisc_destroy().

> >

> > Signed-off-by: Jarek Poplawski

>

> We can't do this.  And at a minimum, the final ->reset() must

> occur in the RCU callback, otherwise asynchronous threads of

> execution could queue packets into this dying qdisc and

> such packets would leak forever.

Could you explain this more? I've thought this synchronize_rcu() is

just to prevent this (and what these comments talk about?):
void dev_deactivate(struct net_device *dev)

{

        bool running;
        netdev_for_each_tx_queue(dev, dev_deactivate_queue, &noop_qdisc);

        dev_deactivate_queue(dev, &dev->rx_queue, &noop_qdisc);
        dev_watchdog_down(dev);
        /* Wait for outstanding qdisc-less dev_queue_xmit calls. */

        synchronize_rcu();
        do {

                while (some_qdisc_is_running(dev, 0))

                        yield();
                /*

                 * Double-check inside queue lock to ensure that all effects

                 * of the queue run are visible when we return.

                 */

                running = some_qdisc_is_running(dev, 1);
                /*

                 * The running flag should never be set at this point because

                 * we've already set dev->qdisc to noop_qdisc *inside* the same

                 * pair of spin locks.  That is, if any qdisc_run starts after

                 * our initial test it should see the noop_qdisc and then

                 * clear the RUNNING bit before dropping the queue lock.  So

                 * if it is set here then we've found a bug.

                 */

        } while (WARN_ON_ONCE(running));

}
Jarek P.

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