On Fri, 2007-10-19 at 13:36 +0800, Herbert Xu wrote:
> On Fri, Oct 19, 2007 at 12:20:25PM +0800, Herbert Xu wrote:
> >
> > In fact this bug exists elsewhere too. For example, the network
> > stack does this in net/sched/sch_generic.c:
> >
> > /* Wait for outstanding qdisc_run calls. */
> > while (test_bit(__LINK_STATE_QDISC_RUNNING, &dev->state))
> > yield();
> >
> > This has the same problem as the current synchronize_irq code.
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index e01d576..b3b7420 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -556,6 +556,7 @@ void dev_deactivate(struct net_device *dev)
> {
> struct Qdisc *qdisc;
> struct sk_buff *skb;
> + int running;
>
> spin_lock_bh(&dev->queue_lock);
> qdisc = dev->qdisc;
> @@ -571,12 +572,31 @@ void dev_deactivate(struct net_device *dev)
>
> dev_watchdog_down(dev);
>
> - /* Wait for outstanding dev_queue_xmit calls. */
> + /* Wait for outstanding qdisc-less dev_queue_xmit calls. */
> synchronize_rcu();
>
> /* Wait for outstanding qdisc_run calls. */
> - while (test_bit(__LINK_STATE_QDISC_RUNNING, &dev->state))
> - yield();
> + do {
> + while (test_bit(__LINK_STATE_QDISC_RUNNING, &dev->state))
> + yield();
> +
Ouch!, is there really no sane locking alternative? Hashed waitqueues
like for the page lock come to mind.
> + /*
> + * Double-check inside queue lock to ensure that all effects
> + * of the queue run are visible when we return.
> + */
> + spin_lock_bh(&dev->queue_lock);
> + running = test_bit(__LINK_STATE_QDISC_RUNNING, &dev->state);
> + spin_unlock_bh(&dev->queue_lock);
> +
> + /*
> + * 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));
> }
>
> void dev_init_scheduler(struct net_device *dev)
-
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