Subject: Re: [PATCH] pkt_sched: Destroy gen estimators under rtnl_lock().

From: Jarek Poplawski <[email-masked]>
Date: Mon, 18 Aug 2008 06:27:47 +0000

On Sun, Aug 17, 2008 at 06:49:21PM -0700, David Miller wrote:
> From: Herbert Xu <[email-masked]>
> Date: Mon, 18 Aug 2008 11:36:33 +1000
> 
> > On Sun, Aug 17, 2008 at 06:35:05PM -0700, David Miller wrote:
> > > I think I see another way out of this:
> > > 
> > > 1) Add __QDISC_STATE_DEACTIVATE.
> > > 
> > > 2) Set it right before dev_deactivate() swaps resets the qdisc
> > >    pointer.
> > > 
> > > 3) Test it in dev_queue_xmit() et al. once the qdisc root lock is
> > >    acquired, and drop lock and resample ->qdisc if
> > >    __QDISC_STATE_DEACTIVATE is set.
> > 
> > Yep this sounds good to me.
> 
> Here it is as a patch below.
> 
> The test being added to __netif_schedule() is just a reminder that
> we have to address the "both bits clear" case somehow, likely with
> Jarko's patch which I unintentionally reimplemented :)

You shouldn't bother with this at all. I'm really pleased if I
sometimes think similarly to you, and this wasn't the most complex
idea to found, btw.

But, there is probably something other to bother here. I didn't get
the final version of this patch nor I can see this on the list, but
in your git there is this change to "goto out_kfree_skb", which
seems to skip rcu_read_unlock_bh().

Otherwise, hmm.. I'm half asleep yet, and only after 1 coffee, so
maybe I'll change my mind soon, but now I've some doubts about these
last changes.

Jarek P.

> 
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index a7abfda..757ab08 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -27,6 +27,7 @@ enum qdisc_state_t
>  {
>  	__QDISC_STATE_RUNNING,
>  	__QDISC_STATE_SCHED,
> +	__QDISC_STATE_DEACTIVATED,
>  };
>  
>  struct qdisc_size_table {
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 600bb23..b88f669 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1341,6 +1341,9 @@ static void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
>  
>  void __netif_schedule(struct Qdisc *q)
>  {
> +	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
> +		return;
> +
>  	if (!test_and_set_bit(__QDISC_STATE_SCHED, &q->state)) {
>  		struct softnet_data *sd;
>  		unsigned long flags;
> @@ -1790,6 +1793,8 @@ gso:
>  	rcu_read_lock_bh();
>  
>  	txq = dev_pick_tx(dev, skb);
> +
> +resample_qdisc:
>  	q = rcu_dereference(txq->qdisc);
>  
>  #ifdef CONFIG_NET_CLS_ACT
> @@ -1800,6 +1805,11 @@ gso:
>  
>  		spin_lock(root_lock);
>  
> +		if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
> +			spin_unlock(root_lock);
> +			goto resample_qdisc;
> +		}
> +
>  		rc = qdisc_enqueue_root(skb, q);
>  		qdisc_run(q);
>  
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 4685746..ff1c455 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -597,6 +597,9 @@ static void transition_one_qdisc(struct net_device *dev,
>  	struct Qdisc *new_qdisc = dev_queue->qdisc_sleeping;
>  	int *need_watchdog_p = _need_watchdog;
>  
> +	if (!(new_qdisc->flags & TCQ_F_BUILTIN))
> +		clear_bit(__QDISC_STATE_DEACTIVATED, &new_qdisc->state);
> +
>  	rcu_assign_pointer(dev_queue->qdisc, new_qdisc);
>  	if (need_watchdog_p && new_qdisc != &noqueue_qdisc)
>  		*need_watchdog_p = 1;
> @@ -640,6 +643,9 @@ static void dev_deactivate_queue(struct net_device *dev,
>  	if (qdisc) {
>  		spin_lock_bh(qdisc_lock(qdisc));
>  
> +		if (!(qdisc->flags & TCQ_F_BUILTIN))
> +			set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
> +
>  		dev_queue->qdisc = qdisc_default;
>  		qdisc_reset(qdisc);
>