Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

[Yunsheng Lin]

On 2020/9/3 9:48, Cong Wang wrote:
> On Wed, Sep 2, 2020 at 6:22 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>
>> On 2020/9/3 8:35, Cong Wang wrote:
>>> On Tue, Sep 1, 2020 at 11:35 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>>>
>>>> On 2020/9/2 12:41, Cong Wang wrote:
>>>>> On Tue, Sep 1, 2020 at 6:42 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> On 2020/9/2 2:24, Cong Wang wrote:
>>>>>>> On Mon, Aug 31, 2020 at 5:59 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>>>>>>>
>>>>>>>> Currently there is concurrent reset and enqueue operation for the
>>>>>>>> same lockless qdisc when there is no lock to synchronize the
>>>>>>>> q->enqueue() in __dev_xmit_skb() with the qdisc reset operation in
>>>>>>>> qdisc_deactivate() called by dev_deactivate_queue(), which may cause
>>>>>>>> out-of-bounds access for priv->ring[] in hns3 driver if user has
>>>>>>>> requested a smaller queue num when __dev_xmit_skb() still enqueue a
>>>>>>>> skb with a larger queue_mapping after the corresponding qdisc is
>>>>>>>> reset, and call hns3_nic_net_xmit() with that skb later.
>>>>>>>
>>>>>>> Can you be more specific here? Which call path requests a smaller
>>>>>>> tx queue num? If you mean netif_set_real_num_tx_queues(), clearly
>>>>>>> we already have a synchronize_net() there.
>>>>>>
>>>>>> When the netdevice is in active state, the synchronize_net() seems to
>>>>>> do the correct work, as below:
>>>>>>
>>>>>> CPU 0:                                       CPU1:
>>>>>> __dev_queue_xmit()                       netif_set_real_num_tx_queues()
>>>>>> rcu_read_lock_bh();
>>>>>> netdev_core_pick_tx(dev, skb, sb_dev);
>>>>>>         .
>>>>>>         .                               dev->real_num_tx_queues = txq;
>>>>>>         .                                       .
>>>>>>         .                                       .
>>>>>>         .                               synchronize_net();
>>>>>>         .                                       .
>>>>>> q->enqueue()                                    .
>>>>>>         .                                       .
>>>>>> rcu_read_unlock_bh()                            .
>>>>>>                                         qdisc_reset_all_tx_gt
>>>>>>
>>>>>>
>>>>>
>>>>> Right.
>>>>>
>>>>>
>>>>>> but dev->real_num_tx_queues is not RCU-protected, maybe that is a problem
>>>>>> too.
>>>>>>
>>>>>> The problem we hit is as below:
>>>>>> In hns3_set_channels(), hns3_reset_notify(h, HNAE3_DOWN_CLIENT) is called
>>>>>> to deactive the netdevice when user requested a smaller queue num, and
>>>>>> txq->qdisc is already changed to noop_qdisc when calling
>>>>>> netif_set_real_num_tx_queues(), so the synchronize_net() in the function
>>>>>> netif_set_real_num_tx_queues() does not help here.
>>>>>
>>>>> How could qdisc still be running after deactivating the device?
>>>>
>>>> qdisc could be running during the device deactivating process.
>>>>
>>>> The main process of changing channel number is as below:
>>>>
>>>> 1. dev_deactivate()
>>>> 2. hns3 handware related setup
>>>> 3. netif_set_real_num_tx_queues()
>>>> 4. netif_tx_wake_all_queues()
>>>> 5. dev_activate()
>>>>
>>>> During step 1, qdisc could be running while qdisc is resetting, so
>>>> there could be skb left in the old qdisc(which will be restored back to
>>>> txq->qdisc during dev_activate()), as below:
>>>>
>>>> CPU 0:                                       CPU1:
>>>> __dev_queue_xmit():                      dev_deactivate_many():
>>>> rcu_read_lock_bh();                      qdisc_deactivate(qdisc);
>>>> q = rcu_dereference_bh(txq->qdisc);             .
>>>> netdev_core_pick_tx(dev, skb, sb_dev);          .
>>>>         .
>>>>         .                               rcu_assign_pointer(dev_queue->qdisc, qdisc_default);
>>>>         .                                       .
>>>>         .                                       .
>>>>         .                                       .
>>>>         .                                       .
>>>> q->enqueue()                                    .
>>>
>>>
>>> Well, like I said, if the deactivated bit were tested before ->enqueue(),
>>> there would be no packet queued after qdisc_deactivate().
>>
>> Only if the deactivated bit testing is also protected by qdisc->seqlock?
>> otherwise there is still window between setting and testing the deactivated bit.
> 
> Can you be more specific here? Why testing or setting a bit is not atomic?

testing a bit or setting a bit separately is atomic.
But testing a bit and setting a bit is not atomic, right?

  cpu0:                   cpu1:
                        testing A bit
setting A bit                .
       .                     .
       .               qdisc enqueuing
qdisc reset

> 
> AFAIU, qdisc->seqlock is an optimization to replace
> __QDISC_STATE_RUNNING, which has nothing to do with deactivate bit.
> 
> Thanks.
> .
>