Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

[Yunsheng Lin]

On 2020/9/18 3:26, Cong Wang wrote:
> On Fri, Sep 11, 2020 at 1:13 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>
>> On 2020/9/11 4:07, Cong Wang wrote:
>>> On Tue, Sep 8, 2020 at 4:06 AM 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.
>>>>
>>>> Reused the existing synchronize_net() in dev_deactivate_many() to
>>>> make sure skb with larger queue_mapping enqueued to old qdisc(which
>>>> is saved in dev_queue->qdisc_sleeping) will always be reset when
>>>> dev_reset_queue() is called.
>>>>
>>>> Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
>>>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>>>> ---
>>>> ChangeLog V2:
>>>>         Reuse existing synchronize_net().
>>>> ---
>>>>  net/sched/sch_generic.c | 48 +++++++++++++++++++++++++++++++++---------------
>>>>  1 file changed, 33 insertions(+), 15 deletions(-)
>>>>
>>>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>>>> index 265a61d..54c4172 100644
>>>> --- a/net/sched/sch_generic.c
>>>> +++ b/net/sched/sch_generic.c
>>>> @@ -1131,24 +1131,10 @@ EXPORT_SYMBOL(dev_activate);
>>>>
>>>>  static void qdisc_deactivate(struct Qdisc *qdisc)
>>>>  {
>>>> -       bool nolock = qdisc->flags & TCQ_F_NOLOCK;
>>>> -
>>>>         if (qdisc->flags & TCQ_F_BUILTIN)
>>>>                 return;
>>>> -       if (test_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state))
>>>> -               return;
>>>> -
>>>> -       if (nolock)
>>>> -               spin_lock_bh(&qdisc->seqlock);
>>>> -       spin_lock_bh(qdisc_lock(qdisc));
>>>>
>>>>         set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
>>>> -
>>>> -       qdisc_reset(qdisc);
>>>> -
>>>> -       spin_unlock_bh(qdisc_lock(qdisc));
>>>> -       if (nolock)
>>>> -               spin_unlock_bh(&qdisc->seqlock);
>>>>  }
>>>>
>>>>  static void dev_deactivate_queue(struct net_device *dev,
>>>> @@ -1165,6 +1151,30 @@ static void dev_deactivate_queue(struct net_device *dev,
>>>>         }
>>>>  }
>>>>
>>>> +static void dev_reset_queue(struct net_device *dev,
>>>> +                           struct netdev_queue *dev_queue,
>>>> +                           void *_unused)
>>>> +{
>>>> +       struct Qdisc *qdisc;
>>>> +       bool nolock;
>>>> +
>>>> +       qdisc = dev_queue->qdisc_sleeping;
>>>> +       if (!qdisc)
>>>> +               return;
>>>> +
>>>> +       nolock = qdisc->flags & TCQ_F_NOLOCK;
>>>> +
>>>> +       if (nolock)
>>>> +               spin_lock_bh(&qdisc->seqlock);
>>>> +       spin_lock_bh(qdisc_lock(qdisc));
>>>
>>>
>>> I think you do not need this lock for lockless one.
>>
>> It seems so.
>> Maybe another patch to remove qdisc_lock(qdisc) for lockless
>> qdisc?
> 
> Yeah, but not sure if we still want this lockless qdisc any more,
> it brings more troubles than gains.
> 
>>
>>
>>>
>>>> +
>>>> +       qdisc_reset(qdisc);
>>>> +
>>>> +       spin_unlock_bh(qdisc_lock(qdisc));
>>>> +       if (nolock)
>>>> +               spin_unlock_bh(&qdisc->seqlock);
>>>> +}
>>>> +
>>>>  static bool some_qdisc_is_busy(struct net_device *dev)
>>>>  {
>>>>         unsigned int i;
>>>> @@ -1213,12 +1223,20 @@ void dev_deactivate_many(struct list_head *head)
>>>>                 dev_watchdog_down(dev);
>>>>         }
>>>>
>>>> -       /* Wait for outstanding qdisc-less dev_queue_xmit calls.
>>>> +       /* Wait for outstanding qdisc-less dev_queue_xmit calls or
>>>> +        * outstanding qdisc enqueuing calls.
>>>>          * This is avoided if all devices are in dismantle phase :
>>>>          * Caller will call synchronize_net() for us
>>>>          */
>>>>         synchronize_net();
>>>>
>>>> +       list_for_each_entry(dev, head, close_list) {
>>>> +               netdev_for_each_tx_queue(dev, dev_reset_queue, NULL);
>>>> +
>>>> +               if (dev_ingress_queue(dev))
>>>> +                       dev_reset_queue(dev, dev_ingress_queue(dev), NULL);
>>>> +       }
>>>> +
>>>>         /* Wait for outstanding qdisc_run calls. */
>>>>         list_for_each_entry(dev, head, close_list) {
>>>>                 while (some_qdisc_is_busy(dev)) {
>>>
>>> Do you want to reset before waiting for TX action?
>>>
>>> I think it is safer to do it after, at least prior to commit 759ae57f1b
>>> we did after.
>>
>> The reference to the txq->qdisc is always protected by RCU, so the synchronize_net()
>> should be enought to ensure there is no skb enqueued to the old qdisc that is saved
>> in the dev_queue->qdisc_sleeping, because __dev_queue_xmit can only see the new qdisc
>> after synchronize_net(), which is noop_qdisc, and noop_qdisc will make sure any skb
>> enqueued to it will be dropped and freed, right?
> 
> Hmm? In net_tx_action(), we do not hold RCU read lock, and we do not
> reference qdisc via txq->qdisc but via sd->output_queue.

Sorry for the delay reply, I seems to miss this.

I assumed synchronize_net() also wait for outstanding softirq to finish, right?

> 
> 
>>
>> If we do any additional reset that is not related to qdisc in dev_reset_queue(), we
>> can move it after some_qdisc_is_busy() checking.
> 
> I am not suggesting to do an additional reset, I am suggesting to move
> your reset after the busy waiting.

There maybe a deadlock here if we reset the qdisc after the some_qdisc_is_busy() checking,
because some_qdisc_is_busy() may require the qdisc reset to clear the skb, so that
some_qdisc_is_busy() can return false. I am not sure this is really a problem, but
sch_direct_xmit() may requeue the skb when dev_hard_start_xmit return TX_BUSY.

> 
> Thanks.
> .
>