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

[Yunsheng Lin]

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()					.
	.					.
rcu_read_unlock_bh()				.

And During step 3, txq->qdisc is pointing to noop_qdisc, so the qdisc_reset()
only reset the noop_qdisc, but not the actual qdisc, which is stored in
txq->qdisc_sleeping, so the actual qdisc may still have skb.

When hns3_link_status_change() call step 4 and 5, it will restore all queue's
qdisc using txq->qdisc_sleeping and schedule all queue with net_tx_action().
The skb enqueued in step 1 may be dequeued and run, which cause the problem.

> 
> 
>>
>>>
>>>>
>>>> Avoid the above concurrent op by calling synchronize_rcu_tasks()
>>>> after assigning new qdisc to dev_queue->qdisc and before calling
>>>> qdisc_deactivate() to make sure skb with larger queue_mapping
>>>> enqueued to old qdisc will always be reset when qdisc_deactivate()
>>>> is called.
>>>
>>> Like Eric said, it is not nice to call such a blocking function when
>>> we have a large number of TX queues. Possibly we just need to
>>> add a synchronize_net() as in netif_set_real_num_tx_queues(),
>>> if it is missing.
>>
>> As above, the synchronize_net() in netif_set_real_num_tx_queues() seems
>> to work when netdevice is in active state, but does not work when in
>> deactive.
> 
> Please explain why deactivated device still has qdisc running?
> 
> At least before commit 379349e9bc3b4, we always test deactivate
> bit before enqueueing. Are you complaining about that commit?
> That commit is indeed suspicious, at least it does not precisely revert
> commit ba27b4cdaaa66561aaedb21 as it claims.

I am not familiar with TCQ_F_CAN_BYPASS.
>From my understanding, the problem is that there is no order between
qdisc enqueuing and qdisc reset.


> 
> 
>>
>> And we do not want skb left in the old qdisc when netdevice is deactived,
>> right?
> 
> Yes, and more importantly, qdisc should not be running after deactivation.
> 
> Thanks.
> .
>