From: Yunsheng Lin <linyunsheng@huawei.com>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>,
Jiri Pirko <jiri@resnulli.us>,
"David Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
"Linux Kernel Network Developers" <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>, <linuxarm@huawei.com>
Subject: Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc
Date: Thu, 3 Sep 2020 09:21:02 +0800 [thread overview]
Message-ID: <cd773132-c98e-18e1-67fd-bbef6babbf0f@huawei.com> (raw)
In-Reply-To: <CAM_iQpXmpMdxF2JDOROaf+Tjk-8dASiXz53K-Ph_q7jVMe0oVw@mail.gmail.com>
On 2020/9/3 8:35, Cong Wang wrote:
> On Tue, Sep 1, 2020 at 11:35 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> On 2020/9/2 12:41, Cong Wang wrote:
>>> On Tue, Sep 1, 2020 at 6:42 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>>>
>>>> On 2020/9/2 2:24, Cong Wang wrote:
>>>>> On Mon, Aug 31, 2020 at 5:59 PM Yunsheng Lin <linyunsheng@huawei.com> 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.
>
>
>> . .
>> 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.
>
> It has almost nothing to do with BYPASS, my point here is all about
> __QDISC_STATE_DEACTIVATED bit, clearly commit 379349e9bc3b4
> removed this bit test for lockless qdisc, whether intentionally or accidentally.
> That bit test existed prior to BYPASS test, see commit ba27b4cdaaa665.
It seems there is no qdisc->seqlock or qdisc->seqlock to protect the
deactivated bit testing in commit ba27b4cdaaa665 too?
>
> Thanks.
> .
>
next prev parent reply other threads:[~2020-09-03 1:21 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-01 0:55 [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc Yunsheng Lin
2020-09-01 6:48 ` Eric Dumazet
2020-09-01 7:27 ` Yunsheng Lin
2020-09-01 18:34 ` David Miller
2020-09-02 1:43 ` Yunsheng Lin
2020-09-01 18:24 ` Cong Wang
2020-09-02 1:42 ` Yunsheng Lin
2020-09-02 4:41 ` Cong Wang
2020-09-02 6:34 ` Yunsheng Lin
2020-09-02 7:32 ` Eric Dumazet
2020-09-02 8:14 ` Yunsheng Lin
2020-09-02 9:20 ` Eric Dumazet
2020-09-03 1:14 ` Yunsheng Lin
2020-09-03 7:24 ` Eric Dumazet
2020-09-04 8:10 ` Yunsheng Lin
2020-09-03 0:35 ` Cong Wang
2020-09-03 1:21 ` Yunsheng Lin [this message]
2020-09-03 1:48 ` Cong Wang
2020-09-03 2:22 ` Yunsheng Lin
2020-09-03 2:53 ` Cong Wang
2020-09-04 1:30 ` John Fastabend
2020-09-04 8:08 ` Yunsheng Lin
2020-09-06 8:52 ` [net] 6fd0d0dede: hwsim.ap_ht40_5ghz_switch.fail kernel test robot
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