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From:   Cong Wang <xiyou.wangcong@gmail.com>
Date:   Tue, 1 Sep 2020 11:24:43 -0700
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Subject: Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc
To:     Yunsheng Lin <linyunsheng@huawei.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
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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.

>
> 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.

Thanks.