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Date:   Wed, 2 Sep 2020 18:48:52 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linuxarm@...wei.com
Subject: Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc

On Wed, Sep 2, 2020 at 6:22 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2020/9/3 8:35, Cong Wang wrote:
> > On Tue, Sep 1, 2020 at 11:35 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
> >>
> >> On 2020/9/2 12:41, Cong Wang wrote:
> >>> On Tue, Sep 1, 2020 at 6:42 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
> >>>>
> >>>> On 2020/9/2 2:24, Cong Wang wrote:
> >>>>> On Mon, Aug 31, 2020 at 5:59 PM Yunsheng Lin <linyunsheng@...wei.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.

Can you be more specific here? Why testing or setting a bit is not atomic?

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

Thanks.

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