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Date:   Thu, 17 Sep 2020 12:26:29 -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,
        John Fastabend <john.fastabend@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc

On Fri, Sep 11, 2020 at 1:13 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2020/9/11 4:07, Cong Wang wrote:
> > On Tue, Sep 8, 2020 at 4:06 AM 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.
> >>
> >> 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@...wei.com>
> >> ---
> >> 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.


>
> 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.

Thanks.

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