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Message-ID: <CAM_iQpU_tbRNO=Lznz_d6YjXmenYhowEfBoOiJgEmo9x8bEevw@mail.gmail.com>
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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