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