From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 100B2C2BB56 for ; Wed, 2 Sep 2020 06:34:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E1AB920758 for ; Wed, 2 Sep 2020 06:34:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726800AbgIBGec (ORCPT ); Wed, 2 Sep 2020 02:34:32 -0400 Received: from szxga04-in.huawei.com ([45.249.212.190]:10790 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726446AbgIBGec (ORCPT ); Wed, 2 Sep 2020 02:34:32 -0400 Received: from DGGEMS403-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id D9877D9EE9AF418DA077; Wed, 2 Sep 2020 14:34:28 +0800 (CST) Received: from [10.74.191.121] (10.74.191.121) by DGGEMS403-HUB.china.huawei.com (10.3.19.203) with Microsoft SMTP Server id 14.3.487.0; Wed, 2 Sep 2020 14:34:20 +0800 Subject: Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc To: Cong Wang CC: Jamal Hadi Salim , Jiri Pirko , "David Miller" , Jakub Kicinski , "Linux Kernel Network Developers" , LKML , References: <1598921718-79505-1-git-send-email-linyunsheng@huawei.com> <511bcb5c-b089-ab4e-4424-a83c6e718bfa@huawei.com> From: Yunsheng Lin Message-ID: Date: Wed, 2 Sep 2020 14:34:21 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.74.191.121] X-CFilter-Loop: Reflected Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2020/9/2 12:41, Cong Wang wrote: > On Tue, Sep 1, 2020 at 6:42 PM Yunsheng Lin wrote: >> >> On 2020/9/2 2:24, Cong Wang wrote: >>> On Mon, Aug 31, 2020 at 5:59 PM Yunsheng Lin 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() . . . 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. > > >> >> And we do not want skb left in the old qdisc when netdevice is deactived, >> right? > > Yes, and more importantly, qdisc should not be running after deactivation. > > Thanks. > . >