netdev - Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

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Date:   Wed, 2 Sep 2020 11:20:28 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Cong Wang <xiyou.wangcong@...il.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 9/2/20 1:14 AM, Yunsheng Lin wrote:
> On 2020/9/2 15:32, Eric Dumazet wrote:
>>
>>
>> On 9/1/20 11:34 PM, Yunsheng Lin wrote:
>>
>>>
>>> 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.
>>
>> Thw qdisc_reset() should be done after rcu grace period, when there is guarantee no enqueue is in progress.
>>
>> qdisc_destroy() already has a qdisc_reset() call, I am not sure why qdisc_deactivate() is also calling qdisc_reset()
> 
> That is a good point.
> Do we allow skb left in qdisc when the qdisc is deactivated state?
> And qdisc_destroy() is not always called after qdisc_deactivate() is called.
> If we allow skb left in qdisc when the qdisc is deactivated state, then it is
> huge change of semantics for qdisc_deactivate(), and I am not sure how many
> cases will affected by this change.

All I am saying is that the qdisc_reset() in qdisc_deactivate() seems
at the wrong place.

This certainly can be deferred _after_ the rcu grace period we already have in dev_deactivate_many()

Something like this nicer patch.

 net/sched/sch_generic.c |   22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)


diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 265a61d011dfaa7ec0f8fb8aaede920784f690c9..0eaa99e4f8de643724c0942ee1a2e9516eed65c0 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,
@@ -1184,6 +1170,9 @@ static bool some_qdisc_is_busy(struct net_device *dev)
                val = (qdisc_is_running(q) ||
                       test_bit(__QDISC_STATE_SCHED, &q->state));
 
+               if (!val)
+                       qdisc_reset(q);
+
                spin_unlock_bh(root_lock);
 
                if (val)
@@ -1213,10 +1202,7 @@ void dev_deactivate_many(struct list_head *head)
                dev_watchdog_down(dev);
        }
 
-       /* Wait for outstanding qdisc-less dev_queue_xmit calls.
-        * This is avoided if all devices are in dismantle phase :
-        * Caller will call synchronize_net() for us
-        */
+       /* Wait for outstanding dev_queue_xmit calls. */
        synchronize_net();
 
        /* Wait for outstanding qdisc_run calls. */