netdev - Re: [PATCH net-next v1 2/5] net: sched: Introduce helpers for qevent blocks

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Date:   Tue, 07 Jul 2020 17:22:36 +0200
From:   Petr Machata <petrm@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>
Subject: Re: [PATCH net-next v1 2/5] net: sched: Introduce helpers for qevent blocks

Cong Wang <xiyou.wangcong@...il.com> writes:

> On Fri, Jun 26, 2020 at 3:46 PM Petr Machata <petrm@...lanox.com> wrote:
>> The function tcf_qevent_handle() should be invoked when qdisc hits the
>> "interesting event" corresponding to a block. This function releases root
>> lock for the duration of executing the attached filters, to allow packets
>> generated through user actions (notably mirred) to be reinserted to the
>> same qdisc tree.
>
> Are you sure releasing the root lock in the middle of an enqueue operation
> is a good idea? I mean, it seems racy with qdisc change or reset path,
> for example, __red_change() could update some RED parameters
> immediately after you release the root lock.

So I had mulled over this for a while. If packets are enqueued or
dequeued while the lock is released, maybe the packet under
consideration should have been hard_marked instead of prob_marked, or
vice versa. (I can't really go to not marked at all, because the fact
that we ran the qevent is a very observable commitment to put the packet
in the queue with marking.) I figured that is not such a big deal.

Regarding a configuration change, for a brief period after the change, a
few not-yet-pushed packets could have been enqueued with ECN marking
even as I e.g. disabled ECN. This does not seem like a big deal either,
these are transient effects.