netdev - Re: [PATCH net-next v1 1/5] net: sched: Pass root lock to Qdisc

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Message-ID: <CAM_iQpUEAt_0Kr1vVfRrpMPz+2arpLA5g4yfWCLe2fhbMTrH7w@mail.gmail.com>
Date:   Mon, 6 Jul 2020 12:21:33 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Petr Machata <petrm@...lanox.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 1/5] net: sched: Pass root lock to Qdisc_ops.enqueue

On Fri, Jun 26, 2020 at 3:46 PM Petr Machata <petrm@...lanox.com> wrote:
>
> A following patch introduces qevents, points in qdisc algorithm where
> packet can be processed by user-defined filters. Should this processing
> lead to a situation where a new packet is to be enqueued on the same port,
> holding the root lock would lead to deadlocks. To solve the issue, qevent
> handler needs to unlock and relock the root lock when necessary.
>
> To that end, add the root lock argument to the qdisc op enqueue, and
> propagate throughout.

Hmm, but why do you pass root lock down to each ->enqueue()?

You can find root lock with sch_tree_lock() (or qdisc_lock() if you don't
care about hierarchy), and you already have qdisc as a parameter of
tcf_qevent_handle().

Thanks.