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

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Date:   Tue, 7 Jul 2020 12:41:41 -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 Tue, Jul 7, 2020 at 8:25 AM Petr Machata <petrm@...lanox.com> wrote:
>
>
> Cong Wang <xiyou.wangcong@...il.com> writes:
>
> > 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().
>
> I know, I wanted to make it clear that the lock may end up being used,
> instead of doing it "stealthily". If you find this inelegant I can push
> a follow-up that converts tcf_qevent_handle() to sch_tree_unlock().

So far only sch_red uses tcf_qevent_handle(), changing the rest
qdisc's just for sch_red seems overkill here.

Of course, if we could eliminate the root lock release, all the pains
would go away.

Thanks.