linux-kernel - Re: [PATCH net] net: hdlc_x25: Use qdisc to queue outgoing LAPB frames

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Date:   Sat, 30 Jan 2021 06:29:20 -0800
From:   Xie He <xie.he.0141@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Martin Schiller <ms@....tdt.de>,
        "David S. Miller" <davem@...emloft.net>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net] net: hdlc_x25: Use qdisc to queue outgoing LAPB frames

On Fri, Jan 29, 2021 at 5:36 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> I'm still struggling to wrap my head around this.
>
> Did you test your code with lockdep enabled? Which Qdisc are you using?
> You're queuing the frames back to the interface they came from - won't
> that cause locking issues?

Hmm... Thanks for bringing this to my attention. I indeed find issues
when the "noqueue" qdisc is used.

When using a qdisc other than "noqueue", when sending an skb:
"__dev_queue_xmit" will call "__dev_xmit_skb";
"__dev_xmit_skb" will call "qdisc_run_begin" to mark the beginning of
a qdisc run, and if the qdisc is already running, "qdisc_run_begin"
will fail, then "__dev_xmit_skb" will just enqueue this skb without
starting qdisc. There is no problem.

When using "noqueue" as the qdisc, when sending an skb:
"__dev_queue_xmit" will try to send this skb directly. Before it does
that, it will first check "txq->xmit_lock_owner" and will find that
the current cpu already owns the xmit lock, it will then print a
warning message "Dead loop on virtual device ..." and drop the skb.

A solution can be queuing the outgoing L2 frames in this driver first,
and then using a tasklet to send them to the qdisc TX queue.

Thanks! I'll make changes to fix this.