netdev - Re: [PATCH net v4 07/12] macvlan: use dynamic lockdep key instead of subclass

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Date:   Tue, 8 Oct 2019 17:13:22 +0900
From:   Taehee Yoo <ap420073@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, linux-wireless@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        j.vosburgh@...il.com, vfalico@...il.com,
        Andy Gospodarek <andy@...yhouse.net>,
        Jiří Pírko <jiri@...nulli.us>,
        Sabrina Dubroca <sd@...asysnail.net>,
        Roopa Prabhu <roopa@...ulusnetworks.com>, saeedm@...lanox.com,
        manishc@...vell.com, rahulv@...vell.com, kys@...rosoft.com,
        haiyangz@...rosoft.com,
        Stephen Hemminger <stephen@...workplumber.org>,
        sashal@...nel.org, hare@...e.de, varun@...lsio.com,
        ubraun@...ux.ibm.com, kgraul@...ux.ibm.com,
        Jay Vosburgh <jay.vosburgh@...onical.com>,
        Cody Schuffelen <schuffelen@...gle.com>, bjorn@...k.no
Subject: Re: [PATCH net v4 07/12] macvlan: use dynamic lockdep key instead of subclass

On Mon, 7 Oct 2019 at 20:41, Johannes Berg <johannes@...solutions.net> wrote:
>

Hi Johannes,

> On Sat, 2019-10-05 at 18:13 +0900, Taehee Yoo wrote:
> >
> > If we place lockdep keys into "struct net_device", this macro would be a
> > little bit modified and reused. And driver code shape will not be huge
> > changed. I think this way is better than this v4 way.
> > So I will try it.
>
> What I was thinking was that if we can do this for every VLAN netdev,
> why shouldn't we do it for *every* netdev unconditionally? Some code
> could perhaps even be simplified if this was just a general part of
> netdev allocation.
>

Your opinion makes sense.
I think there is no critical reason that every netdev shouldn't have
own lockdep keys. By comparison, the benefits are obvious.

> > > But it seems to me the whole nesting also has to be applied here?
> > >
> > > __dev_xmit_skb:
> > >  * qdisc_run_begin()
> > >  * sch_direct_xmit()
> > >    * HARD_TX_LOCK(dev, txq, smp_processor_id());
> > >    * dev_hard_start_xmit() // say this is VLAN
> > >      * dev_queue_xmit() // on real_dev
> > >        * __dev_xmit_skb // recursion on another netdev
> > >
> > > Now if you have VLAN-in-VLAN the whole thing will recurse right?
> > >
> >
> > I have checked on this routine.
> > Only xmit_lock(HARD_TX_LOCK) could be nested. other
> > qdisc locks(runinng, busylock) will not be nested.
>
> OK, I still didn't check it too closely I guess, or got confused which
> lock I should look at.
>
> > This patch already
> > handles the _xmit_lock key. so I think there is no problem.
>
> Right
>
> > But I would like to place four lockdep keys(busylock, address,
> > running, _xmit_lock) into "struct net_device" because of code complexity.
> >
> > Let me know if I misunderstood anything.
>
> Nothing to misunderstand - I was just asking/wondering why the qdisc
> locks were not treated the same way.
>

I'm always thankful for your detailed and careful reviews.

> johannes
>

Thank you,
Taehee Yoo