netdev - Re: [PATCH net 2/2] vlan: move dev_put into vlan_dev

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Message-ID: <CADvbK_f-Zk9X7M87yUi8msAykA9z+5-te3hNXg3TRE+bfpfmBg@mail.gmail.com>
Date:   Fri, 11 Feb 2022 15:58:56 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        network dev <netdev@...r.kernel.org>,
        davem <davem@...emloft.net>,
        Ziyang Xuan <william.xuanziyang@...wei.com>
Subject: Re: [PATCH net 2/2] vlan: move dev_put into vlan_dev_uninit

On Thu, Feb 10, 2022 at 1:59 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 9 Feb 2022 21:49:51 -0800 Eric Dumazet wrote:
> > > Feels like sooner or later we'll run into a scenario when reversing will
> > > cause a problem. Or some data structure will stop preserving the order.
> > >
> > > Do you reckon rewriting netdev_run_todo() will be a lot of effort or
> > > it's too risky?
> >
> > This is doable, and risky ;)
> >
> > BTW, I have the plan of generalizing blackhole_netdev for IPv6,
> > meaning that we could perhaps get rid of the dependency
> > about loopback dev, being the last device in a netns being dismantled.
>
> Oh, I see..
>
> I have no great ideas then, we may need to go back to zeroing
> vlan->real_dev and making sure the caller can deal with that.
> At least for the time being. Xin this was discussed at some
> length in response to the patch under Fixes.
Hi, Jakub,

What if dev->real_dev is freed and zeroed *after* vlan_dev_real_dev()
is called? This issue can still be triggered, right? I don't see any lock
protecting this.

> Feels like sooner or later we'll run into a scenario when reversing will
> cause a problem. Or some data structure will stop preserving the order.
I was checking a few places doing such batch devices freeing, and noticed that:
In rtnl_group_dellink() and __rtnl_kill_links(), it's using for_each_netdev(),
while in default_device_exit_batch(), it's using for_each_netdev_reverse().

shouldn't be in the same order all these places? If yes, which one is the
right one to use?

Thanks.