netdev - Re: [Patch net] netns: avoid allocating idr when dumping info

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Date:	Fri, 27 Feb 2015 22:17:27 -0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [Patch net] netns: avoid allocating idr when dumping info

(Removing Eric from discussion since he expressed he doesn't
care this code at all in a private email.)


On Fri, Feb 27, 2015 at 4:56 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> Why? This relies on rtnetlink to dump the link after creation,
> which is not obvious at all. IOW, imagine if rtnetlink didn't do
> the dump after new link, what would happen?
>
> You are hiding it too deep, without a valid reason. It sounds
> like you want to defer the allocation as late as possible,
> but again it makes no sense: 1) we don't allocate too much,
> just one id in idr; 2) this loses the code readability and hides
> potential bugs.
>

Here we go:

This breaks veth, where veth peers are completely equal,
therefore they are links for each other. So we expect
link-netnsid shown on both side.

Your current impl. relies on rtnl_configure_link() to dump
the link to allocate the netns id after creating a link.
Unfortunately for veth, rtnl_configure_link() is called
before veth->peer is set, therefore the dump of the "peer"
does _not_ actually allocate the netns id, since its
->get_link_net() returns the same net.

I will send a patch together after I figure out another bug
I saw.
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