netdev - Re: [PATCH net-next v1 2/2] bridge: export multicast database via netlink

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Date:	Fri, 30 Nov 2012 15:27:33 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Cong Wang <amwang@...hat.com>
Cc:	netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net-next v1 2/2] bridge: export multicast database via
 netlink

On 11/30/12 at 11:00pm, Cong Wang wrote:
> I don't understand this. nla_put_flag() is used to put a flag (only one
> bit set) into a netlink message, so why should we use it to put
> p->port_no here? And why port_no 0 matters here?

nla_put_flag() will simply add a netlink attribute with no payload,
i.e. just the header. Assuming that port_no == 0 is invalid the
port_no can be used as attribute id as both are 16bit integers.

It will look like this:

MDBA_ROUTERS = {
  {
    .nla_len = 4,
    .nla_type = <port_no_1>,
  },
  {
    .nla_len = 4,
    .nla_type = <port_no_2>,
  }
  [...]
}

If you ever need to extend this you can just add payload to the
per port attribute and nothing will break.

> So I should use net->dev_base_seq + mdb->seq ?

No you can't, mdb->seq is not stable throughout a dump. What you
can do is save mdb->seq in cb->args[] and in case you continue
dumping from the same mdb in the next call to your dump function
you check if it changed and bump cb->seq if it did to trigger an
interrupt.
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