netdev - Re: [RFC] netlink: add socket destruction notification

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Message-ID: <4AF8122F.9060807@trash.net>
Date:	Mon, 09 Nov 2009 13:59:27 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Johannes Berg <johannes@...solutions.net>
CC:	netdev <netdev@...r.kernel.org>, Jouni Malinen <j@...fi>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: [RFC] netlink: add socket destruction notification

Johannes Berg wrote:
> On Fri, 2009-11-06 at 16:37 +0100, Patrick McHardy wrote:
> 
>>>> This seems pretty similar to the NETLINK_URELEASE notifier invoked
>>>> in netlink_release(). Wouldn't that one work as well?
>>> Hmm, it does seem similar, thanks for pointing it out. What exactly does
>>> the condition
>>> 	if (nlk->pid && !nlk->subscriptions) {
>>>
>>> mean though?
>> nlk->pid is non-zero for bound sockets, which is basically any
>> non-kernel socket which has either sent a message or explicitly
>> called bind(). nlk->subscriptions is zero for sockets not bound
>> to multicast groups.
>>
>> So effectively it invokes the notifier for all bound unicast
>> userspace sockets. Not sure why it doesn't invoke the notifier
>> for sockets that are used for both unicast and multicast
>> reception. If that is a problem I think the second condition
>> could be removed.
> 
> Thanks for the explanation. I think we'd need the second condition
> removed, I don't see a reason to force a socket to not also have
> multicast RX if it's used for any of the purposes we're looking at this
> for. Guess we need to audit the callees to determine whether that's ok.

I've already done that. Its currently only used by netfilter
for which this change also makes sense.

> Can you quickly explain the difference between release and destruct?

release is called when the socket is closed, destruct is called
once all references are gone. I think with the synchonous processing
done nowadays they shouldn't make any difference, but release
should be fine in either case.
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