linux-kernel - Re: [PATCH 0/10] af_unix: add multicast and filtering features to AF

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Date:	Thu, 01 Mar 2012 08:00:40 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Javier Martinez Canillas <javier.martinez@...labora.co.uk>
Cc:	Rodrigo Moya <rodrigo.moya@...labora.co.uk>,
	David Laight <David.Laight@...LAB.COM>,
	David Miller <davem@...emloft.net>, javier@...labora.co.uk,
	lennart@...ttering.net, kay.sievers@...y.org,
	alban.crequy@...labora.co.uk, bart.cerneels@...labora.co.uk,
	sjoerd.simons@...labora.co.uk, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/10] af_unix: add multicast and filtering features to
 AF_UNIX

Le jeudi 01 mars 2012 à 14:56 +0100, Javier Martinez Canillas a écrit :

> We could use AF_INET multicast on a local machine but we need some
> ordering and control flow requirements that are not guaranteed on UDP
> multicast over IP. That's why we thought to add a new address family
> AF_MCAST.
> 

It seems application logic and complexity pushed into kernel, for a very
single user (even if used in a lot of products) : D-Bus

> To make it a general local multicast solution and not being too specific
> we added some flags to control its behavior like
> MCAST_MREQ_DROP_WHEN_FULL to decide to either block the sender or drop
> the packet when one receiver has its queue full.

I am only wondering how many lines this is going to add in kernel for a
complete implementation, given your performance expectations, flow
control, reliability, not counting all security issues (ancillary
messages and so on)

In case of IP_MULTICAST_LOOP, we could allow the sender to sleep if
receiver queue is full, with a bit of tweaking in stack (current
implementation uses loopback re-inject, so requires softirq handling).

In fact, we could use a new IP_MULTICAST_LOCAL option, so that sender
processing doesnt trigger a softirq handler at all and is allowed to
sleep if needed. For example skb allocations could use GFP_KERNEL
instead of current GFP_ATOMIC ones in udp mcast .

I dont know, maybe it would be a smaller patch.



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