netdev - Re: [PATCH 4/9] AF_UNIX: find the recipients for multicast messages

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Message-ID: <1290531400.3046.117.camel@edumazet-laptop>
Date:	Tue, 23 Nov 2010 17:56:40 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alban Crequy <alban.crequy@...labora.co.uk>
Cc:	David Miller <davem@...emloft.net>, shemminger@...tta.com,
	gorcunov@...nvz.org, adobriyan@...il.com, lennart@...ttering.net,
	kay.sievers@...y.org, ian.molton@...labora.co.uk,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/9] AF_UNIX: find the recipients for multicast messages

Hmm, just thought about lockless again, and we had same multicast
problem on udp as well.

We are forced to hold a lock (to forbid concurrent deletes), or we might
going through one 'about to be removed' socket and abort the iteration
in the middle. Some sockets would not receive a copy of the message.

(UDP sockets using SLAB_DESTROY_BY_RCU, we could even have multiple
copies sent to some sockets, if the removed socket is re-inserted in
front of chain because of instant reuse)

To have a true lockless path, you would need to restart the full scan if
you notice a delete was done during the iteration, eventually using a
sequence number per chain. That would be expensive, because you have to
undo all the socket accumulation (refcount) done during the lookup,
before restart the thing.

To avoid starvation, getting a lock at the second iteration would be
good ;)

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