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

[Eric Dumazet]

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 ;)



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/