Re: IPoIB: Fix multicast packet drops before join is complete

[David Miller <davem@davemloft.net>]

From: Roland Dreier <rdreier@cisco.com>
Date: Tue, 09 Jun 2009 20:55:57 -0700

> Hmm... how do I do that?  The interface gets an skb that it sees should
> be sent to a multicast group that it is not a member of yet, and so it
> fires off a request to join that group (as a send-only member).  How
> does a netdev block the process that queued up a given skb to send?
> Couldn't the packet have come through a local software bridge or
> something like that, so the original process is long since lost to the
> network stack?

If a facility doesn't exist yet, we're going to have to create
one.  It would need to do a downcall to the device when the
user joins a multicast group on a socket, and then the device
can do whatever magic is necessary to speak multicast immediately
and sleep until it really is available for immediate use.