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

[David Miller <davem@davemloft.net>]

From: Christoph Lameter <cl@linux-foundation.org>
Date: Thu, 11 Jun 2009 11:07:52 -0400 (EDT)

> On Tue, 9 Jun 2009, David Miller wrote:
> 
>> 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.
> 
> Umm?? My patch does that....

No it does not.  It eliminates a limit on the generic intermediate
queue.  It did no blocking, no sleeping, or the process when it
joined the group.

I was suggesting to block the process, at socket context time, when it
joins a multicast group.  That way it wouldn't be able to do any sends
until IPoIB could resolve the multicast join.