# Re: \[PATCH\]\[FIX\] ROCE Multicast: Do not send IGMP leaves for sendonly Multicast groups

- From: Jason Gunthorpe \<jgg@nvidia.com\>
- Date: 2021-09-14 18:57:48 
- List: linux-rdma
- Page: https://ratatoskr.run/linux-rdma/2021/09/15465007
- Thread: https://ratatoskr.run/linux-rdma/2021/09/15464921/t.md

```text
On Wed, Sep 08, 2021 at 01:43:28PM +0200, Christoph Lameter wrote:
> ROCE uses IGMP for Multicast instead of the native Infiniband system where
> joins are required in order to post messages on the Multicast group.
> On Ethernet one can send Multicast messages to arbitrary addresses
> without the need to subscribe to a group.
> 
> So ROCE correctly does not send IGMP joins during rdma_join_multicast().
> 
> F.e. in cma_iboe_join_multicast() we see:
> 
>    if (addr->sa_family == AF_INET) {
>                 if (gid_type == IB_GID_TYPE_ROCE_UDP_ENCAP) {
>                         ib.rec.hop_limit = IPV6_DEFAULT_HOPLIMIT;
>                         if (!send_only) {
>                                 err = cma_igmp_send(ndev, &ib.rec.mgid,
>                                                     true);
>                         }
>                 }
>         } else {
> 
> So the IGMP join is suppressed as it is unnecessary.
> 
> However no such check is done in destroy_mc(). And therefore leaving a
> sendonly multicast group will send an IGMP leave.
> 
> This means that the following scenario can lead to a multicast receiver
> unexpectedly being unsubscribed from a MC group:
> 
> 
> 1. Sender thread does a sendonly join on MC group X. No IGMP join
>    is sent.
> 
> 2. Receiver thread does a regular join on the same MC Group x.
>    IGMP join is sent and the receiver begins to get messages.
> 
> 3. Sender thread terminates and destroys MC group X.
>    IGMP leave is sent and the receiver no longer receives data.
> 
> This patch adds the same logic for sendonly joins to destroy_mc()
> that is also used in cma_iboe_join_multicast().
> 
> Signed-off-by: Christoph Lameter <redacted>

I added the missing fixes line:

    Fixes: ab15c95a17b3 ("IB/core: Support for CMA multicast join flags")

Applied to for-rc, thanks

Jason
```