tipc: receive group membership events via member socket

Like with any other service, group members' availability can be
subscribed for by connecting to be topology server. However, because
the events arrive via a different socket than the member socket, there
is a real risk that membership events my arrive out of synch with the
actual JOIN/LEAVE action. I.e., it is possible to receive the first
messages from a new member before the corresponding JOIN event arrives,
just as it is possible to receive the last messages from a leaving
member after the LEAVE event has already been received.

Since each member socket is internally also subscribing for membership
events, we now fix this problem by passing those events on to the user
via the member socket. We leverage the already present member synch-
ronization protocol to guarantee correct message/event order. An event
is delivered to the user as an empty message where the two source
addresses identify the new/lost member. Furthermore, we set the MSG_OOB
bit in the message flags to mark it as an event. If the event is an
indication about a member loss we also set the MSG_EOR bit, so it can
be distinguished from a member addition event.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/tipc.h b/include/uapi/linux/tipc.h
index 5f7b2c4a09ab..ef41c11a7f38 100644
--- a/include/uapi/linux/tipc.h
+++ b/include/uapi/linux/tipc.h
@@ -238,6 +238,7 @@ struct sockaddr_tipc {
  * Flag values
  */
 #define TIPC_GROUP_LOOPBACK     0x1  /* Receive copy of sent msg when match */
+#define TIPC_GROUP_MEMBER_EVTS  0x2  /* Receive membership events in socket */
 
 struct tipc_group_req {
 	__u32 type;      /* group id */