netlink-socket: Async notifications are incompatible with other operations.

A Netlink socket that receives asynchronous notifications (e.g. from a
multicast group) cannot be used for transactions or dumps, because those
operations would discard asynchronous messages that arrive while waiting
for replies.

This commit documents this issue in a comment on nl_sock_join_mcgroup().
It also removes an internal attempt to avoid mixing multicast reception
with other operations.  The attempt was incomplete, because it only
handled dumps even though ordinary transactions are also problematic.  It
seems better to remove it than to fix it because, first, all of the
existing users in OVS already separate multicast reception from other
operations and, second, an upcoming commit will start using unicast
Netlink for asynchronous notifications, which has the same issues but
doesn't use nl_sock_join_mcgroup().

diff --git a/lib/netlink-socket.c b/lib/netlink-socket.c
index f4635c8..8f9c3d5 100644 (file)
--- a/lib/netlink-socket.c
+++ b/lib/netlink-socket.c
@@ -62,7 +62,6 @@ struct nl_sock
     int fd;
     uint32_t pid;
     int protocol;
-    bool any_groups;
     struct nl_dump *dump;
 };
 
@@ -92,7 +91,6 @@ nl_sock_create(int protocol, struct nl_sock **sockp)
         goto error;
     }
     sock->protocol = protocol;
-    sock->any_groups = false;
     sock->dump = NULL;
 
     retval = alloc_pid(&sock->pid);
@@ -164,6 +162,10 @@ nl_sock_destroy(struct nl_sock *sock)
 /* Tries to add 'sock' as a listener for 'multicast_group'.  Returns 0 if
  * successful, otherwise a positive errno value.
  *
+ * A socket that is subscribed to a multicast group that receives asynchronous
+ * notifications must not be used for Netlink transactions or dumps, because
+ * transactions and dumps can cause notifications to be lost.
+ *
  * Multicast group numbers are always positive.
  *
  * It is not an error to attempt to join a multicast group to which a socket
@@ -181,7 +183,6 @@ nl_sock_join_mcgroup(struct nl_sock *sock, unsigned int multicast_group)
                   multicast_group, strerror(errno));
         return errno;
     }
-    sock->any_groups = true;
     return 0;
 }
 
@@ -536,10 +537,9 @@ nl_dump_start(struct nl_dump *dump,
     nlmsghdr->nlmsg_flags |= NLM_F_DUMP | NLM_F_ACK;
     dump->seq = nlmsghdr->nlmsg_seq;
     dump->buffer = NULL;
-    if (sock->any_groups || sock->dump) {
-        /* 'sock' might belong to some multicast group, or it already has an
-         * ongoing dump.  Clone the socket to avoid possibly intermixing
-         * multicast messages or previous dump results with our results. */
+    if (sock->dump) {
+        /* 'sock' already has an ongoing dump.  Clone the socket because
+         * Netlink only allows one dump at a time. */
         dump->status = nl_sock_clone(sock, &dump->sock);
         if (dump->status) {
             return;