dpif-netdev: Reorder flow mark assignment.

During code-review, it seems like it could be possible to publish a
flow into the cmap, and the hw offload thread assistance could miss
the appropriate flow.  This situation probably didn't occur much in
practice, and the result might be just a dropped packet (although, I'm
not sure that it couldn't also result in a weird duplicate flow
getting installed due to the check in mark_to_flow_find that gets
called during dfc_processing).

This change reorders the assignment to before the cmap_insert.  The
cmap_insert should act as a barrier in this case to ensure that the
write to flow->mark is ready before the read (after the
CMAP_FOR_EACH_WITH_HASH).

Fixes: 241bad1 ("dpif-netdev: associate flow with a mark id")
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Aaron Conole <aconole@redhat.com>

Author: apconole

lib/dpif-netdev.c

@@ -2600,10 +2600,10 @@ mark_to_flow_associate(const uint32_t mark, struct dp_netdev_flow *flow)
     unsigned int tid = netdev_offload_thread_id();
     dp_netdev_flow_ref(flow);
 
+    flow->mark = mark;
     cmap_insert(&dp_offload_threads[tid].mark_to_flow,
                 CONST_CAST(struct cmap_node *, &flow->mark_node),
                 hash_int(mark, 0));
-    flow->mark = mark;
 
     VLOG_DBG("Associated dp_netdev flow %p with mark %u mega_ufid "UUID_FMT,
              flow, mark, UUID_ARGS((struct uuid *) &flow->mega_ufid));