dpif-netdev: Fix rare flow add race condition.

Before this patch, dp_netdev_flow_add() inserted newly minted flows in
the "flow_table" cmap before inserting them into the per core "dpcls"
classifier.  Since dpcls_insert() initializes 'flow->cr.mask', there's
a brief window where the flow is accessible from the cmap, but has a
bogus mask value.

In my testing, under rare instances (i.e. once every 20 minutes with a
very specific flow table and traffic pattern), revalidators core dump
when they call dpif_netdev_flow_dump_next(), which accesses this bogus
mask value from dp_netdev_flow_to_dpif_flow().

By inserting into the per core classifier before the cmap, all the
values are guaranteed to be initialized during flow dumps.  With this
patch, I can no longer reproduce the crash.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>

lib/dpif-netdev.c

@@ -1742,12 +1742,12 @@ dp_netdev_flow_add(struct dp_netdev_pmd_thread *pmd,
     ovs_refcount_init(&flow->ref_cnt);
     ovsrcu_set(&flow->actions, dp_netdev_actions_create(actions, actions_len));
 
-    cmap_insert(&pmd->flow_table,
-                CONST_CAST(struct cmap_node *, &flow->node),
-                dp_netdev_flow_hash(&flow->ufid));
     netdev_flow_key_init_masked(&flow->cr.flow, &match->flow, &mask);
     dpcls_insert(&pmd->cls, &flow->cr, &mask);
 
+    cmap_insert(&pmd->flow_table, CONST_CAST(struct cmap_node *, &flow->node),
+                dp_netdev_flow_hash(&flow->ufid));
+
     if (OVS_UNLIKELY(VLOG_IS_DBG_ENABLED())) {
         struct match match;
         struct ds ds = DS_EMPTY_INITIALIZER;