[ovs-dev] [PATCH ovn v2] ofctrl.c: Avoid repeatedly linking an installed flow and a desired flow.

[Han Zhou]

In update_installed_flows_by_compare() there are two loops. The first
loop iterates the installed flows and find its peer in desired flows to:

1. uninstall flows that are not needed anymore

2. update flows if needed

At the same time, it links the desired flow found for the installed flow
which also set the desired flow as the current active installed flow.

The second loop iterates the desired flows and find its peer in installed
flows to install missing flows. At the same time it will detect if there
are conflict desired flows matching same installed flow then just link
them.

However, currently in the second loop, it blindly link the desired flows to the
installed flows, without checking if it is already linked in the first loop.
Lucky enough, this won't cause any real problem so far, because when there are
conflict flows, the one found in the first loop will be set as active in the
installed_flow, and in the function link_installed_to_desired() checks if it is
already the active desired flow it just does nothing but return.  However, the
check in the link_installed_to_desired() is confusing because a desired_flow
may be linked to the installed_flow already but not the active flow, and the
check is insufficient. It should be rather an assertion and let the caller
ensure that a pair of desired_flow and installed_flow is never linked twice.

For the above reason, this patch does the following changes:

1. Removes the check in link_installed_to_desired() and convert it to an assert.

2. Before calling link_installed_to_desired() in the above mentioned loop,
   check if the desired flow is already installed.

Signed-off-by: Han Zhou <hzhou at ovn.org>
---
v1 -> v2: Addresses Dumitru's comment.
    - Updated the check from checking if the desired flow is active to checking
      if the desired flow is linked to the installed flow.
    - Removed the first check that compares the desired flow with the active
      desired flow in link_installed_to_desired().
    - Updated commit message.

 controller/ofctrl.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/controller/ofctrl.c b/controller/ofctrl.c
index e725c003c..4425d9894 100644
--- a/controller/ofctrl.c
+++ b/controller/ofctrl.c
@@ -806,13 +806,18 @@ desired_flow_set_active(struct desired_flow *d)
     d->installed_flow->desired_flow = d;
 }
 
+/* Adds the desired flow to the list of desired flows that have same match
+ * conditions as the installed flow.
+ *
+ * If the newly added desired flow is the first one in the list, it is also set
+ * as the active one.
+ *
+ * It is caller's responsibility to make sure the link between the pair didn't
+ * exist before. */
 static void
 link_installed_to_desired(struct installed_flow *i, struct desired_flow *d)
 {
-    if (i->desired_flow == d) {
-        return;
-    }
-
+    ovs_assert(i->desired_flow != d);
     if (ovs_list_is_empty(&i->desired_refs)) {
         ovs_assert(!i->desired_flow);
         i->desired_flow = d;
@@ -1705,8 +1710,12 @@ update_installed_flows_by_compare(struct ovn_desired_flow_table *flow_table,
             /* Copy 'd' from 'flow_table' to installed_flows. */
             i = installed_flow_dup(d);
             hmap_insert(&installed_flows, &i->match_hmap_node, i->flow.hash);
+            link_installed_to_desired(i, d);
+        } else if (!d->installed_flow) {
+            /* This is a desired_flow that conflicts with one installed
+             * previously but not linked yet. */
+            link_installed_to_desired(i, d);
         }
-        link_installed_to_desired(i, d);
     }
 }
 
-- 
2.26.2