[ovs-dev] [PATCH] lib/classifier: Minimize critical section.

[Jarno Rajahalme]

classifier_find_rule_exactly() only needs the mutex to protect the
list traversal.  Subtables are already RCU protected.

Locking here can be eliminated completely when RCU list is available.

Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
---
 lib/classifier.c |   20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/lib/classifier.c b/lib/classifier.c
index f30ba95..4a2fd17 100644
--- a/lib/classifier.c
+++ b/lib/classifier.c
@@ -114,8 +114,7 @@ cls_match_alloc(struct cls_rule *rule)
 }
 
 static struct cls_subtable *find_subtable(const struct classifier *cls,
-                                          const struct minimask *)
-    OVS_REQUIRES(cls->mutex);
+                                          const struct minimask *);
 static struct cls_subtable *insert_subtable(struct classifier *cls,
                                             const struct minimask *)
     OVS_REQUIRES(cls->mutex);
@@ -1090,28 +1089,26 @@ classifier_find_rule_exactly(const struct classifier *cls,
     struct cls_match *head, *rule;
     struct cls_subtable *subtable;
 
-    ovs_mutex_lock(&cls->mutex);
     subtable = find_subtable(cls, &target->match.mask);
-    if (!subtable) {
-        goto out;
-    }
-
-    /* Skip if there is no hope. */
-    if (target->priority > subtable->max_priority) {
-        goto out;
+    if (!subtable || target->priority > subtable->max_priority) {
+        return NULL;
     }
 
     head = find_equal(subtable, &target->match.flow,
                       miniflow_hash_in_minimask(&target->match.flow,
                                                 &target->match.mask, 0));
+
+
+    /* Use RCU list instead of locking when one is available. */
+    ovs_mutex_lock(&cls->mutex);
     FOR_EACH_RULE_IN_LIST (rule, head) {
         if (target->priority >= rule->priority) {
             ovs_mutex_unlock(&cls->mutex);
             return target->priority == rule->priority ? rule->cls_rule : NULL;
         }
     }
-out:
     ovs_mutex_unlock(&cls->mutex);
+
     return NULL;
 }
 
@@ -1339,7 +1336,6 @@ cls_cursor_advance(struct cls_cursor *cursor)
 
 static struct cls_subtable *
 find_subtable(const struct classifier *cls, const struct minimask *mask)
-    OVS_REQUIRES(cls->mutex)
 {
     struct cls_subtable *subtable;
 
-- 
1.7.10.4