add comment wrt unnecessary (at this point) fix for #738

Author: cowtowncoder

src/main/java/com/fasterxml/jackson/databind/SerializerProvider.java

@@ -1380,6 +1380,10 @@ protected JsonSerializer<Object> _createUntypedSerializer(JavaType type)
          *    since there's one instance per serialization).
          *   Perhaps not-yet-resolved instance might be exposed too early to callers.
          */
+        // 13-Apr-2018, tatu: Problem does NOT occur any more with late 2.8.x and 2.9.x
+        //    versions, likely due to concurrency fixes for `AnnotatedClass` introspection.
+        //    This sync block could probably be removed; but to minimize any risk of
+        //    regression sync block will only be removed from 3.0.
         synchronized (_serializerCache) {
             // 17-Feb-2013, tatu: Used to call deprecated method (that passed property)
             return (JsonSerializer<Object>)_serializerFactory.createSerializer(this, type);

src/test/java/com/fasterxml/jackson/databind/misc/RaceCondition738Test.java

@@ -56,7 +56,7 @@ public HasSubTypes getHasSubTypes() {
      */
 
     public void testRepeatedly() throws Exception {
-        final int COUNT = 2000;
+        final int COUNT = 3000;
         for (int i = 0; i < COUNT; i++) {
             runOnce(i, COUNT);
         }