8251397: NPE on ClassValue.ClassValueMap.cacheArray

Add release fence to ClassValueMap constructor.

* Release fence guarantees that cacheArray field will published
with a non-null value.
* Without this fix, CacheValueMap.cacheArray can sometimes be
seen as null.

Reviewed-by: shade, psandoz

Author: galderz

src/java.base/share/classes/java/lang/ClassValue.java

@@ -29,6 +29,8 @@
 import java.lang.ref.WeakReference;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import jdk.internal.misc.Unsafe;
+
 import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
 import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;
 
@@ -369,12 +371,22 @@ private static ClassValueMap getMap(Class<?> type) {
     }
 
     private static final Object CRITICAL_SECTION = new Object();
+    private static final Unsafe UNSAFE = Unsafe.getUnsafe();
     private static ClassValueMap initializeMap(Class<?> type) {
         ClassValueMap map;
         synchronized (CRITICAL_SECTION) {  // private object to avoid deadlocks
             // happens about once per type
-            if ((map = type.classValueMap) == null)
-                type.classValueMap = map = new ClassValueMap();
+            if ((map = type.classValueMap) == null) {
+                map = new ClassValueMap();
+                // Place a Store fence after construction and before publishing to emulate
+                // ClassValueMap containing final fields. This ensures it can be
+                // published safely in the non-volatile field Class.classValueMap,
+                // since stores to the fields of ClassValueMap will not be reordered
+                // to occur after the store to the field type.classValueMap
+                UNSAFE.storeFence();
+
+                type.classValueMap = map;
+            }
         }
         return map;
     }