8199462: Use Reference.reachabilityFence in direct ByteBuffer methods

Reviewed-by: psandoz, vlivanov, plevart

src/java.base/share/classes/java/nio/MappedByteBuffer.java

@@ -26,6 +26,7 @@
 package java.nio;
 
 import java.io.FileDescriptor;
+import java.lang.ref.Reference;
 import jdk.internal.misc.Unsafe;
 
 
@@ -166,9 +167,15 @@ public abstract class MappedByteBuffer
         int count = Bits.pageCount(length);
         long a = mappingAddress(offset);
         byte x = 0;
-        for (int i=0; i<count; i++) {
-            x ^= unsafe.getByte(a);
-            a += ps;
+        try {
+            for (int i=0; i<count; i++) {
+                // TODO consider changing to getByteOpaque thus avoiding
+                // dead code elimination and the need to calculate a checksum
+                x ^= unsafe.getByte(a);
+                a += ps;
+            }
+        } finally {
+            Reference.reachabilityFence(this);
         }
         if (unused != 0)
             unused = x;