Make is_oop() MT-safe

src/hotspot/share/gc/shared/collectedHeap.cpp

@@ -229,13 +229,20 @@ bool CollectedHeap::is_oop(oop object) const {
     return false;
   }
 
-  // With compact headers, we can't safely access the Klass* when
-  // the object has been forwarded, because non-full-GC-forwarding
-  // temporarily overwrites the mark-word, and thus the Klass*, with
-  // the forwarding pointer, and here we have no way to make a
-  // distinction between Full-GC and regular GC forwarding.
-  bool can_access_klass = !UseCompactObjectHeaders || !object->is_forwarded();
-  if (can_access_klass && !Metaspace::contains(object->klass_without_asserts())) {
+  if (UseCompactObjectHeaders) {
+    // With compact headers, we can't safely access the Klass* when
+    // the object has been forwarded, because non-full-GC-forwarding
+    // temporarily overwrites the mark-word, and thus the Klass*, with
+    // the forwarding pointer, and here we have no way to make a
+    // distinction between Full-GC and regular GC forwarding.
+    markWord mark = object->mark();
+    if (!mark.is_forwarded()) {
+      Klass* klass = mark.klass();
+      if (!Metaspace::contains(mark.klass_without_asserts())) {
+        return false;
+      }
+    }
+  } else if (!Metaspace::contains(object->klass_without_asserts())) {
     return false;
   }