8210233: Prepare Klass::is_loader_alive() for concurrent class unloading

Reviewed-by: coleenp, pliden

src/hotspot/share/classfile/classLoaderData.hpp

@@ -336,6 +336,8 @@ class ClassLoaderData : public CHeapObj<mtClass> {
   bool claimed() const { return _claimed == 1; }
   bool claim();
 
+  // Computes if the CLD is alive or not. This is safe to call in concurrent
+  // contexts.
   bool is_alive() const;
 
   // Accessors
@@ -377,6 +379,9 @@ class ClassLoaderData : public CHeapObj<mtClass> {
   inline oop class_loader() const;
 
   // Returns true if this class loader data is for a loader going away.
+  // Note that this is only safe after the GC has computed if the CLD is
+  // unloading or not. In concurrent contexts where there are no such
+  // guarantees, is_alive() should be used instead.
   bool is_unloading() const     {
     assert(!(is_the_null_class_loader_data() && _unloading), "The null class loader can never be unloaded");
     return _unloading;

src/hotspot/share/classfile/classLoaderHierarchyDCmd.cpp

@@ -470,7 +470,7 @@ public:
   void do_cld (ClassLoaderData* cld) {
 
     // We do not display unloading loaders, for now.
-    if (cld->is_unloading()) {
+    if (!cld->is_alive()) {
       return;
     }
 

src/hotspot/share/oops/klass.hpp

@@ -656,8 +656,9 @@ protected:
   virtual MetaspaceObj::Type type() const { return ClassType; }
 
   // Iff the class loader (or mirror for unsafe anonymous classes) is alive the
-  // Klass is considered alive.  Has already been marked as unloading.
-  bool is_loader_alive() const { return !class_loader_data()->is_unloading(); }
+  // Klass is considered alive. This is safe to call before the CLD is marked as
+  // unloading, and hence during concurrent class unloading.
+  bool is_loader_alive() const { return class_loader_data()->is_alive(); }
 
   // Load the klass's holder as a phantom. This is useful when a weak Klass
   // pointer has been "peeked" and then must be kept alive before it may