Improve Java volatile field support

Author: pitr-ch

ext/com/concurrent_ruby/ext/SynchronizationLibrary.java

@@ -45,8 +45,6 @@ public IRubyObject allocate(Ruby runtime, RubyClass klazz) {
     @JRubyClass(name = "JavaObject", parent = "AbstractObject")
     public static class JavaObject extends RubyObject {
 
-        private volatile int anVolatileField = 0; // TODO unused on JAVA8
-
         public JavaObject(Ruby runtime, RubyClass metaClass) {
             super(runtime, metaClass);
         }
@@ -118,24 +116,40 @@ public IRubyObject ensureIvarVisibilityBang(ThreadContext context) {
             return context.nil;
         }
 
+        private volatile int anVolatileField = 0; // TODO unused on JAVA8
+        public static final long AN_VOLATILE_FIELD_OFFSET =
+                UnsafeHolder.fieldOffset(JavaObject.class, "anVolatileField");
+
         @JRubyMethod(name = "instance_variable_get_volatile", visibility = Visibility.PROTECTED)
         public IRubyObject instanceVariableGetVolatile(ThreadContext context, IRubyObject name) {
-            int tmp;
-            if (UnsafeHolder.SUPPORTS_FENCES)
+            if (UnsafeHolder.U == null) {
+                synchronized (this) {
+                    return instance_variable_get(context, name);
+                }
+            } else if (UnsafeHolder.SUPPORTS_FENCES) {
                 UnsafeHolder.loadFence();
-            else
-                tmp = anVolatileField;
-            return instance_variable_get(context, name);
+                return instance_variable_get(context, name);
+            } else {
+                UnsafeHolder.U.getIntVolatile(this, AN_VOLATILE_FIELD_OFFSET);
+                return instance_variable_get(context, name);
+            }
         }
 
         @JRubyMethod(name = "instance_variable_set_volatile", visibility = Visibility.PROTECTED)
         public IRubyObject InstanceVariableSetVolatile(ThreadContext context, IRubyObject name, IRubyObject value) {
-            IRubyObject result = instance_variable_set(name, value);
-            if (UnsafeHolder.SUPPORTS_FENCES)
+            if (UnsafeHolder.U == null) {
+                synchronized (this) {
+                    return instance_variable_set(name, value);
+                }
+            } else if (UnsafeHolder.SUPPORTS_FENCES) {
+                IRubyObject result = instance_variable_set(name, value);
                 UnsafeHolder.storeFence();
-            else
-                anVolatileField = 1;
-            return result;
+                return result;
+            } else {
+                UnsafeHolder.U.putIntVolatile(this, AN_VOLATILE_FIELD_OFFSET, 1);
+                IRubyObject result = instance_variable_set(name, value);
+                return result;
+            }
         }
     }
 }

lib/concurrent/synchronization/java_object.rb

@@ -6,10 +6,6 @@ module Synchronization
     if Concurrent.on_jruby?
       require 'jruby'
 
-      unless org.jruby.util.unsafe.UnsafeHolder::SUPPORTS_FENCES
-        raise 'java7 is not supported at the moment, please use java8'
-      end
-
       class JavaObject < AbstractObject
 
         def self.attr_volatile(*names)