Add missing fence and assert ownsGil() where appropriate

Author: steve-s

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/hpy/GraalHPyContext.java

@@ -47,6 +47,7 @@
 import static com.oracle.graal.python.util.PythonUtils.tsArray;
 
 import java.io.IOException;
+import java.lang.invoke.VarHandle;
 import java.lang.ref.Reference;
 import java.lang.ref.ReferenceQueue;
 import java.lang.ref.WeakReference;
@@ -497,7 +498,9 @@ public void execute(PythonContext context) {
     private RootCallTarget getReferenceCleanerCallTarget() {
         if (referenceCleanerCallTarget == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
-            referenceCleanerCallTarget = PythonUtils.getOrCreateCallTarget(new HPyNativeSpaceCleanerRootNode(getContext()));
+            RootCallTarget localTarget = PythonUtils.getOrCreateCallTarget(new HPyNativeSpaceCleanerRootNode(getContext()));
+            VarHandle.storeStoreFence();
+            referenceCleanerCallTarget = localTarget;
         }
         return referenceCleanerCallTarget;
     }

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/code/PCode.java

@@ -591,6 +591,7 @@ public Signature getSignature(Node inliningTarget, InlinedConditionProfile signa
 
     @TruffleBoundary
     synchronized Signature initializeSignature(RootCallTarget rootCallTarget) {
+        assert PythonContext.get(null).ownsGil(); // otherwise this is racy
         if (signature == null) {
             if (rootCallTarget.getRootNode() instanceof PRootNode) {
                 signature = ((PRootNode) rootCallTarget.getRootNode()).getSignature();
@@ -610,6 +611,7 @@ private RootCallTarget getRootCallTarget() {
 
     @TruffleBoundary
     synchronized RootCallTarget initializeCallTarget() {
+        assert PythonContext.get(null).ownsGil(); // otherwise this is racy
         if (callTarget == null) {
             callTarget = (RootCallTarget) callTargetSupplier.get();
             callTargetSupplier = null;