Avoid to scan the finished Continuation Object

There is no need to scan any continuation Objects, which are marked
finished, plus there could be potential race condition between
continuation concurrent scan and continuation free up for finished
Continuation Object.

Signed-off-by: Lin Hu <linhu@ca.ibm.com>

runtime/gc_base/GCExtensions.cpp

@@ -43,6 +43,7 @@
 #include "ReferenceChainWalkerMarkMap.hpp"
 #include "SublistPool.hpp"
 #include "Wildcard.hpp"
+#include "VMHelpers.hpp"
 
 MM_GCExtensions *
 MM_GCExtensions::newInstance(MM_EnvironmentBase *env)
@@ -311,3 +312,30 @@ MM_GCExtensions::releaseNativesForContinuationObject(MM_EnvironmentBase* env, j9
 	}
 #endif /* JAVA_SPEC_VERSION >= 19 */
 }
+
+bool
+MM_GCExtensions::needScanStacksForContinuationObject(J9VMThread *vmThread, j9object_t objectPtr)
+{
+	bool needScan = false;
+#if JAVA_SPEC_VERSION >= 19
+	jboolean started = J9VMJDKINTERNALVMCONTINUATION_STARTED(vmThread, objectPtr);
+	jboolean finished = J9VMJDKINTERNALVMCONTINUATION_FINISHED(vmThread, objectPtr);
+	J9VMContinuation *continuation = J9VMJDKINTERNALVMCONTINUATION_VMREF(vmThread, objectPtr);
+	/**
+	 * We don't scan mounted continuations:
+	 *
+	 * for concurrent GCs, since stack is actively changing. Instead, we scan them during preMount or during root scanning if already mounted at cycle start or during postUnmount (might be indirectly via card cleaning) or during final STW (via root re-scan) if still mounted at cycle end
+	 * for sliding compacts to avoid double slot fixups
+	 *
+	 * For fully STW GCs, there is no harm to scan them, but it's a waste of time since they are scanned during root scanning already.
+	 *
+	 * We don't scan currently scanned either - one scan is enough.
+	 * we don't scan the continuation object before started and after finished - java stack does not exist.
+	 */
+	if (started && !finished) {
+		Assert_MM_true(NULL != continuation);
+		needScan = !VM_VMHelpers::isContinuationMountedOrConcurrentlyScanned(continuation);
+	}
+#endif /* JAVA_SPEC_VERSION >= 19 */
+	return needScan;
+}

runtime/gc_base/GCExtensions.hpp

@@ -1,6 +1,6 @@
 
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -299,6 +299,16 @@ class MM_GCExtensions : public MM_GCExtensionsBase {
 
 	void releaseNativesForContinuationObject(MM_EnvironmentBase* env, j9object_t objectPtr);
 
+	/**
+	 * Check if we need to scan the java stack for the Continuation Object
+	 * Used during main scan phase of GC (object graph traversal) or heap object iteration (in sliding compact).
+	 * Not meant to be used during root scanning (neither strong roots nor weak roots)!
+	 * @param[in] vmThread the current J9VMThread
+	 * @param[in] continuationObject the continuation object
+	 * @return true if we need to scan the java stack
+	 */
+	static bool needScanStacksForContinuationObject(J9VMThread *vmThread, j9object_t objectPtr);
+
 	/**
 	 * Create a GCExtensions object
 	 */

runtime/gc_glue_java/CompactSchemeFixupObject.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -75,7 +75,7 @@ MM_CompactSchemeFixupObject::fixupContinuationNativeSlots(MM_EnvironmentStandard
 	 * mounted Virtual threads later during root fixup, we will skip it during this heap fixup pass
 	 * (hence passing true for scanOnlyUnmounted parameter).
 	 */
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4CompactSchemeFixupObject localData;
 		localData.compactSchemeFixupObject = this;
 		localData.env = env;

runtime/gc_glue_java/HeapWalkerDelegate.cpp

@@ -1,6 +1,6 @@
 
 /*******************************************************************************
- * Copyright (c) 2022, 2022 IBM Corp. and others
+ * Copyright (c) 2022, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -25,6 +25,7 @@
 
 #include "j9.h"
 #include "EnvironmentBase.hpp"
+#include "GCExtensions.hpp"
 #include "ObjectModel.hpp"
 #include "VMHelpers.hpp"
 #include "VMThreadStackSlotIterator.hpp"
@@ -59,7 +60,7 @@ MM_HeapWalkerDelegate::doContinuationNativeSlots(MM_EnvironmentBase *env, omrobj
 {
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
 
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4HeapWalker localData;
 		localData.heapWalker = _heapWalker;
 		localData.env = env;

runtime/gc_glue_java/MarkingDelegate.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2017, 2022 IBM Corp. and others
+ * Copyright (c) 2017, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -261,7 +261,7 @@ void
 MM_MarkingDelegate::scanContinuationNativeSlots(MM_EnvironmentBase *env, omrobjectptr_t objectPtr)
 {
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4MarkingDelegate localData;
 		localData.markingDelegate = this;
 		localData.env = env;

runtime/gc_glue_java/MetronomeDelegate.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2019, 2022 IBM Corp. and others
+ * Copyright (c) 2019, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -1647,7 +1647,7 @@ void
 MM_MetronomeDelegate::scanContinuationNativeSlots(MM_EnvironmentRealtime *env, J9Object *objectPtr)
 {
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4RealtimeMarkingScheme localData;
 		localData.realtimeMarkingScheme = _markingScheme;
 		localData.env = env;

runtime/gc_glue_java/ScavengerDelegate.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2019, 2022 IBM Corp. and others
+ * Copyright (c) 2019, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -345,7 +345,7 @@ MM_ScavengerDelegate::scanContinuationNativeSlots(MM_EnvironmentStandard *env, o
 	bool shouldRemember = false;
 
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4Scavenge localData;
 		localData.scavengerDelegate = this;
 		localData.env = env;

runtime/gc_vlhgc/CopyForwardScheme.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -2322,7 +2322,7 @@ MMINLINE void
 MM_CopyForwardScheme::scanContinuationNativeSlots(MM_EnvironmentVLHGC *env, MM_AllocationContextTarok *reservingContext, J9Object *objectPtr, ScanReason reason)
 {
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4CopyForward localData;
 		localData.copyForwardScheme = this;
 		localData.env = env;

runtime/gc_vlhgc/GlobalMarkCardScrubber.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -192,7 +192,7 @@ bool MM_GlobalMarkCardScrubber::scrubContinuationNativeSlots(MM_EnvironmentVLHGC
 {
 	bool doScrub = true;
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4GlobalMarkCardScrubber localData;
 		localData.globalMarkCardScrubber = this;
 		localData.env = env;

runtime/gc_vlhgc/GlobalMarkingScheme.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -792,7 +792,7 @@ void
 MM_GlobalMarkingScheme::scanContinuationNativeSlots(MM_EnvironmentVLHGC *env, J9Object *objectPtr, ScanReason reason)
 {
 	J9VMThread *currentThread = (J9VMThread *)env->getLanguageVMThread();
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4GlobalMarkingScheme localData;
 		localData.globalMarkingScheme = this;
 		localData.env = env;

runtime/gc_vlhgc/WriteOnceCompactor.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -1240,7 +1240,7 @@ MM_WriteOnceCompactor::fixupContinuationNativeSlots(MM_EnvironmentVLHGC* env, J9
 	 * mounted Virtual threads later during root fixup, we will skip it during this heap fixup pass
 	 * (hence passing true for scanOnlyUnmounted parameter).
 	 */
-	if (VM_VMHelpers::needScanStacksForContinuation(currentThread, objectPtr)) {
+	if (MM_GCExtensions::needScanStacksForContinuationObject(currentThread, objectPtr)) {
 		StackIteratorData4WriteOnceCompactor localData;
 		localData.writeOnceCompactor = this;
 		localData.env = env;

runtime/oti/VMHelpers.hpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2022 IBM Corp. and others
+ * Copyright (c) 1991, 2023 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -2126,37 +2126,6 @@ class VM_VMHelpers
 		return rc;
 	}
 
-	/**
-	 * Check if we need to scan the java stack for the Continuation Object
-	 * Used during main scan phase of GC (object graph traversal) or heap object iteration (in sliding compact).
-	 * Not meant to be used during root scanning (neither strong roots nor weak roots)!
-	 * @param[in] vmThread the current J9VMThread
-	 * @param[in] continuationObject the continuation object
-	 * @param[in] scanOnlyUnmounted if it is true, only scan unmounted continuation object, default is false
-	 * @return true if we need to scan the java stack
-	 */
-	static VMINLINE bool
-	needScanStacksForContinuation(J9VMThread *vmThread, j9object_t continuationObject)
-	{
-		bool needScan = false;
-#if JAVA_SPEC_VERSION >= 19
-		jboolean started = J9VMJDKINTERNALVMCONTINUATION_STARTED(vmThread, continuationObject);
-		J9VMContinuation *continuation = J9VMJDKINTERNALVMCONTINUATION_VMREF(vmThread, continuationObject);
-		/**
-		 * We don't scan mounted continuations:
-		 *
-		 * for concurrent GCs, since stack is actively changing. Instead, we scan them during preMount or during root scanning if already mounted at cycle start or during postUnmount (might be indirectly via card cleaning) or during final STW (via root re-scan) if still mounted at cycle end
-		 * for sliding compacts to avoid double slot fixups
-		 *
-		 * For fully STW GCs, there is no harm to scan them, but it's a waste of time since they are scanned during root scanning already.
-		 *
-		 * We don't scan currently scanned either - one scan is enough.
-		 */
-		needScan = started && (NULL != continuation) && (!isContinuationMountedOrConcurrentlyScanned(continuation));
-#endif /* JAVA_SPEC_VERSION >= 19 */
-		return needScan;
-	}
-
 	static VMINLINE UDATA
 	setVMState(J9VMThread *currentThread, UDATA newState)
 	{