8256811: Delayed/missed jdwp class unloading events

Reviewed-by: andrew
Backport-of: 54854d9300479c22c416fd9d2fdb5c29fc1884bc

Author: adinn

src/jdk.jdwp.agent/share/native/libjdwp/classTrack.c

@@ -37,6 +37,7 @@
 #include "util.h"
 #include "bag.h"
 #include "classTrack.h"
+#include "eventHandler.h"
 
 #define NOT_TAGGED 0
 
@@ -46,64 +47,14 @@
 static jvmtiEnv* trackingEnv;
 
 /*
- * A bag containing all the deleted classes' signatures. Must be accessed under
- * classTrackLock.
+ * Invoke the callback when classes are freed.
  */
-struct bag* deletedSignatures;
-
-/*
- * Lock to keep integrity of deletedSignatures.
- */
-static jrawMonitorID classTrackLock;
-
-/*
- * Invoke the callback when classes are freed, find and record the signature
- * in deletedSignatures. Those are only used in addPreparedClass() by the
- * same thread.
- */
-static void JNICALL
+void JNICALL
 cbTrackingObjectFree(jvmtiEnv* jvmti_env, jlong tag)
 {
-    debugMonitorEnter(classTrackLock);
-    if (deletedSignatures == NULL) {
-      debugMonitorExit(classTrackLock);
-      return;
-    }
-    *(char**)bagAdd(deletedSignatures) = (char*)jlong_to_ptr(tag);
-
-    debugMonitorExit(classTrackLock);
-}
-
-/*
- * Called after class unloads have occurred.
- * The signatures of classes which were unloaded are returned.
- */
-struct bag *
-classTrack_processUnloads(JNIEnv *env)
-{
-    if (deletedSignatures == NULL) {
-      return NULL;
-    }
-
-    /* Allocate new bag outside classTrackLock lock to avoid deadlock.
-     *
-     * Note: jvmtiAllocate/jvmtiDeallocate() may be blocked by ongoing safepoints.
-     * It is dangerous to call them (via bagCreateBag/bagDestroyBag()) while holding monitor(s),
-     * because jvmti may post events, e.g. JVMTI_EVENT_OBJECT_FREE at safepoints and event processing
-     * code may acquire the same monitor(s), e.g. classTrackLock in cbTrackingObjectFree(),
-     * which can lead to deadlock.
-     */
-    struct bag* new_bag = bagCreateBag(sizeof(char*), 10);
-    debugMonitorEnter(classTrackLock);
-    struct bag* deleted = deletedSignatures;
-    deletedSignatures = new_bag;
-    debugMonitorExit(classTrackLock);
-    return deleted;
+    eventHandler_synthesizeUnloadEvent((char*)jlong_to_ptr(tag), getEnv());
 }
 
-/*
- * Add a class to the prepared class table.
- */
 void
 classTrack_addPreparedClass(JNIEnv *env_unused, jclass klass)
 {
@@ -162,8 +113,6 @@ setupEvents()
 void
 classTrack_initialize(JNIEnv *env)
 {
-    deletedSignatures = NULL;
-    classTrackLock = debugMonitorCreate("Deleted class tag lock");
     trackingEnv = getSpecialJvmti();
     if (trackingEnv == NULL) {
         EXIT_ERROR(AGENT_ERROR_INTERNAL, "Failed to allocate tag-tracking jvmtiEnv");
@@ -195,44 +144,3 @@ classTrack_initialize(JNIEnv *env)
         EXIT_ERROR(error,"loaded classes array");
     }
 }
-
-/*
- * Called to activate class-tracking when a listener registers for EI_GC_FINISH.
- */
-void
-classTrack_activate(JNIEnv *env)
-{
-    // Allocate bag outside classTrackLock lock to avoid deadlock.
-    // See comments in classTrack_processUnloads() for details.
-    struct bag* new_bag = bagCreateBag(sizeof(char*), 1000);
-    debugMonitorEnter(classTrackLock);
-    deletedSignatures = new_bag;
-    debugMonitorExit(classTrackLock);
-}
-
-static jboolean
-cleanDeleted(void *signatureVoid, void *arg)
-{
-    char* sig = *(char**)signatureVoid;
-    jvmtiDeallocate(sig);
-    return JNI_TRUE;
-}
-
-/*
- * Called when agent detaches.
- */
-void
-classTrack_reset(void)
-{
-    debugMonitorEnter(classTrackLock);
-    struct bag* to_delete = deletedSignatures;
-    deletedSignatures = NULL;
-    debugMonitorExit(classTrackLock);
-
-    // Deallocate bag outside classTrackLock to avoid deadlock.
-    // See comments in classTrack_processUnloads() for details.
-    if (to_delete != NULL) {
-      bagEnumerateOver(to_delete, cleanDeleted, NULL);
-      bagDestroyBag(to_delete);
-    }
-}

src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c

@@ -792,7 +792,6 @@ debugInit_reset(JNIEnv *env)
     threadControl_reset();
     util_reset();
     commonRef_reset(env);
-    classTrack_reset();
 
     /*
      * If this is a server, we are now ready to accept another connection.

src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c

@@ -457,16 +457,10 @@ reportEvents(JNIEnv *env, jbyte sessionID, jthread thread, EventIndex ei,
     }
 }
 
-/* A bagEnumerateFunction.  Create a synthetic class unload event
- * for every class no longer present.  Analogous to event_callback
- * combined with a handler in a unload specific (no event
- * structure) kind of way.
- */
-static jboolean
-synthesizeUnloadEvent(void *signatureVoid, void *envVoid)
+/* Create a synthetic class unload event for the specified signature. */
+jboolean
+eventHandler_synthesizeUnloadEvent(char *signature, JNIEnv *env)
 {
-    JNIEnv *env = (JNIEnv *)envVoid;
-    char *signature = *(char **)signatureVoid;
     char *classname;
     HandlerNode *node;
     jbyte eventSessionID = currentSessionID;
@@ -560,39 +554,10 @@ event_callback(JNIEnv *env, EventInfo *evinfo)
     currentException = JNI_FUNC_PTR(env,ExceptionOccurred)(env);
     JNI_FUNC_PTR(env,ExceptionClear)(env);
 
-    /* See if a garbage collection finish event happened earlier.
-     *
-     * Note: The "if" is an optimization to avoid entering the lock on every
-     *       event; garbageCollected may be zapped before we enter
-     *       the lock but then this just becomes one big no-op.
-     */
-    if ( garbageCollected > 0 ) {
-        struct bag *unloadedSignatures = NULL;
-
-        /* We want to compact the hash table of all
-         * objects sent to the front end by removing objects that have
-         * been collected.
-         */
+    /* See if a garbage collection finish event happened earlier. */
+    if ( garbageCollected > 0) {
         commonRef_compact();
-
-        /* We also need to simulate the class unload events. */
-
-        debugMonitorEnter(handlerLock);
-
-        /* Clear garbage collection counter */
         garbageCollected = 0;
-
-        /* Analyze which class unloads occurred */
-        unloadedSignatures = classTrack_processUnloads(env);
-
-        debugMonitorExit(handlerLock);
-
-        /* Generate the synthetic class unload events and/or just cleanup.  */
-        if ( unloadedSignatures != NULL ) {
-            (void)bagEnumerateOver(unloadedSignatures, synthesizeUnloadEvent,
-                             (void *)env);
-            bagDestroyBag(unloadedSignatures);
-        }
     }
 
     thread = evinfo->thread;
@@ -1627,9 +1592,6 @@ installHandler(HandlerNode *node,
 
     node->handlerID = external? ++requestIdCounter : 0;
     error = eventFilterRestricted_install(node);
-    if (node->ei == EI_GC_FINISH) {
-        classTrack_activate(getEnv());
-    }
     if (error == JVMTI_ERROR_NONE) {
         insert(getHandlerChain(node->ei), node);
     }

src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.h

@@ -76,6 +76,7 @@ void eventHandler_reset(jbyte sessionID);
 void eventHandler_lock(void);
 void eventHandler_unlock(void);
 
+jboolean eventHandler_synthesizeUnloadEvent(char *signature, JNIEnv *env);
 
 jclass getMethodClass(jvmtiEnv *jvmti_env, jmethodID method);
 

test/hotspot/jtreg/ProblemList.txt

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016, 2021, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2016, 2022, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it