commonref.c

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/*
 * @(#)commonRef.c	1.24 06/10/25
 *
 * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
 * 
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License version
 * 2 only, as published by the Free Software Foundation. 
 * 
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License version 2 for more details (a copy is
 * included at /legal/license.txt). 
 * 
 * You should have received a copy of the GNU General Public License
 * version 2 along with this work; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA 
 * 
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
 * Clara, CA 95054 or visit www.sun.com if you need additional
 * information or have any questions. 
 */

#include "util.h"
#include "commonRef.h"

#define ALL_REFS -1

/*
 * Each object sent to the front end is tracked with the RefNode struct 
 * (see util.h).
 * External to this module, objects are identified by a jlong id which is 
 * simply the sequence number. A weak reference is usually used so that
 * the presence of a debugger-tracked object will not prevent
 * its collection. Once an object is collected, its RefNode may be 
 * deleted and the weak ref inside may be reused (these may happen in
 * either order). Using the sequence number
 * as the object id prevents ambiguity in the object id when the weak ref 
 * is reused. The RefNode* is stored with the object as it's JVMTI Tag.
 *
 * The ref member is changed from weak to strong when
 * gc of the object is to be prevented.
 * Whether or not it is strong, it is never exported from this module.
 *
 * A reference count of each jobject is also maintained here. It tracks
 * the number times an object has been referenced through 
 * commonRef_refToID. A RefNode is freed once the reference
 * count is decremented to 0 (with commonRef_release*), even if the 
 * correspoding object has not been collected.
 *
 * One hash table is maintained. The mapping of ID to jobject (or RefNode*)
 * is handled with one hash table that will re-size itself as the number
 * of RefNode's grow.
 */

/* Initial hash table size (must be power of 2) */
#define HASH_INIT_SIZE 512
/* If element count exceeds HASH_EXPAND_SCALE*hash_size we expand & re-hash */
#define HASH_EXPAND_SCALE 8
/* Maximum hash table size (must be power of 2) */
#define HASH_MAX_SIZE  (1024*HASH_INIT_SIZE)

/* Map a key (ID) to a hash bucket */
static jint 
hashBucket(jlong key) 
{
    /* Size should always be a power of 2, use mask instead of mod operator */
    /*LINTED*/
    return ((jint)key) & (gdata->objectsByIDsize-1);
}

/* Generate a new ID */
static jlong 
newSeqNum(void)
{
    return gdata->nextSeqNum++;
}

/* Create a fresh RefNode structure, create a weak ref and tag the object */
static RefNode *
createNode(JNIEnv *env, jobject ref) 
{
    RefNode   *node;
    jobject    weakRef;
    jvmtiError error;
  
    /* Could allocate RefNode's in blocks, not sure it would help much */
    node = (RefNode*)jvmtiAllocate((int)sizeof(RefNode));     
    if (node == NULL) {
        return NULL;
    }
    
    /* Create weak reference to make sure we have a reference */
    weakRef = JNI_FUNC_PTR(env,NewWeakGlobalRef)(env, ref);
    if (weakRef == NULL) {
        jvmtiDeallocate(node);
        return NULL;
    }

    /* Set tag on weakRef */
    error = JVMTI_FUNC_PTR(gdata->jvmti, SetTag)
                          (gdata->jvmti, weakRef, (jlong)((jint)node));
    if ( error != JVMTI_ERROR_NONE ) {
        JNI_FUNC_PTR(env,DeleteWeakGlobalRef)(env, weakRef);
        jvmtiDeallocate(node);
        return NULL;
    }

    /* Fill in RefNode */
    node->ref      = weakRef;
    node->isStrong = JNI_FALSE;
    node->count    = 1;
    node->seqNum   = newSeqNum();

    /* Count RefNode's created */
    gdata->objectsByIDcount++;
    return node;
}

/* Delete a RefNode allocation, delete weak/global ref and clear tag */
static void 
deleteNode(JNIEnv *env, RefNode *node)
{
    LOG_MISC(("Freeing %d (%x)\n", (int)node->seqNum, node->ref));

    if ( node->ref != NULL ) {
        /* Clear tag */
        (void)JVMTI_FUNC_PTR(gdata->jvmti,SetTag)
                            (gdata->jvmti, node->ref, NULL_OBJECT_ID);
        if (node->isStrong) {
            JNI_FUNC_PTR(env,DeleteGlobalRef)(env, node->ref);
        } else {
            JNI_FUNC_PTR(env,DeleteWeakGlobalRef)(env, node->ref);
        }
    }
    gdata->objectsByIDcount--;
    jvmtiDeallocate(node);
}

/* Change a RefNode to have a strong reference */
static jobject
strengthenNode(JNIEnv *env, RefNode *node) 
{
    if (!node->isStrong) {
        jobject strongRef;
        
        strongRef = JNI_FUNC_PTR(env,NewGlobalRef)(env, node->ref);
        /*
         * NewGlobalRef on a weak ref will return NULL if the weak 
         * reference has been collected or if out of memory. 
         * We need to distinguish those two occurrences.
         */
        if ((strongRef == NULL) && !isSameObject(env, node->ref, NULL)) {
            EXIT_ERROR(AGENT_ERROR_NULL_POINTER,"NewGlobalRef");
        } 
        if (strongRef != NULL) {
            JNI_FUNC_PTR(env,DeleteWeakGlobalRef)(env, node->ref);
            node->ref      = strongRef;
            node->isStrong = JNI_TRUE; 
        }
        return strongRef;
    } else {
        return node->ref;
    }
}

/* Change a RefNode to have a weak reference */
static jweak
weakenNode(JNIEnv *env, RefNode *node) 
{
    if (node->isStrong) {
        jweak weakRef;
        
        weakRef = JNI_FUNC_PTR(env,NewWeakGlobalRef)(env, node->ref);
        if (weakRef != NULL) {
            JNI_FUNC_PTR(env,DeleteGlobalRef)(env, node->ref);
            node->ref      = weakRef;
            node->isStrong = JNI_FALSE;
        }
        return weakRef;
    } else {
        return node->ref;
    }
}

/*
 * Returns the node which contains the common reference for the 
 * given object. The passed reference should not be a weak reference
 * managed in the object hash table (i.e. returned by commonRef_idToRef)
 * because no sequence number checking is done.
 */
static RefNode *
findNodeByRef(JNIEnv *env, jobject ref) 
{
    jvmtiError error;
    jlong      tag;

    tag   = NULL_OBJECT_ID;
    error = JVMTI_FUNC_PTR(gdata->jvmti,GetTag)(gdata->jvmti, ref, &tag);
    if ( error == JVMTI_ERROR_NONE ) {
        RefNode   *node;
        
        node = (RefNode*)((jint)tag);
        return node;
    }
    return NULL;
}

/* Locate and delete a node based on ID */
static void 
deleteNodeByID(JNIEnv *env, jlong id, jint refCount) 
{
    jint     slot;
    RefNode *node;
    RefNode *prev;
    
    slot = hashBucket(id);
    node = gdata->objectsByID[slot];
    prev = NULL;

    while (node != NULL) {
        if (id == node->seqNum) {
            if (refCount != ALL_REFS) {
                node->count -= refCount;
            } else {
                node->count = 0;
            }
            if (node->count <= 0) {
                if ( node->count < 0 ) {
                    EXIT_ERROR(AGENT_ERROR_INTERNAL,"RefNode count < 0");
                }
                /* Detach from id hash table */
                if (prev == NULL) {
                    gdata->objectsByID[slot] = node->next;
                } else {
                    prev->next = node->next;
                }
                deleteNode(env, node);
            }
            break;
        }
        prev = node;
        node = node->next;
    }
}

/*
 * Returns the node stored in the object hash table for the given object
 * id. The id should be a value previously returned by 
 * commonRef_refToID.
 *
 *  NOTE: It is possible that a match is found here, but that the object
 *        is garbage collected by the time the caller inspects node->ref.
 *        Callers should take care using the node->ref object returned here.
 *
 */
static RefNode *
findNodeByID(JNIEnv *env, jlong id) 
{
    jint     slot;
    RefNode *node;
    RefNode *prev;

    slot = hashBucket(id);
    node = gdata->objectsByID[slot];
    prev = NULL;

    while (node != NULL) {
        if ( id == node->seqNum ) {
            if ( prev != NULL ) {
                /* Re-order hash list so this one is up front */
                prev->next = node->next;
                node->next = gdata->objectsByID[slot];
                gdata->objectsByID[slot] = node;
            }
            break;
        }
        node = node->next;
    }
    return node;
}

/* Initialize the hash table stored in gdata area */
static void 
initializeObjectsByID(int size) 
{
    /* Size should always be a power of 2 */
    if ( size > HASH_MAX_SIZE ) size = HASH_MAX_SIZE;
    gdata->objectsByIDsize  = size;
    gdata->objectsByIDcount = 0;
    gdata->objectsByID      = (RefNode**)jvmtiAllocate((int)sizeof(RefNode*)*size);
    (void)memset(gdata->objectsByID, 0, (int)sizeof(RefNode*)*size);
}

/* hash in a RefNode */
static void
hashIn(RefNode *node)
{
    jint     slot;
    
    /* Add to id hashtable */
    slot                     = hashBucket(node->seqNum);