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4243978: (ref) Race condition in Reference.enqueue()

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4268317: (ref) Reference.isEnqueued() can return true when instance not enqueued

The reference handler now declares, and assumes, that the discovered field, rather than the next field, is (to be) used to link the entries in the pending list, thus allowing a reference object to be safely enqueued even while it is in the pending state. Also added slightly modified regression tests from the two bug reports.

Reviewed-by: mchung, alanb, jcoomes
This commit is contained in:
Y. Srinivas Ramakrishna 2011-10-31 17:38:15 -07:00
parent becccc16ec
commit e9207384c5
5 changed files with 302 additions and 14 deletions
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31
jdk/src/share/classes/java/lang/ref/Reference.java
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@ -27,7 +27,6 @@ package java.lang.ref;
import sun.misc.Cleaner;
/**
* Abstract base class for reference objects. This class defines the
* operations common to all reference objects. Because reference objects are
@ -69,7 +68,7 @@ public abstract class Reference<T> {
* null.
*
* Pending: queue = ReferenceQueue with which instance is registered;
* next = Following instance in queue, or this if at end of list.
* next = this
*
* Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
* in queue, or this if at end of list.
@ -81,17 +80,28 @@ public abstract class Reference<T> {
* the next field is null then the instance is active; if it is non-null,
* then the collector should treat the instance normally.
*
* To ensure that concurrent collector can discover active Reference
* To ensure that a concurrent collector can discover active Reference
* objects without interfering with application threads that may apply
* the enqueue() method to those objects, collectors should link
* discovered objects through the discovered field.
* discovered objects through the discovered field. The discovered
* field is also used for linking Reference objects in the pending list.
*/
private T referent; /* Treated specially by GC */
ReferenceQueue<? super T> queue;
/* When active: NULL
* pending: this
* Enqueued: next reference in queue (or this if last)
* Inactive: this
*/
Reference next;
/* When active: next element in a discovered reference list maintained by GC (or this if last)
* pending: next element in the pending list (or null if last)
* otherwise: NULL
*/
transient private Reference<T> discovered; /* used by VM */
@ -106,7 +116,8 @@ public abstract class Reference<T> {
/* List of References waiting to be enqueued. The collector adds
* References to this list, while the Reference-handler thread removes
* them. This list is protected by the above lock object.
* them. This list is protected by the above lock object. The
* list uses the discovered field to link its elements.
*/
private static Reference pending = null;
@ -120,14 +131,12 @@ public abstract class Reference<T> {
public void run() {
for (;;) {
Reference r;
synchronized (lock) {
if (pending != null) {
r = pending;
Reference rn = r.next;
pending = (rn == r) ? null : rn;
r.next = r;
pending = r.discovered;
r.discovered = null;
} else {
try {
lock.wait();
@ -201,10 +210,8 @@ public abstract class Reference<T> {
* been enqueued
*/
public boolean isEnqueued() {
/* In terms of the internal states, this predicate actually tests
whether the instance is either Pending or Enqueued */
synchronized (this) {
return (this.queue != ReferenceQueue.NULL) && (this.next != null);
return (this.next != null && this.queue == ReferenceQueue.ENQUEUED);
}
}

3
jdk/src/share/javavm/export/jvm.h
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@ -1424,7 +1424,8 @@ typedef struct {
*/
unsigned int thread_park_blocker : 1;
unsigned int post_vm_init_hook_enabled : 1;
unsigned int : 30;
unsigned int pending_list_uses_discovered_field : 1;
unsigned int : 29;
unsigned int : 32;
unsigned int : 32;
} jdk_version_info;

2
jdk/src/share/native/common/jdk_util.c
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@ -101,5 +101,5 @@ JDK_GetVersionInfo0(jdk_version_info* info, size_t info_size) {
// Advertise presence of sun.misc.PostVMInitHook:
// future optimization: detect if this is enabled.
info->post_vm_init_hook_enabled = 1;
info->pending_list_uses_discovered_field = 1;
}

79
jdk/test/java/lang/ref/ReferenceEnqueue.java Normal file
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@ -0,0 +1,79 @@
/*
* Copyright (c) 2011, 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
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code 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 in the LICENSE file that
* accompanied this code).
*
* 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/* @test
* @bug 4268317
* @summary Test if Reference.enqueue() works properly with GC
*/
import java.lang.ref.*;
public class ReferenceEnqueue {
public static void main(String args[]) throws Exception {
for (int i=0; i < 5; i++)
new WeakRef().run();
System.out.println("Test passed.");
}
static class WeakRef {
final ReferenceQueue<Object> queue = new ReferenceQueue<Object>();
final Reference<Object> ref;
final int iterations = 1000;
WeakRef() {
this.ref = new WeakReference<Object>(new Object(), queue);
}
void run() throws InterruptedException {
System.gc();
for (int i = 0; i < iterations; i++) {
System.gc();
if (ref.isEnqueued()) {
break;
}
Thread.sleep(100);
}
if (ref.isEnqueued() == false) {
// GC have not enqueued refWeak for the timeout period
System.out.println("Reference not enqueued yet");
return;
}
if (ref.enqueue() == true) {
// enqueue() should return false since
// ref is already enqueued by the GC
throw new RuntimeException("Error: enqueue() returned true;"
+ " expected false");
}
if (queue.poll() == null) {
// poll() should return ref enqueued by the GC
throw new RuntimeException("Error: poll() returned null;"
+ " expected ref object");
}
}
}
}

201
jdk/test/java/lang/ref/ReferenceEnqueuePending.java Normal file
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@ -0,0 +1,201 @@
/* Copyright (c) 2011, 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
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code 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 in the LICENSE file that
* accompanied this code).
*
* 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/* @test
* @bug 4243978
* @summary Test if Reference.enqueue() works properly with pending references
*/
import java.lang.ref.*;
public class ReferenceEnqueuePending {
static class NumberedWeakReference extends WeakReference<Integer> {
// Add an integer to identify the weak reference object.
int number;
NumberedWeakReference(Integer referent, ReferenceQueue<Integer> q, int i) {
super(referent, q);
number = i;
}
}
final static boolean debug = System.getProperty("test.debug") != null;
final static int iterations = 1000;
final static int gc_trigger = 99;
static int[] a = new int[2 * iterations];
// Keep all weak references alive with the following array.
static NumberedWeakReference[] b = new NumberedWeakReference[iterations];
public static void main(String[] argv) throws Exception {
if (debug) {
System.out.println("Starting the test.");
}
// Raise thread priority to match the referenceHandler
// priority, so that they can race also on a uniprocessor.
raisePriority();
ReferenceQueue<Integer> refQueue = new ReferenceQueue<>();
// Our objective is to let the mutator enqueue
// a Reference object that may already be in the
// pending state because of having been identified
// as weakly reachable at a previous garbage collection.
// To this end, we create many Reference objects, each with a
// a unique integer object as its referant.
// We let the referents become eligible for collection,
// while racing with the garbage collector which may
// have pended some of these Reference objects.
// Finally we check that all of the Reference objects
// end up on the their queue. The test was originally
// submitted to show that such races could break the
// pending list and/or the reference queue, because of sharing
// the same link ("next") for maintaining both lists, thus
// losing some of the Reference objects on either queue.
Integer obj = new Integer(0);
NumberedWeakReference weaky = new NumberedWeakReference(obj, refQueue, 0);
for (int i = 1; i < iterations; i++) {
// Create a new object, dropping the onlY strong reference to
// the previous Integer object.
obj = new Integer(i);
// Trigger gc each gc_trigger iterations.
if ((i % gc_trigger) == 0) {
forceGc(0);
}
// Enqueue every other weaky.
if ((i % 2) == 0) {
weaky.enqueue();
}
// Remember the Reference objects, for testing later.
b[i - 1] = weaky;
// Get a new weaky for the Integer object just
// created, which may be explicitly enqueued in
// our next trip around the loop.
weaky = new NumberedWeakReference(obj, refQueue, i);
}
// Do a final collection to discover and process all
// Reference objects created above, allowing enough time
// for the ReferenceHandler thread to queue the References.
forceGc(100);
forceGc(100);
// Verify that all WeakReference objects ended up queued.
checkResult(refQueue, obj, iterations-1);
System.out.println("Test passed.");
}
private static void checkResult(ReferenceQueue<Integer> queue,
Integer obj,
int expected) {
if (debug) {
System.out.println("Reading the queue");
}
// Empty the queue and record numbers into a[];
NumberedWeakReference weakRead = (NumberedWeakReference) queue.poll();
int length = 0;
while (weakRead != null) {
a[length++] = weakRead.number;
weakRead = (NumberedWeakReference) queue.poll();
}
if (debug) {
System.out.println("Reference Queue had " + length + " elements");
}
// Use the last Reference object of those created above, so as to keep it "alive".
System.out.println("I must write " + obj + " to prevent compiler optimizations.");
// verify the queued references: all but the last Reference object
// should have been in the queue.
if (debug) {
System.out.println("Start of final check");
}
// Sort the first "length" elements in array "a[]".
sort(length);
boolean fail = (length != expected);
for (int i = 0; i < length; i++) {
if (a[i] != i) {
if (debug) {
System.out.println("a[" + i + "] is not " + i + " but " + a[i]);
}
fail = true;
}
}
if (fail) {
printMissingElements(length, expected);
throw new RuntimeException("TEST FAILED: only " + length
+ " reference objects have been queued out of "
+ expected);
}
}
private static void printMissingElements(int length, int expected) {
System.out.println("The following numbers were not found in the reference queue: ");
int missing = 0;
int element = 0;
for (int i = 0; i < length; i++) {
while ((a[i] != element) & (element < expected)) {
System.out.print(element + " ");
if (missing % 20 == 19) {
System.out.println(" ");
}
missing++;
element++;
}
element++;
}
System.out.print("\n");
}
private static void forceGc(long millis) throws InterruptedException {
Runtime.getRuntime().gc();
Thread.sleep(millis);
}
// Bubble sort the first "length" elements in array "a".
private static void sort(int length) {
int hold;
if (debug) {
System.out.println("Sorting. Length=" + length);
}
for (int pass = 1; pass < length; pass++) { // passes over the array
for (int i = 0; i < length - pass; i++) { // a single pass
if (a[i] > a[i + 1]) { // then swap
hold = a[i];
a[i] = a[i + 1];
a[i + 1] = hold;
}
} // End of i loop
} // End of pass loop
}
// Raise thread priority so as to increase the
// probability of the mutator succeeding in enqueueing
// an object that is still in the pending state.
// This is (probably) only required for a uniprocessor.
static void raisePriority() {
Thread tr = Thread.currentThread();
tr.setPriority(Thread.MAX_PRIORITY);
}
} // End of class ReferenceEnqueuePending