8016304: ThreadMXBean.getDeadlockedThreads reports bogus deadlocks on JDK 8

Reviewed-by: dcubed, mgronlun

hotspot/src/share/vm/services/threadService.cpp

@@ -327,27 +327,30 @@ DeadlockCycle* ThreadService::find_deadlocks_at_safepoint(bool concurrent_locks)
     while (waitingToLockMonitor != NULL || waitingToLockBlocker != NULL) {
       cycle->add_thread(currentThread);
       if (waitingToLockMonitor != NULL) {
-        currentThread = Threads::owning_thread_from_monitor_owner(
-                          (address)waitingToLockMonitor->owner(),
-                          false /* no locking needed */);
-        if (currentThread == NULL) {
-          // This function is called at a safepoint so the JavaThread
-          // that owns waitingToLockMonitor should be findable, but
-          // if it is not findable, then the previous currentThread is
-          // blocked permanently. We record this as a deadlock.
-          num_deadlocks++;
+        address currentOwner = (address)waitingToLockMonitor->owner();
+        if (currentOwner != NULL) {
+          currentThread = Threads::owning_thread_from_monitor_owner(
+                            currentOwner,
+                            false /* no locking needed */);
+          if (currentThread == NULL) {
+            // This function is called at a safepoint so the JavaThread
+            // that owns waitingToLockMonitor should be findable, but
+            // if it is not findable, then the previous currentThread is
+            // blocked permanently. We record this as a deadlock.
+            num_deadlocks++;
 
-          cycle->set_deadlock(true);
+            cycle->set_deadlock(true);
 
-          // add this cycle to the deadlocks list
-          if (deadlocks == NULL) {
-            deadlocks = cycle;
-          } else {
-            last->set_next(cycle);
+            // add this cycle to the deadlocks list
+            if (deadlocks == NULL) {
+              deadlocks = cycle;
+            } else {
+              last->set_next(cycle);
+            }
+            last = cycle;
+            cycle = new DeadlockCycle();
+            break;
           }
-          last = cycle;
-          cycle = new DeadlockCycle();
-          break;
         }
       } else {
         if (concurrent_locks) {

hotspot/test/serviceability/threads/TestFalseDeadLock.java

@@ -0,0 +1,95 @@
+/*
+ * Copyright (c) 2013, 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.
+ */
+
+import java.lang.management.ManagementFactory;
+import java.lang.management.ThreadMXBean;
+import java.util.Random;
+
+/*
+ * @test
+ * @bug 8016304
+ * @summary Make sure no deadlock is reported for this program which has no deadlocks.
+ * @run main/othervm TestFalseDeadLock
+ */
+
+/*
+ * This test will not provoke the bug every time it is run since the bug is intermittent.
+ * The test has a fixed running time of 5 seconds.
+ */
+
+public class TestFalseDeadLock {
+    private static ThreadMXBean bean;
+    private static volatile boolean running = true;
+    private static volatile boolean found = false;
+
+    public static void main(String[] args) throws Exception {
+        bean = ManagementFactory.getThreadMXBean();
+        Thread[] threads = new Thread[500];
+        for (int i = 0; i < threads.length; i++) {
+            Test t = new Test();
+            threads[i] = new Thread(t);
+            threads[i].start();
+        }
+        try {
+            Thread.sleep(5000);
+        } catch (InterruptedException ex) {
+        }
+        running = false;
+        for (Thread t : threads) {
+            t.join();
+        }
+        if (found) {
+            throw new Exception("Deadlock reported, but there is no deadlock.");
+        }
+    }
+
+    public static class Test implements Runnable {
+        public void run() {
+            Random r = new Random();
+            while (running) {
+                try {
+                    synchronized (this) {
+                        wait(r.nextInt(1000) + 1);
+                    }
+                } catch (InterruptedException ex) {
+                }
+                recurse(2000);
+            }
+            if (bean.findDeadlockedThreads() != null) {
+                System.out.println("FOUND!");
+                found = true;
+            }
+        }
+
+        private void recurse(int i) {
+            if (!running) {
+                // It is important for the test to call println here
+                // since there are locks inside that path.
+                System.out.println("Hullo");
+            }
+            else if (i > 0) {
+                recurse(i - 1);
+            }
+        }
+    }
+}