[jboss-jira] [Red Hat JIRA] (AG-145) Active waiting deadlock in StampedCopyOnWriteArrayList

Active waiting deadlock in StampedCopyOnWriteArrayList ------------------------------------------------------ Key: AG-145 URL: https://issues.redhat.com/browse/AG-145 Project: Agroal Issue Type: Bug Affects Versions: 1.8 Reporter: Rene Böing Assignee: Luis Barreiro Priority: Critical Fix For: 1.9 Attachments: image-2020-07-31-08-39-28-630.png, image-2020-07-31-08-40-41-968.png While using agroal connection pool, we discovered some rare deadlock, which are causing 100% cpu on some threads. These deadlocks occur in the StampedCopyOnWriteArrayList class, when there is more than one thread trying to remove the same object.   A simple reproducer in junit (fails nearly every time on my machine):   {code:java} @Test public void testThis() { ExecutorService service = Executors.newFixedThreadPool(10); StampedCopyOnWriteArrayList<Object> list = new StampedCopyOnWriteArrayList<>(Object.class); Object o = new Object(); list.add(new Object()); list.add(new Object()); list.add(new Object()); list.add(new Object()); list.add(o); list.add(new Object()); List<Runnable> runnerList = new ArrayList<>(10); List<Future> futureList = new ArrayList<>(10); for (int i = 0; i < 10; i++) { runnerList.add(new Runnable() { @Override public void run() { list.remove(o); System.out.println("Removed success!"); } }); } for (Runnable r : runnerList) { futureList.add(service.submit(r)); } for (Future r : futureList) { try { r.get(10000, TimeUnit.MILLISECONDS); } catch (InterruptedException e) { e.printStackTrace(); } catch (ExecutionException e) { e.printStackTrace(); } catch (TimeoutException e) { System.out.println("Seems like we have a deadlock!"); } } } {code}   Originally this deadlock seems to occur, when agroal tries to flush a connection due to the config parameter <property name="hibernate.agroal.maxLifetime_m">60</property> If at the same time another thread using this connection calls session.close there is a possibility in the ConnectionPool.class getting called twice. The parameter goes through the following path: !image-2020-07-31-08-39-28-630.png!   The parallel session.close call does not find a checked_out connection and tries to flush it instead, hence two Threads are getting into the deadlock situation: !image-2020-07-31-08-40-41-968.png!   Kind regards, Rene