RFC 7038914: VM could throw uncaught OOME in ReferenceHandler thread (original) (raw)

Peter Levart peter.levart at gmail.com
Thu May 9 08:35:57 UTC 2013

On 05/09/2013 07:53 AM, David Holmes wrote:

Hi Thomas,

On 9/05/2013 1:28 AM, Thomas Schatzl wrote: Hi,

please review the latest webrev for this patch that is available at http://cr.openjdk.java.net/~tschatzl/7038914/webrev.2/ As mentioned, it incorporates the fix and reproducer from Peter Levart. Fix is fine. I'm not sure about the test (sorry I didn't pay it attention earlier). Have you instrumented the code to verify that the test actually triggers an OOME? It may be possible that if the OOME did kill the reference thread that we wouldn't necessarily detect it using the sleeps etc. Also I don't understand the need for the actual weakRef usage and System.gc() etc. If the heap is full then the interrupt will wake the reference handler thread and the allocation will trigger the OOME. It doesn't matter if there are any references to process. The main logic might then reduce to: referenceHandlerThread.interrupt(); for (int i = 0; i < 10; i++) { if (!referenceHandlerThread.isAlive()) throw new Exception("Reference-handler thread died"); Thread.sleep(1000); } which just gives it 10s to die else assumes all ok. ? But I can live with existing test (it just might vacuously pass) Cheers, David

Hi David, Thomas,

Yes, this was just a left-over from my bug reproducer that demonstrated the situation where WeakReference was cleared, but nothing was enqueue-d into the ReferenceQueue. Reference Handler thread otherwise just sleeps in lock.wait() most of the time if there's nothing to do and interrupting it's thread will trigger allocation of InterruptedException and consequently OOME nevertheless.

One think to watch: Reference Handler thread is started in the j.l.r.Reference static initializer, so the Reference class must be initialized. I don't know if there is any guarantee that this is done before entering main(). So there should be something explicit in main() method that ensures it.

Also, throwing test-fail exception should be done after releasing the heap or we'll just trigger another OOME without any message to describe the situation.

Waiting in a loop for ReferenceHandler thread is not needed since the test will normally succeed and so the whole loop will execute to the end. Only when the test fails the loop will exit faster. In my experience, the thread dies between 10-20ms after interrupting, so waiting for about 500ms is enough I think. Also no System.gc() and additional waiting is needed to report the outcome - just releasing the reference to 'waste' variable is enuugh in my experience to restore the heap into working condition.

Here's the updated test that does all that:

public class OOMEInReferenceHandler { static Object[] fillHeap() { Object[] first = null, last = null; int size = 1 << 20; while (size > 0) { try { Object[] array = new Object[size]; if (first == null) { first = array; } else { last[0] = array; } last = array; } catch (OutOfMemoryError oome) { size = size >>> 1; } } return first; }

 public static void main(String[] args) throws Exception {
     // ensure ReferenceHandler thread is started
     new WeakReference<Object>(new Object());

     ThreadGroup tg = Thread.currentThread().getThreadGroup();
     for (ThreadGroup tgn = tg;
          tgn != null;
          tg = tgn, tgn = tg.getParent());

     Thread[] threads = new Thread[tg.activeCount()];
     Thread referenceHandlerThread = null;
     int n = tg.enumerate(threads);
     for (int i = 0; i < n; i++) {
         if("Reference Handler".equals(threads[i].getName())) {
             referenceHandlerThread = threads[i];
         }
     }

     if (referenceHandlerThread == null) {
         throw new IllegalStateException("Couldn't find Reference

Handler thread."); }

     Object waste = fillHeap();

     referenceHandlerThread.interrupt();

     Thread.sleep(500L);

     // release waste so we can allocate and throw normal exceptions
     waste = null;

      // check that the handler is still alive
     if (!referenceHandlerThread.isAlive()) {
         throw new Exception("Reference Handler thread died.");
     }
 }

}

Regards, Peter

Bugs.sun.com: http://bugs.sun.com/viewbug.do?bugid=7038914

JIRA: https://jbs.oracle.com/bugs/browse/JDK-7038914 Testing: JPRT, with the new reproducer passing In need of reviewers and a sponsor for this patch; as mentioned earlier I will set Peter (plevart) as author for this patch, i.e. send a patchset to the sponsor for this change with that author set. Thanks, Thomas

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