RFR: JDK-8148992: VM can hang on exit if root region scanning is initiated but not executed

Per Liden per.liden at oracle.com
Mon Feb 8 11:33:16 UTC 2016


Hi Bengt,

On 2016-02-08 11:15, Bengt Rutisson wrote:
>
> Hi all,
>
> Could I have a couple of reviews for this change?
>
> http://cr.openjdk.java.net/~brutisso/8148992/webrev.00

Looks good.

/Per

> https://bugs.openjdk.java.net/browse/JDK-8148992
>
> There are some more details in the bug report, but here's the most
> relevant text:
>
> The reason for the hang is that during shutdown we don't check the root
> region scanning.
>
> The ConcurrentMark loop starts like this:
>
>    while (!_should_terminate) {
>      // wait until started is set.
>      sleepBeforeNextCycle();
>      if (_should_terminate) {
>        break;
>      }
>
> If _should_terminate is true we just exit without notifying any waiters
> on the root region lock. If a GC happens during shutdown the GC will
> hang waiting for the root region scanning to finish but the
> ConcurrentMark thread has just exited and will not do any root region
> scanning.
>
> I can trigger this behavior by adding a sleep in the above code:
>
>    while (!_should_terminate) {
>      // wait until started is set.
>      sleepBeforeNextCycle();
>      if (_should_terminate) {
>        for (int i = 0; i < 10; i++) {
>          os::naked_short_sleep(999);
>        }
>        break;
>      }
>
> and running this small java program:
>
> import java.util.LinkedList;
>
> public class Repro2 {
>
>      public static LinkedList<byte[]> dummyStore = new LinkedList<>();
>
>      public static void main(String[] args) throws Exception {
>          System.out.println("Started");
>          for (int i = 0; i < 1024*16; i++) {
>              dummyStore.add(new byte[1024]);
>          }
>          System.out.println("Triggered one YC");
>
>          Thread thread = new Thread(()->System.exit(0));
>          thread.start();
>          Thread.sleep(100);
>
>          for (int i = 0; i < 1024*16; i++) {
>              dummyStore.add(new byte[1024]);
>          }
>          System.out.println("Triggered Initial mark");
>
>          System.gc(); // Full GG
>
>          System.out.println("Done.");
>      }
> }
>
>
> Running with the sleep added and the following command line:
>
> java -Xmx16m -Xmx64m -XX:InitiatingHeapOccupancyPercent=0 Repro2
>
> makes the VM hang every time on my workstation.
>
> If I add a "cancel_scan()" method and call it before the ConcurrentMark
> thread is giving up, the VM does not hang anymore. That is, running with
> this code makes the VM sleep a while during shutdown but it does not hang:
>
>
>    while (!_should_terminate) {
>      // wait until started is set.
>      sleepBeforeNextCycle();
>      if (_should_terminate) {
>        for (int i = 0; i < 10; i++) {
>          os::naked_short_sleep(999);
>        }
>        _cm->root_regions()->cancel_scan();
>        break;
>      }


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