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

Bengt Rutisson bengt.rutisson at oracle.com
Mon Feb 8 11:35:03 UTC 2016


Hi Per,

On 2016-02-08 12:33, Per Liden wrote:
> 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.

Thanks!
Bengt

>
> /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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