RFR: JDK-8151336: Root region scanning should be cancelled and disabled when the ConcurrentMarkThread::run_service() exits

[Per Liden]

Hi Bengt,

> On 15 Mar 2016, at 16:11, Bengt Rutisson <bengt.rutisson at oracle.com> wrote:
> 
> 
> Hi again everyone,
> 
> Here's an updated webrev:
> http://cr.openjdk.java.net/~brutisso/8151336/webrev.01
> 
> I removed the extra _has_terminated state and am instead using the ConcurrentMarkThread::should_terminate() state to make sure we don't start a new initial mark if the concurrent mark thread is about to terminate or has already terminated.
> 
> The _should_terminate state is set in ConcurrentGCThread::stop() where we first take the Terminator_lock with a safepoint check. So, we should be able to rely on this value inside of do_collection_pause_at_safepoint().

This looks correct as far as I can tell. Reviewed.

cheers,
Per

> 
> Thanks,
> Bengt
> 
> On 2016-03-15 12:55, Bengt Rutisson wrote:
>> 
>> Hi again,
>> 
>> I am working on an update to this webrev. So, please hold off with reviews of the patch below.
>> 
>> Thanks,
>> Bengt
>> 
>> On 2016-03-14 12:23, Bengt Rutisson wrote:
>>> 
>>> Hi all,
>>> 
>>> Could I have a couple of reviews for this change?
>>> 
>>> http://cr.openjdk.java.net/~brutisso/8151336/webrev.00/
>>> https://bugs.openjdk.java.net/browse/JDK-8151336
>>> 
>>> The run_service() loop in ConcurrentMarkThread looks like this:
>>> 
>>>  while (!_should_terminate) {
>>>    // wait until started is set.
>>>    sleepBeforeNextCycle();
>>>    if (_should_terminate) {
>>>      _cm->root_regions()->cancel_scan();
>>>      break;
>>>    }
>>>    // do all the work
>>> }
>>> 
>>> If a full GC is triggered while the ConcurrentMarkThread is in "do all the work" the full GC will wait for the root region scan. But if we at the same time decide to exit the VM we will set _should_terminate to true, which will cause the while() loop to not continue and we won't get to the cancel_scan call.
>>> 
>>> The proposed fix moves the call to cancel_scan() until after the while loop has exited. That way we will always cancel any pending root region scanning.
>>> 
>>> I also introduce a _has_terminated state for G1CMRootRegions to makes sure that another root region scanning can't be initialized once the CM thread has exited. This also called for a rename of cancel_scan() to terminate().
>>> 
>>> Thanks,
>>> Bengt
>> 
>