RFR: 8251570: 8215624 causes assert(worker_id <' _n_workers) failed: Invalid worker_id

[Stefan Karlsson]

Hi all,

Please review this patch to fix a newly introduced bug where there can 
be a mismatch between the number of parallel heap inspection threads and 
the number of used GC threads

https://cr.openjdk.java.net/~stefank/8251570/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8251570

The assert triggers because of this code:
      ParallelObjectIterator* poi = 
Universe::heap()->parallel_object_iterator(parallel_thread_num);
      if (poi != NULL) {
        ParHeapInspectTask task(poi, cit, filter);
       Universe::heap()->run_task(&task);

G1's parallel_object_iterator sets up a HeapRegionClaimer with 
parallel_thread_num, while run_task uses active_workers() of the used 
WorkGang.

The original patch also has another, latent bug. Both ZGC and Shenandoah 
uses the wrong WorkGang in run_task. Currently, non of those GCs provide 
a parallel_object_iterator, so the code isn't executed ATM.

The proposed fix for this is to simplify the code and introduce a 
concrete function CollectedHeap::run_task_at_safepoint that takes the 
number of requested workers as a parameter, and dispatches the task 
through the CollectedHeap::get_safepoint_workers() workers (if provided 
by the GC).

Testing:
- Locally with the failing tests and all GCs
- tier1-tier5 on Linux

Thanks,
StefanK