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

[Kim Barrett]

> On Aug 14, 2020, at 10:40 AM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> 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

Looks good.