[8u40] RFR: 8064556: G1: ParallelGCThreads=0 may cause assert(!MetadataOnStackMark::has_buffer_for_thread(Thread::current())) failed: Should be empty

[Stefan Karlsson]

Hi all,

Please review this patch to weaken the failing assert:
  http://cr.openjdk.java.net/~stefank/8064556/webrev.01
  https://bugs.openjdk.java.net/browse/JDK-8064556

The asserts in pre_work_verification and post_work_verification were put 
in place to make sure that the worker threads entered and exited with 
clean slates. However, in JDK 8 this code path can be taken by the 
VMThread, which will have registered Metadata when running the 
MetadataOnStackMark constructor in the unloading phase of 
ConcurrentMark::weakRefsWork. This is different from JDK 9, which always 
use worker threads to execute this path.

The patch has been verified with the failing test that found this issue.

thanks,
StefanK