This simplest way to demonstrate this is to run:

$ java -XX:G1ConcRefinementThreads=1 -XX:+UnlockDiagnosticVMOptions -XX:+G1TraceConcRefinement Hello
G1-Refine-stop
G1-Refine-stop

Those two lines correspond to two separate threads coming to ConcurrentG1RefineThread::stop. I understand this is because of:

  // We need one extra thread to do the young gen rset size sampling.
  _n_threads = _n_worker_threads + 1;

...but this is not what user supplied for the number of refinement threads. This is at very least confusing, and potentially complicates GC fine-tuning.