src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

rev 2891 : 7120038: G1: ParallelGCThreads==0 is broken
Summary: Running G1 with ParallelGCThreads==0 results in various crashes and asserts. Most of these are caused by unguarded references to the worker threads array or an incorrect number of active workers.
Reviewed-by:

*** 3721,3732 ****
          }
  
          double end_time_sec = os::elapsedTime();
          double pause_time_ms = (end_time_sec - start_time_sec) * MILLIUNITS;
          g1_policy()->record_pause_time_ms(pause_time_ms);
!         int active_gc_threads = workers()->active_workers();
!         g1_policy()->record_collection_pause_end(active_gc_threads);
  
          MemoryService::track_memory_usage();
  
          // In prepare_for_verify() below we'll need to scan the deferred
          // update buffers to bring the RSets up-to-date if
--- 3721,3733 ----
          }
  
          double end_time_sec = os::elapsedTime();
          double pause_time_ms = (end_time_sec - start_time_sec) * MILLIUNITS;
          g1_policy()->record_pause_time_ms(pause_time_ms);
!         int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
!                                 workers()->active_workers() : 1);
!         g1_policy()->record_collection_pause_end(active_workers);
  
          MemoryService::track_memory_usage();
  
          // In prepare_for_verify() below we'll need to scan the deferred
          // update buffers to bring the RSets up-to-date if
*** 5246,5257 ****
    // object discovered by the STW ref processor.
  
    int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
                          workers()->active_workers() : 1);
  
!   assert(active_workers == workers()->active_workers(),
           "Need to reset active_workers");
    set_par_threads(active_workers);
    G1ParPreserveCMReferentsTask keep_cm_referents(this, active_workers, _task_queues);
  
    if (G1CollectedHeap::use_parallel_gc_threads()) {
      workers()->run_task(&keep_cm_referents);
--- 5247,5260 ----
    // object discovered by the STW ref processor.
  
    int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
                          workers()->active_workers() : 1);
  
!   assert(!G1CollectedHeap::use_parallel_gc_threads() ||
!            active_workers == workers()->active_workers(),
             "Need to reset active_workers");
+ 
    set_par_threads(active_workers);
    G1ParPreserveCMReferentsTask keep_cm_referents(this, active_workers, _task_queues);
  
    if (G1CollectedHeap::use_parallel_gc_threads()) {
      workers()->run_task(&keep_cm_referents);
*** 5385,5401 ****
                                       workers()->active_workers(),
                                       Threads::number_of_non_daemon_threads());
      assert(UseDynamicNumberOfGCThreads ||
             n_workers == workers()->total_workers(),
             "If not dynamic should be using all the  workers");
      set_par_threads(n_workers);
    } else {
      assert(n_par_threads() == 0,
             "Should be the original non-parallel value");
      n_workers = 1;
    }
-   workers()->set_active_workers(n_workers);
  
    G1ParTask g1_par_task(this, _task_queues);
  
    init_for_evac_failure(NULL);
  
--- 5388,5404 ----
                                       workers()->active_workers(),
                                       Threads::number_of_non_daemon_threads());
      assert(UseDynamicNumberOfGCThreads ||
             n_workers == workers()->total_workers(),
             "If not dynamic should be using all the  workers");
+     workers()->set_active_workers(n_workers);
      set_par_threads(n_workers);
    } else {
      assert(n_par_threads() == 0,
             "Should be the original non-parallel value");
      n_workers = 1;
    }
  
    G1ParTask g1_par_task(this, _task_queues);
  
    init_for_evac_failure(NULL);
  
*** 5413,5422 ****
--- 5416,5426 ----
             workers()->active_workers() == workers()->total_workers(),
             "If not dynamic should be using all the  workers");
      workers()->run_task(&g1_par_task);
    } else {
      StrongRootsScope srs(this);
+     g1_par_task.set_for_termination(n_workers);
      g1_par_task.work(0);
    }
  
    double par_time = (os::elapsedTime() - start_par) * 1000.0;
    g1_policy()->record_par_time(par_time);
*** 6070,6079 ****
--- 6074,6084 ----
  }
  
  void G1CollectedHeap::set_par_threads() {
    // Don't change the number of workers.  Use the value previously set
    // in the workgroup.
+   assert(G1CollectedHeap::use_parallel_gc_threads(), "shouldn't be here otherwise");
    int n_workers = workers()->active_workers();
    assert(UseDynamicNumberOfGCThreads ||
             n_workers == workers()->total_workers(),
        "Otherwise should be using the total number of workers");
    if (n_workers == 0) {