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  1. JDK
  2. JDK-8129626

G1: set_in_progress() and clear_started() needs a barrier on non-TSO platforms

    Details

    • Type: Bug
    • Status: Resolved
    • Priority: P2
    • Resolution: Fixed
    • Affects Version/s: 9
    • Fix Version/s: 9
    • Component/s: hotspot
    • Labels:
      None
    • Subcomponent:
      gc
    • Resolved In Build:
      b74

      Backports

        Description

        ConcurrentMarkThread::during_cycle() is implemented as:

        bool during_cycle() { return started() || in_progress(); }

        So, it checks both ConcurrentMarkThread::_started and ConcurrentMarkThread::_in_progress and they are meant to overlap. That is, we should not set _started to false until after we have set _in_progress to true. This is done in sleepBeforeNextCycle():

        void ConcurrentMarkThread::sleepBeforeNextCycle() {
          // We join here because we don't want to do the "shouldConcurrentMark()"
          // below while the world is otherwise stopped.
          assert(!in_progress(), "should have been cleared");

          MutexLockerEx x(CGC_lock, Mutex::_no_safepoint_check_flag);
          while (!started() && !_should_terminate) {
            CGC_lock->wait(Mutex::_no_safepoint_check_flag);
          }

          if (started()) {
            set_in_progress();
            clear_started();
          }
        }

        On non-TSO platforms there is a risk that the write to _in_progress (from set_in_progress()) is seen by other threads after the write to _started (in clear_started()). In that case there is a window when during_cycle() may return false even though we are in a concurrent cycle.

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                • Assignee:
                  brutisso Bengt Rutisson (Inactive)
                  Reporter:
                  brutisso Bengt Rutisson (Inactive)
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                  • Created:
                    Updated:
                    Resolved: