barrier_->Sync();
        ASSERT(AtomicOperations::LoadRelaxed(num_busy_) == 0); // (1)

        // Check if we have any pending properties with marked keys.
        // Those might have been marked by another marker.
        more_to_mark = visitor.ProcessPendingWeakProperties();
        if (more_to_mark) {
          // We have more work to do. Notify others.
          AtomicOperations::FetchAndIncrement(num_busy_);  // (2)
        }

        // Wait for all other markers to finish processing their pending
        // weak properties and decide if they need to continue marking.
        // Caveat: we need two barriers here to make this decision in lock step
        // between all markers and the main thread.
        barrier_->Sync();

        barrier_->Sync();
#if defined(DEBUG)
        ASSERT(AtomicOperations::LoadRelaxed(num_busy_) == 0); // (1)
        barrier->Sync();
#endif