code: final void externalPush(ForkJoinTask<?> task) {
          int r;                            // initialize caller's probe
          if ((r = ThreadLocalRandom.getProbe()) == 0) {
              ThreadLocalRandom.localInit();
              r = ThreadLocalRandom.getProbe();
          }
          for (;;) {
              WorkQueue q; int wl, k, stat;
              int rs = runState;
              WorkQueue[] ws = workQueues;
              if (rs <= 0 || ws == null || (wl = ws.length) <= 0)
                  tryInitialize(true);
              else if ((q = ws[k = (wl - 1) & r & SQMASK]) == null)
                  tryCreateExternalQueue(k);
              else if ((stat = q.sharedPush(task)) < 0)
                  break;
              else if (stat == 0) {
                  signalWork();
                  break;
              }
              else                          // move if busy
                  r = ThreadLocalRandom.advanceProbe(r);
          }
      }
docs: Adds the given task to a submission queue at submitter's
  current queue. Also performs secondary initialization upon the
  first submission of the first task to the pool, and detects
  first submission by an external thread and creates a new shared
  queue if the one at index if empty or contended.
  
  @param task the task. Caller must ensure non-null.