code: public void clear() {
          mPutLock.lock();
          mPollLock.lock();
          try {
              mSize.set(0);
              mHead = new Node(null);
              mTail = new Node(null);
              mHead.mNext = mTail;
          }
          finally {
              mPollLock.unlock();
              mPutLock.unlock();
          }
      }
docs: Clears the contents of the queue.   This is a blocking operation.