If a thread is in upgradeable mode, and there are no threads waiting to enter write mode, any number of other threads can enter read mode, even if there are threads waiting to enter upgradeable mode.

• 如果有一个线程处于可升级读模式，并且没有任何线程等待进入写模式，那么任意数量的线程可以进入读模式，即使有线程在等待进入可升级读模式。