他不可能是最闪光的家伙，但是主人锁是有力的。
He may not be the flashiest guy, but that Master Lock is powerful.
当调用如果线程已持有锁，增量递归层次一，并返回。如果线程没有等待直到主人锁释放锁。
When invoked if this thread already owns the lock, increment the recursion level by one, and return immediately. if thread doesn't own the lock it waits until owner release lock.
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