There are at least three variations of the readers-writers problems.

The first readers-writers problem

No readers will be kept waiting unless a writer has already obtained permission to use the shared object.

No readers should wait for other readers to finish simply because a writer is waiting.

//读取数据的进程将一直读，除了有进程已经得到进行写操作的允许。

//可能有好几个读取数据的进程在同时访问数据，但是