> For the complete documentation index, see [llms.txt](https://1790865014.gitbook.io/ucore-step-by-step/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://1790865014.gitbook.io/ucore-step-by-step/intro-10/7_lab7.md).

# lab7

本实验主要需要阅读代码和回答问题。

1. `kern/sync/sem.c`的信号量实现中，出现了这样的暂时禁用中断的代码：

   ```c
   static __noinline uint32_t __down(semaphore_t *sem, uint32_t wait_state) {
       bool intr_flag;
       local_intr_save(intr_flag);
       /*some code*/
       local_intr_restore(intr_flag);
   ```

   我们其实从正确的框架里删除了两句`local_intr_restore(intr_flag)`和一句`local_intr_save(intr_flag);`，请你找出是哪里删掉了，并举出删掉之后不能正常工作的一种情况。(**4分**，每找出一处1分，举出不能正常工作的情况1分)

   * 注：现在还没删这几处。
   * 同学们其实可以轻松地找到正确框架的代码，然后通过对比得到哪里是删掉的。这是否对自己分析的同学不太公平？（这个分析的过程可能还比较恶心）。可行的方式是不是，我们直接告诉同学们正确框架在哪里，只要你不想自己分析我们就允许你直接diff不同文件来找bug， 如果你觉得我们的作业有意思，就自己分析哪里应该加？
2. 证明/说明为什么我们给出的信号量实现的哲学家问题不会出现死锁。不必特别严谨，但要能说服你自己/助教？。（**3分**)
3. 证明/说明为什么我们给出的条件变量实现的哲学家问题不会出现死锁。不必特别严谨，但要能说服你自己/助教？(**3分**)
4. 给出为用户态进程/线程提供信号量机制的设计方案，并比较说明给内核级提供信号量机制的异同。（**3分**)
5. 给出为用户态进程/线程提供条件变量机制的设计方案，并比较说明给内核级提供条件变量机制的异同。（**3分**）
6. 能否不基于信号量机制来完成条件变量？如果不能，请给出理由，如果能，请给出设计说明和具体实现。(**4分**)


---

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