Chapter 6（Operating System）

Chapter 6 Deadlock

pthread_join: 阻塞当前的线程，直到另外一个线程运行结束

Mutex

•Used to guarantee that one thread “excludes” all other threads while it executes the critical section.

Deadlock: each process in the set is blocked awaiting an event that can only be triggered by another blocked process in the set

Resource Categories

• Reusable : can be safely used by only one process at a time and is not depleted by that use

• Consumable: one that can be created (produced) and destroyed (consumed)

Condition

必要条件

1. Mutual Exclusion

2. Hold-and-Wait: a process may hold allocated resources while awaiting assignment of others

3. No preemption(抢占)：no resource can be forcibly removed from a process holding it

   第四个条件：Circular Wait 存在一个闭合的进程链，每个进程至少占有此链中下一个进程所需的一个资源

Dealing with Deadlock

Prevent

Avoid

• Resource Allocation Denial：如果一个进程的增加资源请求会导致死锁，则不允许这一资源分配

  •Safe state is one in which there is at least one sequence of resource allocations to processes that does not result in a deadlock

  •Unsafe state is a state that is not safe

• Process Initiation Denial: 如果一个进程的请求会导致死锁，则不启动该进程

  

  会产生死锁 因为四个进程都需要至少一个R1

Detect

•Ignore any process not requesting more resources

recovery

1. 取消所有死锁进程
2. 将每个死锁进程备份到一些先前定义的检查点，并重新启动所有进程
3. 连续中止死锁进程，直到死锁不再存在
4. 依次抢占资源，直到死锁不再存在