Deadlock

简化自这篇博客

定义：

a deadlock is a state in which each member of a group of actions, is waiting for some other member to release a lock.

发生条件：

• Mutual Exclusion – There should be at least one non-shareable resource.//有不可共享资源

• Hold and Wait – A deadlock will occur if a process can hold a resource and then wait for another. If only every resource can wait for a resource (without holding), there won’t be any deadlock as no one is holding on to any resource.// 进程所获得的资源在未使用完毕之前，资源申请者不能强行地从资源占有者手中夺取资源，而只能由该资源的占有者进程自行释放

• No preemption – //占有又申请

• Circular Wait – If each process holds a resource and wait for another resource, which is held by another waiting process, and if it finally forms a circular wait sequence, a deadlock will occur.

预防方法：

• 当一个进程已占有了某些资源，它又申请新的资源，但不能立即被满足时，它必须释放所占有的全部资源，以后再重新申请。它所释放的资源可以分配给其它进程。
• 可以实行资源预先分配策略。即进程在运行前一次性地向系统申请它所需要的全部资源。如果某个进程所需的全部资源得不到满足，则不分配任何资源，此进程暂不运行。
• 打破循环等待条件，实行资源有序分配策略。采用这种策略，即把资源事先分类编号，按号分配，使进程在申请，占用资源时不会形成环路。所有进程对资源的请求必须严格按资源序号递增的顺序提出。
• 安全序列，银行家算法Banker's Algorithm 各种矩阵