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---
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title: "[学习笔记]逐步解析Peterson算法为什么不会死等"
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pubDate: 2025-06-10
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description: '关于Peterson算法的学习笔记'
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author: "三叶"
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image:
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url: "https://files.seeusercontent.com/2026/03/25/sTq9/pasted-image-1774456630694.webp"
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alt: "zako2"
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tags: ["peterson算法", "操作系统"]
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---
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在OS课程上看到一个有意思的算法——Peterson算法,那么为什么Peterson算法能满足“忙则等待”、“空闲让进”、“有限等待”这三个条件呢?
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先上伪代码:
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```
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bool flag[2]; // 表示进入临界区意愿的数组,初始值都为false
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int turn = 0; // turn表示优先让哪个进程进入临界区
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// P0进程
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flag[0] = true; // 首先将自己的flag设置为1,声明自己需要使用临界区
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turn = 1; // 让P1优先执行,如果P1需要临界区
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while (flag[1] && turn == 1); // 检查P1是否需要临界区
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CRITICAL_SECTION;
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flag[0] = false;
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REMAINDER_SECTION;
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// P1进程
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flag[1] = true;
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turn = 0;
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while (flag[0] && turn == 0);
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CRITICAL_SECTION;
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flag[1] = false;
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REMAINDER_SECTION;
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```
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让我们来分情况分析。
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现在,我们假设P0和P1在并发执行,并且非常巧的是,他们一起完成了第一步,他们俩的flag都是false。
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此时:
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> **\[情况1\]**
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> 若P0先上处理机,将turn设置为1;
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> P1上处理机,将turn设置为0;
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> P0上处理机,P0检查flag和turn,发现trun被改为0,不满足等待条件,P0进入临界区;
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> P1上处理机,P1检查flag和turn,发现flag\[0\]为true且turn未被改变,P1等待。
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> P0上处理机,且P0结束使用临界区,将flag\[1\]改为false;
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> P1上处理机,发现flag\[0\]为false,结束等待,进入临界区;
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>
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> **\[情况2\]**
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> 若P0先上处理机,将turn设置为1;
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> P0继续使用处理机,发现flag\[0\]为true,turn仍然为1,满足等待条件,P0等待;
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> P1上处理机,将turn改为0;
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> P1继续使用处理机,发现flag\[1\]为true,turn仍然为0,满足等待条件,P1等待;
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> P0上处理机,发现turn变为0,不满足等待条件,P0解除等待进入临界区;
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> P1上处理机,发现等待条件仍满足,P1继续等待;
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> P0上处理机,且P0结束使用处理机,将flag\[0\]改为false;
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> P1上处理机,发现flag\[0\] == false, 不满足等待条件,结束等待,进入临界区;
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_YES, IT WORKS ON MY MACHINE._
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