XV6源代码阅读-同步机制

柔情只为你懂 2021-11-04 15:48 197阅读 0赞

[XV6源代码阅读-同步机制][XV6_-]

## Exercise1 源代码阅读 ##

锁部分：spinlock.h/spinlock.c以及相关其他文件代码

// Mutual exclusion lock.
    struct spinlock {
      uint locked; // 0未被占用, 1已被占用
      
      // For debugging:
      char *name;        // Name of lock.
      struct cpu *cpu;   // The cpu holding the lock.
      uint pcs[10];      // The call stack (an array of program counters)
                         // that locked the lock.
    };
    
    // 初始化自旋锁
    void initlock(struct spinlock *lk, char *name)
    {
      lk->name = name;
      lk->locked = 0;
      lk->cpu = 0;
    }
    
    // Acquire the lock.
    // Loops (spins) until the lock is acquired.
    // Holding a lock for a long time may cause
    // other CPUs to waste time spinning to acquire it.
    void acquire(struct spinlock *lk)
    {
      // 关中断
      pushcli(); // disable interrupts to avoid deadlock.
      if(holding(lk)) // 判断锁的持有是否为当前cpu
        panic("acquire");
    
      // The xchg is atomic.
      // It also serializes, so that reads after acquire are not
      // reordered before it. 
      while(xchg(&lk->locked, 1) != 0); // 拿不到锁开始自旋
    
      // Record info about lock acquisition for debugging.
      lk->cpu = cpu;
      getcallerpcs(&lk, lk->pcs);
    }
    
    // Release the lock.
    void release(struct spinlock *lk)
    {
      if(!holding(lk))
        panic("release");
    
      lk->pcs[0] = 0;
      lk->cpu = 0;
    
      // The xchg serializes, so that reads before release are 
      // not reordered after it.  The 1996 PentiumPro manual (Volume 3,
      // 7.2) says reads can be carried out speculatively and in
      // any order, which implies we need to serialize here.
      // But the 2007 Intel 64 Architecture Memory Ordering White
      // Paper says that Intel 64 and IA-32 will not move a load
      // after a store. So lock->locked = 0 would work here.
      // The xchg being asm volatile ensures gcc emits it after
      // the above assignments (and after the critical section).
      xchg(&lk->locked, 0);
    
      popcli();
    }

## Exercise2 带着问题阅读 ##

1.什么是临界区? 什么是同步和互斥? 什么是竞争状态? 临界区操作时中断是否应该开启? 中断会有什么影响? XV6的锁是如何实现的，有什么操作? xchg 是什么指令，该指令有何特性?

*  临界区(Critical Section)：访问临界区的那段代码，多个进程/线程必须互斥进入临界区；
 *  同步(Synchronization)：指多个进程/线程能够按照程序员期望的方式来协调执行顺序，为了实现这个目的，必须要借助于同步机制(如信号量，条件变量，管程等)；
 *  互斥(Mutual Exclusion)：互斥的目的是保护临界区；
 *  竞争状态：竞争是基于并发环境下的，单个进程/线程不存在竞争，在并发环境下，多个进程/线程都需要请求某资源的时候，只有竞争到该资源的进程/线程才能够执行，释放资源后，剩余进程/线程按照预定的算法策略重新竞争；
 *  操作临界区必须关中断，对临界区的操作是原子性的；
 *  中断影响：中断降低了并发性能，同时中断也会导致频繁的上下文切换，上下文切换会导致tlb快表失效，因此要尽可能的缩减中断处理的时间；
 *  自旋锁(Spinlock)：xv6中利用该数据结构实现多个进程/线程同步和互斥访问临界区。当进程/线程请求锁失败时进入循环，直至锁可用并成功拿到后返回，对于单cpu系统自旋锁浪费CPU资源，不利于并发，自旋锁的优势体现在多CPU系统下，XV6支持多CPU。主要接口有void initlock(struct spinlock \* lk, char \* name)、void initlock(struct spinlock \* lk, char \* name)、void release(struct spinlock \* lk)；
 *  xchg：xchg()函数使用GCC的内联汇编语句，该函数中通过xchg原子性交换spinlock.locked和newval，并返回spinlock.locked原来的值。当返回值为1时，说明其他线程占用了该锁，继续循环等待；当返回值为0时，说明其他地方没有占用该锁，同时locked本设置成1了，所以该锁被此处占用。

// x86.h 调用方式如xchg(&lk->locked, 1)
    static inline uint xchg(volatile uint *addr, uint newval)
    {
      uint result;
      
      // The + in "+m" denotes a read-modify-write operand.
      asm volatile("lock; xchgl %0, %1" :
                   "+m" (*addr), "=a" (result) :
                   "1" (newval) :
                   "cc");
      return result;
    }

2.基于XV6的spinlock, 请给出实现信号量、读写锁、信号机制的设计方案(三选二，请写出相应的伪代码)?

*  信号量实现

struct semaphore {
      int value;
      struct spinlock lock;
      struct proc *queue[NPROC]; // 进程等待队列,这是一个循环队列
      int end;   // 队尾
      int start; // 队头
    };
    
    // 初始化信号量
    void sem_init(struct semaphore *s, int value) {
      s->value = value;
      initlock(&s->lock, "semaphore_lock");
      end = start = 0;
    }
    
    void sem_wait(struct semaphore *s) {
      acquire(&s->lock); // 竞争锁,如果竞争不到进入自旋
      s->value--; 
      if (s->value < 0) {
        s->queue[s->end] = myproc(); // myproc()获取当前进程, 放入队尾
        s->end = (s->end + 1) % NPROC; // 循环队列计算新的队尾
        // 1. 释放锁(下一个sem_wait的进程才能进入acquire),
        // 2. 然后进入睡眠等待, 被唤醒时重新竞争锁
        sleep(myproc(), &s->lock); 
      }
      release(&s->lock);
    }
    
    void sem_signal(struct semaphore *s) {
      acquire(&s->lock); // 竞争锁
      s->value++;
      if (s->value <= 0) {
        wakeup(s->queue[s->start]); // 唤醒循环队列头的进程
        s->queue[s->start] = 0; 
        s->start = (s->start + 1) % NPROC; // 重新计算队头
      }
      release(&s->lock);
    }
    
    // proc.h
    // Per-process state
    struct proc {
      uint sz;                     // Size of process memory (bytes)
      pde_t* pgdir;                // Page table
      char *kstack;                // Bottom of kernel stack for this process
      enum procstate state;        // Process state
      volatile int pid;            // Process ID
      struct proc *parent;         // Parent process
      struct trapframe *tf;        // Trap frame for current syscall
      struct context *context;     // swtch() here to run process
      void *chan;                  // If non-zero, sleeping on chan
      int killed;                  // If non-zero, have been killed
      struct file *ofile[NOFILE];  // Open files
      struct inode *cwd;           // Current directory
      char name[16];               // Process name (debugging)
    };

## 参考文献 ##

\[1\] [xv6锁-博客园][xv6_-]  
\[2\] [xv6锁-xchg][xv6_-xchg]  
\[3\] [xv6锁-CSDN][xv6_-CSDN]  
\[4\] [xv6整体报告-百度文库][xv6_- 1]

posted on 2019-06-09 16:30 [荒野之萍][Link 1] 阅读( ...) 评论( ...) [编辑][Link 2] 收藏

转载于:https://www.cnblogs.com/icoty23/p/10993878.html

[XV6_-]: https://www.cnblogs.com/icoty23/p/10993878.html
[xv6_-]: https://www.cnblogs.com/hehao98/p/10678493.html
[xv6_-xchg]: https://www.cnblogs.com/hygblog/p/9361888.html
[xv6_-CSDN]: https://blog.csdn.net/qq_25426415/article/details/54631192
[xv6_- 1]: https://wenku.baidu.com/view/339ba16e7e21af45b307a8e6.html
[Link 1]: https://www.cnblogs.com/icoty23/
[Link 2]: https://i.cnblogs.com/EditPosts.aspx?postid=10993878