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
[2] xv6锁-xchg
[3] xv6锁-CSDN
[4] xv6整体报告-百度文库
