xv6 锁

　　在xv6 中锁对象是 spinlock，spinlock中的locked为1的时候表示被占用，为0的时候锁空闲。

struct spinlock {
  uint locked;       // Is the lock held?
  ......
};

　　使用 acquire获取锁

void acquire(struct spinlock *lk)
{
  ........
  while(xchg(&lk->locked, 1) != 0);
  ......
}

　　该函数中通过xchg原子性交换locked和1，并返回locked的原来的值。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。

　　

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");
  /*
    //最终汇编类似于
    movq    *addr, %rdx
    movl    newval, %eax
    lock; xchgl (%rdx), %eax
    movl    %eax, result
    
  */
  return result;
}

　　xchg通过lock xchg实现原子性的交换，把*addr的老值放入eax中，然后在赋值给result。

　　释放锁

void release(struct spinlock *lk)
{
  ...
  // Release the lock, equivalent to lk->locked = 0.
  // This code can't use a C assignment, since it might
  // not be atomic. A real OS would use C atomics here.
  asm volatile("movl $0, %0" : "+m" (lk->locked) : );
  ...
}