xv6 鎖

 

　　在xv6 中鎖對象是 spinlock，spinlock中的locked爲1的時候表示被佔用，爲0的時候鎖空閒。

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

 

　　使用 acquire獲取鎖

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

　　該函數中經過xchg原子性交換locked和1，並返回locked的原來的值。當返回值爲1時，說明其餘線程佔用了該鎖，繼續循環等待；當返回值爲0時，說明其餘地方沒有佔用該鎖，同時locked本設置成1了，因此該鎖被此處佔用。

　　

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");
  /*
    //最終彙編相似於
    movq    *addr, %rdx
    movl    newval, %eax
    lock; xchgl (%rdx), %eax
    movl    %eax, result
    
  */
  return result;
}

　　xchg經過lock xchg實現原子性的交換，把*addr的老值放入eax中，而後在賦值給result。

 

　　釋放鎖

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) : );
  ...
}