struct spinlock {

  uint locked;       // Is the lock held?

  ......

};

void acquire(struct spinlock *lk)

{

  ........

  while(xchg(&lk->locked, ) != );

  ......

}

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) :

               "" (newval) :

               "cc");

  /*

    //最终汇编类似于

    movq    *addr, %rdx

    movl    newval, %eax

    lock; xchgl (%rdx), %eax

    movl    %eax, result

  */

  return result;

}

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) : );

  ...

}