struct semaphore 
{ 
atomic_t count; 
int sleepers; 
wait_queue_head_t wait; 
};

static inline void down(struct semaphore*sem) 
{ 
/* 1 */sem->count--; //为原子操作 
if(sem->count<0) 
{ 
struct task_struct *tsk = current; 
DECLARE_WAITQUEUE(wait, tsk); 
tsk->state = TASK_UNINTERRUPTIBLE; 
add_wait_queue_exclusive(&sem->wait, &wait); 
spin_lock_irq(&semaphore_lock); 
/* 2 */ sem->sleepers++; 
for (;;) { 
int sleepers = sem->sleepers; 
/* 
* Add "everybody else" into it. They aren't 
* playing, because we own the spinlock. 
*/ 
/* 3 */ if (!atomic_add_negative(sleepers - 1, &sem->count)) { 
/* 4 */ sem->sleepers = 0; //这时sem->count=0 
break; 
} 
/* 4 */ sem->sleepers = 1; /* us - see -1 above */ // 这时sem 
->count 
=-1 
spin_unlock_irq(&semaphore_lock); 
schedule(); 
tsk->state = TASK_UNINTERRUPTIBLE; 
spin_lock_irq(&semaphore_lock); 
} 
spin_unlock_irq(&semaphore_lock); 
remove_wait_queue(&sem->wait, &wait); 
tsk->state = TASK_RUNNING; 
wake_up(&sem->wait); 
} 
}

void up(struct semaphore*sem) 
{ 
sem->count++; //为原子操作 
if(sem->count<=0) 
{ 
//唤醒等待队列中的一个符合条件的进程（因为每个进程都加了TASK_EXCLUSIVE标志） 
。 
};