【每天一个小知识】Linux信号量

int mmc_init_queue(struct mmc_queue *mq, struct mmc_card *card,
		   spinlock_t *lock, const char *subname)
{
    /* 省略··· */
    sema_init(&mq->thread_sem, 1);
    /* 省略··· */
}

// sema_init(struct semaphore, int); 以指定计数值都初始化动态创建信号量

void mmc_queue_suspend(struct mmc_queue *mq)
{
	struct request_queue *q = mq->queue;
	unsigned long flags;

	if (!(mq->flags & MMC_QUEUE_SUSPENDED)) {
		mq->flags |= MMC_QUEUE_SUSPENDED;

		spin_lock_irqsave(q->queue_lock, flags);
		blk_stop_queue(q);
		spin_unlock_irqrestore(q->queue_lock, flags);

		down(&mq->thread_sem);
	}
}

/**
 * mmc_queue_resume - resume a previously suspended MMC request queue
 * @mq: MMC queue to resume
 */
void mmc_queue_resume(struct mmc_queue *mq)
{
	struct request_queue *q = mq->queue;
	unsigned long flags;

	if (mq->flags & MMC_QUEUE_SUSPENDED) {
		mq->flags &= ~MMC_QUEUE_SUSPENDED;

		up(&mq->thread_sem);

		spin_lock_irqsave(q->queue_lock, flags);
		blk_start_queue(q);
		spin_unlock_irqrestore(q->queue_lock, flags);
	}
}

// 尝试获取信号量，如果信号量已被占用，则进入不可中断睡眠状态
down(struct semaphore *) 
// 释放信号量，如果睡眠队列不为空，则唤醒其中一个任务
up(struct semaphore *)

static int mmc_queue_thread(void *d)
{
	struct mmc_queue *mq = d;
	struct request_queue *q = mq->queue;

	current->flags |= PF_MEMALLOC;

	down(&mq->thread_sem);
	do {
		struct request *req = NULL;
		unsigned int cmd_flags = 0;

		spin_lock_irq(q->queue_lock);
		set_current_state(TASK_INTERRUPTIBLE);
		req = blk_fetch_request(q);
		mq->mqrq_cur->req = req;
		spin_unlock_irq(q->queue_lock);

		if (req || mq->mqrq_prev->req) {
			set_current_state(TASK_RUNNING);
			cmd_flags = req ? req->cmd_flags : 0;
			mq->issue_fn(mq, req);
			cond_resched();
			if (mq->flags & MMC_QUEUE_NEW_REQUEST) {
				mq->flags &= ~MMC_QUEUE_NEW_REQUEST;
				continue; /* fetch again */
			}

			/*
			 * Current request becomes previous request
			 * and vice versa.
			 * In case of special requests, current request
			 * has been finished. Do not assign it to previous
			 * request.
			 */
			if (cmd_flags & MMC_REQ_SPECIAL_MASK)
				mq->mqrq_cur->req = NULL;

			mq->mqrq_prev->brq.mrq.data = NULL;
			mq->mqrq_prev->req = NULL;
			swap(mq->mqrq_prev, mq->mqrq_cur);
		} else {
			if (kthread_should_stop()) {
				set_current_state(TASK_RUNNING);
				break;
			}
			up(&mq->thread_sem);
			schedule();
			down(&mq->thread_sem);
		}
	} while (1);
	up(&mq->thread_sem);

	return 0;
}