rt_mq_send 函数问题

rt_err_t rt_mq_send (rt_mq_t mq, void* buffer, rt_size_t size)
{
    register rt_ubase_t temp;
    struct rt_mq_message *msg;
    /* greater than one message size */
    if (size > mq->msg_size) return -RT_ERROR;
#ifdef RT_USING_HOOK
    if (rt_object_put_hook != RT_NULL) rt_object_put_hook(&(mq->parent.parent));
#endif
    /* disable interrupt */
    temp = rt_hw_interrupt_disable();
    /* get a free list, there must be an empty item */
    msg = (struct rt_mq_message*)mq->msg_queue_free;
    /* message queue is full */
    if (msg == RT_NULL)
    {
        /* enable interrupt */
        rt_hw_interrupt_enable(temp);
        return -RT_EFULL;
    }
    /* move free list pointer */
    mq->msg_queue_free = msg->next;
    /* enable interrupt */
    rt_hw_interrupt_enable(temp);
    /* copy buffer */
    rt_memcpy(msg + 1, buffer, size);
    /* disable interrupt */
    temp = rt_hw_interrupt_disable();
    /* link msg to message queue */
    if (mq->msg_queue_tail != RT_NULL)
    {
        /* if the tail exists, */
        ((struct rt_mq_message*)mq->msg_queue_tail)->next = msg;  //这里指定tail的下个msg
    }
    /* the msg is the new tail of list, the next shall be NULL */
    msg->next = RT_NULL;
    /* set new tail */
    mq->msg_queue_tail = msg;                   //这里怎么又把msg替代tail呢？
    /* if the head is empty, set head */
    if (mq->msg_queue_head == RT_NULL)mq->msg_queue_head = msg; //如果head,tail都为空，那么msg不是占用两个空间？
    /* increase message entry */
    mq->entry ++;
    /* resume suspended thread */
    if( !rt_list_isempty(&mq->parent.suspend_thread) )
    {
        rt_ipc_object_resume(&(mq->parent));
        /* enable interrupt */
        rt_hw_interrupt_enable(temp);
        rt_schedule();
        return RT_EOK;
    }
    /* enable interrupt */
    rt_hw_interrupt_enable(temp);
    return RT_EOK;
}