register_filesystem函数解析

register_filesystem: register a new filesystem.
    1.Adds the file system passed to the list of file systems the kernel is aware of for mount and other syscalls. Returns 0 on success,or a negative errno code on an error.
    2.The &struct file_system_type that is passed is linked into the kernel structures and must not be freed until the file system has been unregistered.

int register_filesystem(struct file_system_type * fs)
{
    int res = 0;
    struct file_system_type ** p;

    BUG_ON(strchr(fs->name, '.'));
    if (fs->next)
        return -EBUSY;
    write_lock(&file_systems_lock);
    p = find_filesystem(fs->name, strlen(fs->name));
    if (*p)
        res = -EBUSY;
    else
        *p = fs;
    write_unlock(&file_systems_lock);
    return res;
}

    其中9与15行是保证对

static struct file_system_type *file_systems;

写操作的唯一性。
    其中10行，从file_system中查找是否有与&fs同名字文件系统，如果有返回指向结构的指针的指针，如果没有返回返回执行file_systems单链表最后元素的(*file_system_type)->next的地址。
    

static struct file_system_type **find_filesystem(const char *name, unsigned len)
{
    struct file_system_type **p;
    for (p=&file_systems; *p; p=&(*p)->next)
        if (strlen((*p)->name) == len &&
            strncmp((*p)->name, name, len) == 0)
            break;
    return p;
}

总结：从上面源码的分析可知，register_filesystem(fs);功能是将fs对象添加到全局的file_systems对象链表中。如果file_systems对象链表中有与fs同名字的对象，则返回错误的值，否则返回0.