近来在阅读linux-2.6.37中cgroup的相关代码，读到include/linux/cgroup.h的开头时犯了嘀咕：
#include <linux/sched.h>
#include <linux/cpumask.h>
#include <linux/nodemask.h>
#include <linux/rcupdate.h>
#include <linux/cgroupstats.h>
#include <linux/prio_heap.h>
#include <linux/rwsem.h>
#include <linux/idr.h>
#ifdef CONFIG_CGROUPS
struct cgroupfs_root;
struct cgroup_subsys;
struct inode;
struct cgroup;
struct css_id;
extern int cgroup_init_early(void);
extern int cgroup_init(void);
extern void cgroup_lock(void);
extern int cgroup_lock_is_held(void);
extern bool cgroup_lock_live_group (struct cgroup *cgrp);
......
在此头文件的后面有cgroupfs_root、cgroup_subsys、cgroup这三个结构的定义，为何此处需要这么几行代码？
将此处的"strct cgroup;“注释掉之后，重新编译内核报出如下错误和警告：
kernel/cgroup.c:1910: error: conflicting types for ‘cgroup_lock_live_group’
include/linux/cgroup.h:32: error: previous declaration of ‘cgroup_lock_live_group’ was here
kernel/cgroup.c:1918: error: conflicting types for ‘cgroup_lock_live_group’
include/linux/cgroup.h:32: error: previous declaration of ‘cgroup_lock_live_group’ was here
include/linux/cgroup.h:32: warning: ‘struct cgroup’ declared inside parameter list
include/linux/cgroup.h:32: warning: its scope is only this definition or declaration, which is probably not what you want
由此我们可以知道，由于在cgroup.h文件中，struct cgroup结构定义在函数声明
extern bool cgroup_lock_live_group (struct cgroup *cgrp);
之后，所以编译的时候将struct cgroup作为在参数列表中定义的空结构处理了，这样在编译cgroup.c的时候便与真实的cgroup_lock_live_group函数定义冲突。