linux内核链表分析

linux内核的链表一般都是双向循环链表，双向循环链表的效率是最高的，找头节点，尾节点，直接前驱，直接后继时间复杂度都是O(1)，而使用单链表，单向循环链表或其他形式的链表是不能完成的。

linux内核链表最大特点就是它的通用性，不必因为结构体中的数据域的不通而单独为操作链表设计一套方案.

linux内核在linux/list.h文件中定义了内核通用链表list_head类型基本结构：

/*
 * Simple doubly linked list implementation.
 *
 * Some of the internal functions ("__xxx") are useful when
 * manipulating whole lists rather than single entries, as
 * sometimes we already know the next/prev entries and we can
 * generate better code by using them directly rather than
 * using the generic single-entry routines.
 */
struct list_head {
struct list_head *next, *prev;
};
list_head定义了一个双向链表。
然后对链表头进行初始化，可使用两种方法进行。
/*
 * Simple doubly linked list implementation.
 *
 * Some of the internal functions ("__xxx") are useful when
 * manipulating whole lists rather than single entries, as
 * sometimes we already know the next/prev entries and we can
 * generate better code by using them directly rather than
 * using the generic single-entry routines.
 */

struct list_head {
struct list_head *next, *prev;
};

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
struct list_head name =LIST_HEAD_INIT(name)

static inline void INIT_LIST_HEAD(struct list_head *list)
{
list->next = list;
list->prev = list;
}

LIST_HEAD和INIT_LIST_HEAD都是对表头进行初始化，使next
和 prev指向自己。

**
 * list_empty - tests whether a list is empty
 * @head: the list to test.
 */
static inline int list_empty(const struct list_head *head)
{
return head->next == head;
}

**
 * list_empty - tests whether a list is empty
 * @head: the list to test.
 */
static inline int list_empty(const struct list_head *head)
{
return head->next == head;
}

通过这个函数看这个链表的前节点和后节点是否相等判断是否为空。

插入

向一个链表插入有两种方式：一是在头节点后面插入新节点，二是在链表末尾插入新节点。

(1)在头节点后面插入新节点

/*
 * Insert a new entry between two known consecutive entries.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
#ifndef CONFIG_DEBUG_LIST
static inline void __list_add(struct list_head *new,
     struct list_head *prev, struct list_head *next)
{
next->prev = new;
new->next = next;
new->prev = prev;
prev->next = new;
}
#else
extern void __list_add(struct list_head *new,
      struct list_head *prev, struct list_head *next);
#endif

/**
 * list_add - add a new entry
 * @new: new entry to be added
 * @head: list head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 */
static inline void list_add(struct list_head *new, struct list_head *head)
{
__list_add(new, head, head->next);
}

(2)在链表末尾插入新节点

/**
 * list_add_tail - add a new entry
 * @new: new entry to be added
 * @head: list head to add it before
 *
 * Insert a new entry before the specified head.
 * This is useful for implementing queues.
 */
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
__list_add(new, head->prev, head);
}

/*
 * Delete a list entry by making the prev/next entries
 * point to each other.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_del(struct list_head *prev, struct list_head *next)
{
next->prev = prev;
prev->next = next;
}

 删除
/**
 * list_del - deletes entry from list.
 * @entry: the element to delete from the list.
 * Note: list_empty() on entry does not return true after this, the entry is
 * in an undefined state.
 */
#ifndef CONFIG_DEBUG_LIST
static inline void list_del(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
entry->next = (void *)0xDEADBEEF;
entry->prev = (void *)0xBEEFDEAD;
}
#else
extern void list_del(struct list_head *entry);
#endif

遍历
/**
 * list_first_entry - get the first element from a list
 * @ptr: the list head to take the element from.
 * @type: the type of the struct this is embedded in.
 * @member: the name of the list_struct within the struct.
 *
 * Note, that list is expected to be not empty.
 */
#define list_first_entry(ptr, type, member) \
list_entry((ptr)->next, type, member)
从头节点的下一个节点开始遍历，一直末尾。
删除
/**
 * list_del - deletes entry from list.
 * @entry: the element to delete from the list.
 * Note: list_empty() on entry does not return true after this, the entry is
 * in an undefined state.
 */
#ifndef CONFIG_DEBUG_LIST
static inline void list_del(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
entry->next = (void *)0xDEADBEEF;
entry->prev = (void *)0xBEEFDEAD;
}
#else
extern void list_del(struct list_head *entry);
#endif

测试代码
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/list.h>

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("List Module");
MODULE_ALIAS("List module");

struct student
{
    char name[100];
    int num;
    struct list_head list;
};

struct student *pstudent;
struct student *tmp_student;
struct list_head student_list;
struct list_head *pos;

int mylist_init(void)
{
int i = 0;

INIT_LIST_HEAD(&student_list);

pstudent = kmalloc(sizeof(struct student)*5,GFP_KERNEL);
memset(pstudent,0,sizeof(struct student)*5);

for(i=0;i<5;i++)
{
       sprintf(pstudent[i].name,"Student%d",i+1);
pstudent[i].num = i+1; 
list_add( &(pstudent[i].list), &student_list);
} 

list_for_each(pos,&student_list)
{
tmp_student = list_entry(pos,struct student,list);
printk("<0>student %d name: %s\n",tmp_student->num,tmp_student->name);
}

return 0;
}

void mylist_exit(void)
{ 
int i ;
/* 实验：将for换成list_for_each来遍历删除结点，观察要发生的现象，并考虑解决办法 */
for(i=0;i<5;i++)
{
list_del(&(pstudent[i].list));     
}

kfree(pstudent);
}

module_init(mylist_init);
module_exit(mylist_exit);

这里采用头插法，也可改成为插法测试结果。