Linux内核--链表结构

一、前言
    Linux内核链表结构是一种双向循环链表结构，与传统的链表结构不同，Linux内核链表结构仅包含前驱和后继指针，不包含数据域。使用链表结构，仅需在结构体成员中包含list_head*成员就行；链表结构的定义在linux/list.h头文件。

二、链表初始化

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;
}
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宏LIST_HEAD_INIT(name)和LIST_HEAD(name)的作用在于初始化一个链表头节点，并使其前驱指针和后继指针指向自身；内联函数INIT_LIST_HEAD同理；

三、添加节点

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;
}
static inline void list_add(struct list_head *new, struct list_head *head)
{
    __list_add(new, head, head->next);
}
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
    __list_add(new, head->prev, head);
}
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list_add：在头节点后插入节点，图示如下，node2为新增的节点：

list_add_tail在头节点前插入节点，图示如下，node2为新增的节点：

四、删除节点

static inline void __list_del(struct list_head * prev, struct list_head * next)
{
    next->prev = prev;
    prev->next = next;
}
static inline void list_del(struct list_head *entry)
{
    __list_del(entry->prev, entry->next);
    entry->next = LIST_POISON1;
    entry->prev = LIST_POISON2;
}
static inline void list_del_init(struct list_head *entry)
{
    __list_del(entry->prev, entry->next);
    INIT_LIST_HEAD(entry);
}
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list_del：删除链表中的entry节点，entry节点的前驱后继指针指向LIST_POSITION1和LIST_POSITION2两个特殊值，这样设置是为了保证不在链表中的节点项不可访问，对LIST_POSITION1和LIST_POSITION2的访问都将引起页故障。
list_del_init：删除原链表中的entry节点，然后重新初始化entry节点为头节点（使其前驱后继指针都指向自身）。

/*
 * Architectures might want to move the poison pointer offset
 * into some well-recognized area such as 0xdead000000000000,
 * that is also not mappable by user-space exploits:
 */
#ifdef CONFIG_ILLEGAL_POINTER_VALUE
# define POISON_POINTER_DELTA _AC(CONFIG_ILLEGAL_POINTER_VALUE, UL)
#else
# define POISON_POINTER_DELTA 0
#endif
 
/*
 * These are non-NULL pointers that will result in page faults
 * under normal circumstances, used to verify that nobody uses
 * non-initialized list entries.
 */
#define LIST_POISON1  ((void *) 0x00100100 + POISON_POINTER_DELTA)
#define LIST_POISON2  ((void *) 0x00200200 + POISON_POINTER_DELTA)
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链表删除的图示如下：

五、节点替换

static inline void list_replace(struct list_head *old,
                struct list_head *new)
{
    new->next = old->next;
    new->next->prev = new;
    new->prev = old->prev;
    new->prev->next = new;
}
 
static inline void list_replace_init(struct list_head *old,
                    struct list_head *new)
{
    list_replace(old, new);
    INIT_LIST_HEAD(old);
}
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list_replace：将旧节点替换为新节点，函数头两句对应下图2，新节点next指针指向node1，node1节点的prev指针指向新节点。后两句对应图3，新节点prev指针指向head，head节点的next指针指向新节点。此时old节点的next和prev指针指向仍保留着；
list_replace_init：将旧节点替换为新节点，并将旧节点重新初始化为头节点（前驱后继指针指向自身），对应下图4。


六、移动节点

static inline void list_move(struct list_head *list, struct list_head *head)
{
    __list_del(list->prev, list->next);
    list_add(list, head);
}
static inline void list_move_tail(struct list_head *list,
                  struct list_head *head)
{
    __list_del(list->prev, list->next);
    list_add_tail(list, head);
}
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list_move：将list节点移动至head节点后（对应下图示的node1节点移动）；

list_move_tail：将list节点移动至head节点前（对应下图示的node2节点移动）；

七、尾节点判断

static inline int list_is_last(const struct list_head *list,
                const struct list_head *head)
{
    return list->next == head;
}
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链表的最后一个节点特性：其后继指针next必将指向头节点head

八、链表空判断

static inline int list_empty(const struct list_head *head)
{
    return head->next == head;
}
static inline int list_empty_careful(const struct list_head *head)
{
    struct list_head *next = head->next;
    return (next == head) && (next == head->prev);
}
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list_empty和list_empty_careful都是判断链表是否为空。list_empty判断节点的后继指针next是否指向自身；list_empty_careful判断节点的后继指针和前驱指针是否均指向自身，其可用来判断链表是否为空且当前是否正在被修改。

九、链表旋转

static inline void list_rotate_left(struct list_head *head)
{
    struct list_head *first;
 
    if (!list_empty(head)) {
        first = head->next;
        list_move_tail(first, head);
    }
}
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list_rotate_left：链表节点向左移动，原先左边的节点向右移。相当于与前一节点互换位置。图示如下：

十、判断链表是否仅含单个节点

static inline int list_is_singular(const struct list_head *head)
{
    return !list_empty(head) && (head->next == head->prev);
}
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判断条件为链表不为空，且头指针的前驱和后继均指向同个节点

十一、合并链表

static inline void __list_splice(const struct list_head *list,
                 struct list_head *prev,
                 struct list_head *next)
{
    struct list_head *first = list->next;
    struct list_head *last = list->prev;
 
    first->prev = prev;
    prev->next = first;
 
    last->next = next;
    next->prev = last;
}
 
/**
 * list_splice - join two lists, this is designed for stacks
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice(const struct list_head *list,
                struct list_head *head)
{
    if (!list_empty(list))
        __list_splice(list, head, head->next);
}
 
/**
 * list_splice_tail - join two lists, each list being a queue
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice_tail(struct list_head *list,
                struct list_head *head)
{
    if (!list_empty(list))
        __list_splice(list, head->prev, head);
}
 
/**
 * list_splice_init - join two lists and reinitialise the emptied list.
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * The list at @list is reinitialised
 */
static inline void list_splice_init(struct list_head *list,
                    struct list_head *head)
{
    if (!list_empty(list)) {
        __list_splice(list, head, head->next);
        INIT_LIST_HEAD(list);
    }
}
 
/**
 * list_splice_tail_init - join two lists and reinitialise the emptied list
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * Each of the lists is a queue.
 * The list at @list is reinitialised
 */
static inline void list_splice_tail_init(struct list_head *list,
                     struct list_head *head)
{
    if (!list_empty(list)) {
        __list_splice(list, head->prev, head);
        INIT_LIST_HEAD(list);
    }
}
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链表初始状态：

first->prev = prev;

prev->next = first;

这里prev即head节点

last->next = next;

next->prev = last;

这里next即node1节点

INIT_LIST_HEAD(list);

最后一步，把list节点重新初始化为头节点，使其前驱后继指针指向自身。

上述图示描述了list_splice_init的链表合并过程，函数的作用是把list链表（除list节点自身）插入到head节点后（即head和head->next之间），并重新初始化list节点；

list_splice_tail_init则是与list_splice_init的区别仅是插入的位置不同，其是插入到head节点之前（即head->prev和head之间）。

linux中定义了很多优美的宏，值得我们深入学习。如下：
一、container_of和offsetof

首先介绍两个很好用的宏container_of和offsetof。offsetof宏用于计算结构体成员基于结构体首地址的偏移量，container_of宏用于获取结构体首地址（根据成员指针）。

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
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offsetof宏接受两个入参，分别为结构体类型和结构体成员名，该宏将0强制转换成结构体类型的指针，并取其成员的地址。结构体首地址为0，对应成员的地址即成员相对结构体首地址的偏移量。

/**
 * container_of - cast a member of a structure out to the containing structure
 * @ptr:    the pointer to the member.
 * @type:    the type of the container struct this is embedded in.
 * @member:    the name of the member within the struct.
 *
 */
#define container_of(ptr, type, member) ({            \
    const typeof(((type *)0)->member) * __mptr = (ptr);    \
    (type *)((char *)__mptr - offsetof(type, member)); })
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container_of宏接受三个入参，指向结构体成员的指针ptr，结构体类型type，结构体成员名member。该宏首先定义一个结构体成员类型的指针_mptr，类型的获取通过typeof，_mptr = ptr，并将_mptr强转为char*型，减去offsetof计算的偏移量，即得到结构体首地址。

二、list_entry

/**
 * list_entry - get the struct for this entry
 * @ptr:    the &struct list_head pointer.
 * @type:    the type of the struct this is embedded in.
 * @member:    the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
    container_of(ptr, type, member)
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list_entry即根据结构体成员指针ptr取得结构体首地址，如下例子使用：

/** 结构体定义 **/
struct student{
    int id;
    char name[20];
    list_head node;
}；
 
struct student stu;
char* ptr = &stu.node;
 
/** 宏使用如下 **/
struct student* s = list_entry(ptr, struct student, node);
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三、list_first_entry

/**
 * 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)
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ptr为链表头节点指针，type为结构体类型，member为结构体内成员名（结构体的链表成员）。list_first_entry宏取得链表首个节点的结构体首地址（头节点不算在内）。
四、list_for_each

#define list_for_each(pos, head) \
	for (pos = (head)->next; pos != (head); pos = pos->next)
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从链表首节点（不包含头节点）开始往后遍历。

六、list_for_each_prev

#define list_for_each_prev(pos, head) \
	for (pos = (head)->prev; pos != (head); pos = pos->prev)
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从链表首节点（不包含头节点）开始往前遍历。
七、list_for_each_safe

/**
 * list_for_each_safe - iterate over a list safe against removal of list entry
 * @pos:    the &struct list_head to use as a loop cursor.
 * @n:        another &struct list_head to use as temporary storage
 * @head:    the head for your list.
 */
#define list_for_each_safe(pos, n, head) \
    for (pos = (head)->next, n = pos->next; pos != (head); \
        pos = n, n = pos->next)
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list_for_each的加强版，支持遍历过程的节点删除操作，提高安全性。使用变量n提前保存节点pos的后继，避免遍历过程pos节点删除后，指向错误。

八、list_for_each_prev_safe

/**
 * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
 * @pos:    the &struct list_head to use as a loop cursor.
 * @n:        another &struct list_head to use as temporary storage
 * @head:    the head for your list.
 */
#define list_for_each_prev_safe(pos, n, head) \
	for (pos = (head)->prev, n = pos->prev; \
	     pos != (head); \
	     pos = n, n = pos->prev)
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list_for_each_prev的加强版，支持遍历过程的节点删除操作。

九、list_for_each_entry

/**
 * list_for_each_entry	-	iterate over list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 */
#define list_for_each_entry(pos, head, member)				\
	for (pos = list_first_entry(head, typeof(*pos), member);	\
	     &pos->member != (head);					\
	     pos = list_next_entry(pos, member))
 
/**
 * list_for_each_entry_reverse - iterate backwards over list of given type.
 * @pos:    the type * to use as a loop cursor.
 * @head:    the head for your list.
 * @member:    the name of the list_struct within the struct.
 */
#define list_for_each_entry_reverse(pos, head, member)            \
    for (pos = list_entry((head)->prev, typeof(*pos), member);    \
         prefetch(pos->member.prev), &pos->member != (head);     \
         pos = list_entry(pos->member.prev, typeof(*pos), member))
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list_for_each_entry：从链表首节点（不包含头节点）开始往后遍历，pos指向的是结构体，而不是结构体内的链表节点成员。与list_for_each不同，list_for_each遍历的是链表节点，而list_for_each_entry遍历的是由链表节点串起来的结构体链表。

list_for_each_entry_reverse：与list_for_each_entry相反，是往前遍历。