Linux 内核链表剖析（二十）

上节博客中，我们讲到了 Linux 中的宏定义 offsetof 与 container_of 宏。那么本节我们的课程目标就是一直 Linux 内核链表，使其适用于非 GNU 编译器，分析 Linux 内核中链表的基本实现。

我们首先来看看 Linux 内核链表的位置及其依赖：

1、位置：{linux-2.6.39}\\include\linux\list.h

2、依赖：

#include

#include

#include

#include

在移植时需要注意的事项：

1、清楚文件间的依赖：剥离依赖文件中与链表实现相关的代码

2、清楚平台相关代码（GNU C）：（{}），typeof，__builtin_prefetch，static inline

我们下来看看 list.h 的源码是怎样写的

#ifndef _LINUX_LIST_H#define _LINUX_LIST_H// #include // #include // #include // #include #ifndef offsetof#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)#endif#ifndef container_of#define container_of(ptr, type, member) ((type *)((char *)ptr - offsetof(type,member)))#endif#define prefetch(x) ((void)x)#define LIST_POISON1  (NULL)#define LIST_POISON2  (NULL)struct list_head {    struct list_head *next, *prev;};struct hlist_head {    struct hlist_node *first;};struct hlist_node {    struct hlist_node *next, **pprev;};/* * 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. */#define LIST_HEAD_INIT(name) { &(name), &(name) }#define LIST_HEAD(name) \    struct list_head name = LIST_HEAD_INIT(name)static void INIT_LIST_HEAD(struct list_head *list){    list->next = list;    list->prev = list;}/* * 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_LISTstatic void __list_add(struct list_head *node,                  struct list_head *prev,                  struct list_head *next){    next->prev = node;    node->next = next;    node->prev = prev;    prev->next = node;}#elseextern void __list_add(struct list_head *node,                  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 void list_add(struct list_head *node, struct list_head *head){    __list_add(node, head, head->next);}/** * 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 void list_add_tail(struct list_head *node, struct list_head *head){    __list_add(node, 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 void __list_del(struct list_head * prev, struct list_head * next){    next->prev = prev;    prev->next = next;}/** * 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_LISTstatic void __list_del_entry(struct list_head *entry){    __list_del(entry->prev, entry->next);}static void list_del(struct list_head *entry){    __list_del(entry->prev, entry->next);    entry->next = LIST_POISON1;    entry->prev = LIST_POISON2;}#elseextern void __list_del_entry(struct list_head *entry);extern void list_del(struct list_head *entry);#endif/** * list_del_init - deletes entry from list and reinitialize it. * @entry: the element to delete from the list. */static void list_del_init(struct list_head *entry){    __list_del_entry(entry);    INIT_LIST_HEAD(entry);}/** * list_move - delete from one list and add as another's head * @list: the entry to move * @head: the head that will precede our entry */static void list_move(struct list_head *list, struct list_head *head){    __list_del_entry(list);    list_add(list, head);}/** * list_move_tail - delete from one list and add as another's tail * @list: the entry to move * @head: the head that will follow our entry */static void list_move_tail(struct list_head *list,                  struct list_head *head){    __list_del_entry(list);    list_add_tail(list, head);}/** * list_is_last - tests whether @list is the last entry in list @head * @list: the entry to test * @head: the head of the list */static int list_is_last(const struct list_head *list,                const struct list_head *head){    return list->next == head;}/** * list_empty - tests whether a list is empty * @head: the list to test. */static int list_empty(const struct list_head *head){    return head->next == head;}/** * list_empty_careful - tests whether a list is empty and not being modified * @head: the list to test * * Description: * tests whether a list is empty _and_ checks that no other CPU might be * in the process of modifying either member (next or prev) * * NOTE: using list_empty_careful() without synchronization * can only be safe if the only activity that can happen * to the list entry is list_del_init(). Eg. it cannot be used * if another CPU could re-list_add() it. */static int list_empty_careful(const struct list_head *head){    struct list_head *next = head->next;    return (next == head) && (next == head->prev);}/** * list_rotate_left - rotate the list to the left * @head: the head of the list */static void list_rotate_left(struct list_head *head){    struct list_head *first;    if (!list_empty(head)) {        first = head->next;        list_move_tail(first, head);    }}/** * list_is_singular - tests whether a list has just one entry. * @head: the list to test. */static int list_is_singular(const struct list_head *head){    return !list_empty(head) && (head->next == head->prev);}static void __list_cut_position(struct list_head *list,        struct list_head *head, struct list_head *entry){    struct list_head *new_first = entry->next;    list->next = head->next;    list->next->prev = list;    list->prev = entry;    entry->next = list;    head->next = new_first;    new_first->prev = head;}/** * list_cut_position - cut a list into two * @list: a new list to add all removed entries * @head: a list with entries * @entry: an entry within head, could be the head itself *    and if so we won't cut the list * * This helper moves the initial part of @head, up to and * including @entry, from @head to @list. You should * pass on @entry an element you know is on @head. @list * should be an empty list or a list you do not care about * losing its data. * */static void list_cut_position(struct list_head *list,        struct list_head *head, struct list_head *entry){    if (list_empty(head))        return;    if (list_is_singular(head) &&        (head->next != entry && head != entry))        return;    if (entry == head)        INIT_LIST_HEAD(list);    else        __list_cut_position(list, head, entry);}static 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 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 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 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 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);    }}/** * 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)/** * 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)/** * list_for_each    -    iterate over a list * @pos:    the &struct list_head to use as a loop cursor. * @head:    the head for your list. */#define list_for_each(pos, head) \    for (pos = (head)->next; prefetch(pos->next), pos != (head); \            pos = pos->next)/** * __list_for_each    -    iterate over a list * @pos:    the &struct list_head to use as a loop cursor. * @head:    the head for your list. * * This variant differs from list_for_each() in that it's the * simplest possible list iteration code, no prefetching is done. * Use this for code that knows the list to be very short (empty * or 1 entry) most of the time. */#define __list_for_each(pos, head) \    for (pos = (head)->next; pos != (head); pos = pos->next)/** * list_for_each_prev    -    iterate over a list backwards * @pos:    the &struct list_head to use as a loop cursor. * @head:    the head for your list. */#define list_for_each_prev(pos, head) \    for (pos = (head)->prev; prefetch(pos->prev), pos != (head); \            pos = pos->prev)/** * 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)/** * 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; \         prefetch(pos->prev), pos != (head); \         pos = n, n = pos->prev)/** * 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_struct within the struct. */#define list_for_each_entry(pos, head, member)                \    for (pos = list_entry((head)->next, typeof(*pos), member);    \         prefetch(pos->member.next), &pos->member != (head);     \         pos = list_entry(pos->member.next, typeof(*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))/** * list_prepare_entry - prepare a pos entry for use in list_for_each_entry_continue() * @pos:    the type * to use as a start point * @head:    the head of the list * @member:    the name of the list_struct within the struct. * * Prepares a pos entry for use as a start point in list_for_each_entry_continue(). */#define list_prepare_entry(pos, head, member) \    ((pos) ? : list_entry(head, typeof(*pos), member))/** * list_for_each_entry_continue - continue iteration 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. * * Continue to iterate over list of given type, continuing after * the current position. */#define list_for_each_entry_continue(pos, head, member)         \    for (pos = list_entry(pos->member.next, typeof(*pos), member);    \         prefetch(pos->member.next), &pos->member != (head);    \         pos = list_entry(pos->member.next, typeof(*pos), member))/** * list_for_each_entry_continue_reverse - iterate backwards from the given point * @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. * * Start to iterate over list of given type backwards, continuing after * the current position. */#define list_for_each_entry_continue_reverse(pos, head, member)        \    for (pos = list_entry(pos->member.prev, typeof(*pos), member);    \         prefetch(pos->member.prev), &pos->member != (head);    \         pos = list_entry(pos->member.prev, typeof(*pos), member))/** * list_for_each_entry_from - iterate over list of given type from the current point * @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. * * Iterate over list of given type, continuing from current position. */#define list_for_each_entry_from(pos, head, member)             \    for (; prefetch(pos->member.next), &pos->member != (head);    \         pos = list_entry(pos->member.next, typeof(*pos), member))/** * list_for_each_entry_safe - iterate over list of given type safe against removal of list entry * @pos:    the type * to use as a loop cursor. * @n:        another type * to use as temporary storage * @head:    the head for your list. * @member:    the name of the list_struct within the struct. */#define list_for_each_entry_safe(pos, n, head, member)            \    for (pos = list_entry((head)->next, typeof(*pos), member),    \        n = list_entry(pos->member.next, typeof(*pos), member);    \         &pos->member != (head);                     \         pos = n, n = list_entry(n->member.next, typeof(*n), member))/** * list_for_each_entry_safe_continue - continue list iteration safe against removal * @pos:    the type * to use as a loop cursor. * @n:        another type * to use as temporary storage * @head:    the head for your list. * @member:    the name of the list_struct within the struct. * * Iterate over list of given type, continuing after current point, * safe against removal of list entry. */#define list_for_each_entry_safe_continue(pos, n, head, member)         \    for (pos = list_entry(pos->member.next, typeof(*pos), member),         \        n = list_entry(pos->member.next, typeof(*pos), member);        \         &pos->member != (head);                        \         pos = n, n = list_entry(n->member.next, typeof(*n), member))/** * list_for_each_entry_safe_from - iterate over list from current point safe against removal * @pos:    the type * to use as a loop cursor. * @n:        another type * to use as temporary storage * @head:    the head for your list. * @member:    the name of the list_struct within the struct. * * Iterate over list of given type from current point, safe against * removal of list entry. */#define list_for_each_entry_safe_from(pos, n, head, member)             \    for (n = list_entry(pos->member.next, typeof(*pos), member);        \         &pos->member != (head);                        \         pos = n, n = list_entry(n->member.next, typeof(*n), member))/** * list_for_each_entry_safe_reverse - iterate backwards over list safe against removal * @pos:    the type * to use as a loop cursor. * @n:        another type * to use as temporary storage * @head:    the head for your list. * @member:    the name of the list_struct within the struct. * * Iterate backwards over list of given type, safe against removal * of list entry. */#define list_for_each_entry_safe_reverse(pos, n, head, member)        \    for (pos = list_entry((head)->prev, typeof(*pos), member),    \        n = list_entry(pos->member.prev, typeof(*pos), member);    \         &pos->member != (head);                     \         pos = n, n = list_entry(n->member.prev, typeof(*n), member))/** * list_safe_reset_next - reset a stale list_for_each_entry_safe loop * @pos:    the loop cursor used in the list_for_each_entry_safe loop * @n:        temporary storage used in list_for_each_entry_safe * @member:    the name of the list_struct within the struct. * * list_safe_reset_next is not safe to use in general if the list may be * modified concurrently (eg. the lock is dropped in the loop body). An * exception to this is if the cursor element (pos) is pinned in the list, * and list_safe_reset_next is called after re-taking the lock and before * completing the current iteration of the loop body. */#define list_safe_reset_next(pos, n, member)                \    n = list_entry(pos->member.next, typeof(*pos), member)/* * Double linked lists with a single pointer list head. * Mostly useful for hash tables where the two pointer list head is * too wasteful. * You lose the ability to access the tail in O(1). */#define HLIST_HEAD_INIT { .first = NULL }#define HLIST_HEAD(name) struct hlist_head name = {  .first = NULL }#define INIT_HLIST_HEAD(ptr) ((ptr)->first = NULL)static void INIT_HLIST_NODE(struct hlist_node *h){    h->next = NULL;    h->pprev = NULL;}static int hlist_unhashed(const struct hlist_node *h){    return !h->pprev;}static int hlist_empty(const struct hlist_head *h){    return !h->first;}static void __hlist_del(struct hlist_node *n){    struct hlist_node *next = n->next;    struct hlist_node **pprev = n->pprev;    *pprev = next;    if (next)        next->pprev = pprev;}static void hlist_del(struct hlist_node *n){    __hlist_del(n);    n->next = LIST_POISON1;    n->pprev = LIST_POISON2;}static void hlist_del_init(struct hlist_node *n){    if (!hlist_unhashed(n)) {        __hlist_del(n);        INIT_HLIST_NODE(n);    }}static void hlist_add_head(struct hlist_node *n, struct hlist_head *h){    struct hlist_node *first = h->first;    n->next = first;    if (first)        first->pprev = &n->next;    h->first = n;    n->pprev = &h->first;}/* next must be != NULL */static void hlist_add_before(struct hlist_node *n,                    struct hlist_node *next){    n->pprev = next->pprev;    n->next = next;    next->pprev = &n->next;    *(n->pprev) = n;}static void hlist_add_after(struct hlist_node *n,                    struct hlist_node *next){    next->next = n->next;    n->next = next;    next->pprev = &n->next;    if(next->next)        next->next->pprev  = &next->next;}/* after that we'll appear to be on some hlist and hlist_del will work */static void hlist_add_fake(struct hlist_node *n){    n->pprev = &n->next;}/* * Move a list from one list head to another. Fixup the pprev * reference of the first entry if it exists. */static void hlist_move_list(struct hlist_head *old,                   struct hlist_head *node){    node->first = old->first;    if (node->first)        node->first->pprev = &node->first;    old->first = NULL;}#define hlist_entry(ptr, type, member) container_of(ptr,type,member)#define hlist_for_each(pos, head) \    for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); \         pos = pos->next)#define hlist_for_each_safe(pos, n, head) \    for (pos = (head)->first; pos && ({ n = pos->next; 1; }); \         pos = n)/** * hlist_for_each_entry    - iterate over list of given type * @tpos:    the type * to use as a loop cursor. * @pos:    the &struct hlist_node to use as a loop cursor. * @head:    the head for your list. * @member:    the name of the hlist_node within the struct. */#define hlist_for_each_entry(tpos, pos, head, member)             \    for (pos = (head)->first;                     \         pos && ({ prefetch(pos->next); 1;}) &&             \        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \         pos = pos->next)/** * hlist_for_each_entry_continue - iterate over a hlist continuing after current point * @tpos:    the type * to use as a loop cursor. * @pos:    the &struct hlist_node to use as a loop cursor. * @member:    the name of the hlist_node within the struct. */#define hlist_for_each_entry_continue(tpos, pos, member)         \    for (pos = (pos)->next;                         \         pos && ({ prefetch(pos->next); 1;}) &&             \        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \         pos = pos->next)/** * hlist_for_each_entry_from - iterate over a hlist continuing from current point * @tpos:    the type * to use as a loop cursor. * @pos:    the &struct hlist_node to use as a loop cursor. * @member:    the name of the hlist_node within the struct. */#define hlist_for_each_entry_from(tpos, pos, member)             \    for (; pos && ({ prefetch(pos->next); 1;}) &&             \        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \         pos = pos->next)/** * hlist_for_each_entry_safe - iterate over list of given type safe against removal of list entry * @tpos:    the type * to use as a loop cursor. * @pos:    the &struct hlist_node to use as a loop cursor. * @n:        another &struct hlist_node to use as temporary storage * @head:    the head for your list. * @member:    the name of the hlist_node within the struct. */#define hlist_for_each_entry_safe(tpos, pos, n, head, member)          \    for (pos = (head)->first;                     \         pos && ({ n = pos->next; 1; }) &&                  \        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \         pos = n)#endif

我们来看看 Linux 内核链表的实现：

1、带头结点的双向循环链表，且头结点为表中成员；

2、头结点的 next 指针指向首结点；

3、头结点的 prev 指针指向尾结点。

关系如下图所示

下来我们来看看 Linux 内核链表的结点定义，如下

那么问题来了，数据放在那里呢？所以说，下来我们要使用 struct list_head 自定义链表结点。如下

下来我们来加下自定义的链表结点

/** * list_replace - replace old entry by new one * @old : the element to be replaced * @new : the new element to insert * * If @old was empty, it will be overwritten. */static void list_replace(struct list_head *old,                struct list_head *node){    node->next = old->next;    node->next->prev = node;    node->prev = old->prev;    node->prev->next = node;}static void list_replace_init(struct list_head *old,                    struct list_head *node){    list_replace(old, node);    INIT_LIST_HEAD(old);}

下来我们来看看 Linux 内核链表的插入、删除、遍历等操作。

A、插入操作：a> 在链表头部插入：list_add(new,head)、b> 在链表尾部插入：list_add_tail(new,head)；如下

B、删除操作：如下

C、遍历操作：a> 正向遍历：list_for_each(pos,head)、b> 逆向遍历：list_for_each_prev(pos,head)；如下

下来我们来测试下代码，测试代码如下

#include #include #include "LinuxList.h"void list_demo_1(){    struct Node    {        struct list_head head;        int value;    };    struct Node l = {0};    struct list_head* list = (struct list_head*)&l;    struct list_head* slider = NULL;    int i = 0;    INIT_LIST_HEAD(list);    printf("Insert begin ...\n");    for(i=0; i<5; i++)    {        struct Node* n = (struct Node*)malloc(sizeof(struct Node));        n->value = i;        list_add_tail((struct list_head*)n, list);    }    list_for_each(slider, list)    {        printf("%d\n", ((struct Node*)slider)->value);    }    printf("Insert end ...\n");    printf("Delete begin ...\n");    list_for_each(slider, list)    {        if( ((struct Node*)slider)->value == 3 )        {            list_del(slider);            free(slider);            break;        }    }    list_for_each(slider, list)    {        printf("%d\n", ((struct Node*)slider)->value);    }    printf("Delete end ...\n");}void list_demo_2(){    struct Node    {        int value;        struct list_head head;    };    struct Node l = {0};    struct list_head* list = &l.head;    struct list_head* slider = NULL;    int i = 0;    INIT_LIST_HEAD(list);    printf("Insert begin ...\n");    for(i=0; i<5; i++)    {        struct Node* n = (struct Node*)malloc(sizeof(struct Node));        n->value = i;        list_add(&n->head, list);    }    list_for_each(slider, list)    {        printf("%d\n", list_entry(slider, struct Node, head)->value);    }    printf("Insert end ...\n");    printf("Delete begin ...\n");    list_for_each(slider, list)    {        struct Node* n = list_entry(slider, struct Node, head);        if( n->value == 3 )        {            list_del(slider);            free(n);            break;        }    }    list_for_each(slider, list)    {        printf("%d\n", list_entry(slider, struct Node, head)->value);    }    printf("Delete end ...\n");}int main(){    list_demo_1();    list_demo_2();    return 0;}

我们编译看看结果

输出结果如我们所想的那样，现在移植已经完成。经过今天对 Linux 内核中链表的移植，总结如下：1、Linux 内核链表移植时需要剔除依赖以及平台相关代码；2、Linux 内核链表是带头节点的双向循环链表；3、使用 Linux 内核链表时需要自定义链表节点：a> 将 struct list_head 作为结点结构体的第一个成员或最后一个成员；b> struct list_head 作为最后一个成员时，需要使用 list_entry 宏；c> list_entry 的定义中使用了 container_of 宏。