文章目錄

新年開工第一篇，有點空閒翻博客，發現寫過關於container_of宏定義的博客之後，怎麼能不寫鏈表操作呢，豈不是斷篇了？那就把鏈表操作的筆記呀搬上來吧。
自從學C語言時接觸到鏈表後，直到再學Linux以前，一直以爲鏈表就那樣操作了，在結構體中定義指向結構體的指針，可以實現單向鏈表、雙向鏈表、循環鏈表等，自從看到Linux的鏈表之後，就顛覆了我的認知，原來鏈表還可以這麼搞；Linux的鏈表可以說做到了以不變應萬變。
Linux內核定義的鏈表結構不帶數據域，只需要兩個指針完成鏈表的操作，當然要通過鏈表指針找到實際的數據結構，就需要藉助container_of宏了。

鏈表指針結構體

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

鏈表指針結構體內部只定義了兩個指向結構體本身的指針；當需要用鏈表結構時，只需要在自定義結構體中定義一個鏈表類型的數據即可。

struct usbcamera_node
{
	struct list_head node;
	int channel;
	char id[32];
	int usb_port;
	//V4L2
	char devname[32];
	int fd;
	struct v4l2_format fmt;
	struct v4l2_streamparm parm;
	struct v4l2_requestbuffers req;
	struct buffer *buffers;
	int n_buffers;
	int poll_index[MAX_CHANNEL];
};

將struct list_head node定義爲結構體的第一項成員，還有一個用處就是，可以將指向list_head指針，進行強制轉換爲自定義結構體的指針。
定義一個鏈表頭結點並初始化

#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;
}

鏈表頭節點的初始化，即鏈表的前向與後向均指向自身；在使用LIST_HEAD定義時，已經初始化，也可是使用內聯函數INIT_LIST_HEAD對鏈表進行初始化。

鏈表的插入

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);
}


static inline void __list_add_rcu(struct list_head * new,
		struct list_head * prev, struct list_head * next)
{
	new->next = next;
	new->prev = prev;
	asm volatile ("":::"memory");//smp_wmb();
	next->prev = new;
	prev->next = new;
}
static inline void list_add_rcu(struct list_head *new, struct list_head *head)
{
	__list_add_rcu(new, head, head->next);
}
static inline void list_add_tail_rcu(struct list_head *new,
					struct list_head *head)
{
	__list_add_rcu(new, head->prev, head);
}

__list_add、__list_add_rcu是實際的操作函數，不同之處在於RCU方式下的執行順序不會被優化
list_add、list_add_rcu，用於將新節點添加到鏈表指定節點位置之後
list_add_tail、list_add_tail_rcu，用於將新節點添加到鏈表最後

鏈表的刪除

#define LIST_POISON1  ((void *) 0x00100100)
#define LIST_POISON2  ((void *) 0x00200200)
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_rcu(struct list_head *entry)
{
	__list_del(entry->prev, entry->next);
	entry->prev = LIST_POISON2;
}

從鏈表中刪除一個節點，需要改變該節點前向節點的後向節點、後向結點的前向節點。最後設置該節點的前驅節點和後繼結點指向LIST_POSITION1和LIST_POSITION2兩個特殊值，這樣設置是爲了保證不在鏈表中的節點項不可訪問，對LIST_POSITION1和LIST_POSITION2的訪問都將引起頁故障
entry->prev = LIST_POISON2; 給entry節點的prev指針賦值，此處沒給next賦值，就是爲了保證讀線程的完整性，已經獲取此條目地址的遍歷程序不會中斷，沒有獲取此條目的會獲得新的next地址。這裏也符合rcu理念，保證鏈表遍歷完整性。

鏈表的替換

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);
}

/**
 * list_replace_rcu - replace old entry by new one
 * @old : the element to be replaced
 * @new : the new element to insert
 *
 * The @old entry will be replaced with the @new entry atomically.
 * Note: @old should not be empty.
 */
static inline void list_replace_rcu(struct list_head *old,
				struct list_head *new)
{
	new->next = old->next;
	new->prev = old->prev;
	asm volatile ("":::"memory");//smp_wmb();
	new->next->prev = new;
	new->prev->next = new;
	old->prev = LIST_POISON2;
}

list_replace：用於替換新舊節點
list_replace_init：用於替換新舊節點，並初始化舊節點
list_replace_rcu：這裏用rcu保護機制給新節點的prev與next賦值，保證新節點的完整性，防止某些編譯器優化代碼執行順序，保證讀線程獲取的old節點地址要麼是舊的，要麼是已經賦好值得新指針。old->prev = LIST_POISON2，給old節點的prev賦值，保持next不變，防止已經獲取其地址的讀線程遍歷中斷，這也是rcu機制完整性思想。

鏈表的移動

/**
 * 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 inline void list_move(struct list_head *list, struct list_head *head)
{
	__list_del(list->prev, list->next);
	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 inline void list_move_tail(struct list_head *list,
				  struct list_head *head)
{
	__list_del(list->prev, list->next);
	list_add_tail(list, head);
}

list_move：將節點移動到鏈表頭部
list_move_tail：將節點移動到鏈表尾部

鏈表的遍歷

/**
 * 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; \
             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_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
 * @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
 * @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
 * @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_for_each_rcu	-	iterate over an rcu-protected list
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define list_for_each_rcu(pos, head) \
	for (pos = (head)->next; \
		prefetch(rcu_dereference(pos)->next), pos != (head); \
        	pos = pos->next)

#define __list_for_each_rcu(pos, head) \
	for (pos = (head)->next; \
		rcu_dereference(pos) != (head); \
        	pos = pos->next)

/**
 * list_for_each_safe_rcu
 * @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.
 *
 * Iterate over an rcu-protected list, safe against removal of list entry.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define list_for_each_safe_rcu(pos, n, head) \
	for (pos = (head)->next; \
		n = rcu_dereference(pos)->next, pos != (head); \
		pos = n)

/**
 * list_for_each_entry_rcu	-	iterate over rcu 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.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define list_for_each_entry_rcu(pos, head, member) \
	for (pos = list_entry((head)->next, typeof(*pos), member); \
		prefetch(rcu_dereference(pos)->member.next), \
			&pos->member != (head); \
		pos = list_entry(pos->member.next, typeof(*pos), member))


/**
 * list_for_each_continue_rcu
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 *
 * Iterate over an rcu-protected list, continuing after current point.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define list_for_each_continue_rcu(pos, head) \
	for ((pos) = (pos)->next; \
		prefetch(rcu_dereference((pos))->next), (pos) != (head); \
        	(pos) = (pos)->next)

程序訪問的變量如果都能在系統內存cache中則能提升性能，prefetch是內核中一個預熱內存函數，這樣下次遍歷時就能高效命中內存cache，從而提升程序性能；上面的代碼中遍歷鏈表時下次訪問的內存爲pos->next，故在每次遍歷時對pos->next進行預熱，從而提升性能。
ptr：結構體成員list_head的指針
type：結構體數據類型
member：包含list_head結構體的成員member的名稱
pos：指向當前結點的指針
head：指向雙向鏈表頭的指針
n：臨時用來保存指向pos的下一個結點的指針
list_for_each()：此種編歷時需要刪除節點時，list_del(pos)將pos的前後指針指向undefined state，導致kernel panic，list_del_init(pos)將pos前後指針指向自身，導致死循環。
list_for_each_safe()：首先將pos的後指針緩存到n，處理一個流程後再賦回pos，避免了這種情況發生。因此之遍歷鏈表不刪除結點時，可以使用list_for_each()，而當由刪除結點操作時，則要使用list_for_each_safe()。

好了鏈表的操作就寫這麼多了，未曾用到的就不寫了。

Linux鏈表操作

文章目錄

鏈表指針結構體

鏈表的插入

鏈表的刪除

鏈表的替換

鏈表的移動

鏈表的遍歷

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linux以太網驅動總結