内核版本：linux-6.6.69 longterm

链表

双向链表

Linux内核实现了一套纯链表的封装，链表节点数据结构只有指针区而没有数据区，另外还封装了各种操作函数，如创建节点函数、插入节点函数、删除节点函数、遍历节点函数等。

linux 内核中的链表在文件include/linux/types.h中定义类型，在include/linux/list.h中定义操作。

Linux内核链表使用struct list_head数据结构来描述。

// include/linux/types.h
struct list_head {
	struct list_head *next, *prev;
};

struct list_head数据结构不包含链表节点的数据区，通常是嵌入其他数据结构。
链表头的初始化有两种方法，一种是静态初始化，另一种动态初始化。把next和prev指针都初始化并指向自己，这样便初始化了一个带头节点的空链表。

// include/linux/list.h
#define LIST_HEAD_INIT(name) { &(name), &(name) }

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

/**
 * INIT_LIST_HEAD - Initialize a list_head structure
 * @list: list_head structure to be initialized.
 *
 * Initializes the list_head to point to itself.  If it is a list header,
 * the result is an empty list.
 */
static inline void INIT_LIST_HEAD(struct list_head *list)
{
	WRITE_ONCE(list->next, list);
	WRITE_ONCE(list->prev, list);
}

添加节点到一个链表中，内核提供了几个接口函数，如list_add()是把一个节点添加到表头，list_add_tail()是插入表尾。

// include/linux/list.h
/*
 * Insert a new entry between two known consecutive entries.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next)
{
	if (!__list_add_valid(new, prev, next))
		return;

	next->prev = new;
	new->next = next;
	new->prev = prev;
	WRITE_ONCE(prev->next, new);
}

/**
 * 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);
}


/**
 * 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);
}

遍历节点的接口函数。

// include/linux/list.h
/**
 * 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; !list_is_head(pos, (head)); pos = pos->next)

这个宏只是遍历一个一个节点的当前位置，那么如何获取节点本身的数据结构呢？这里还需要使用list_entry()宏。

// include/linux/list.h
/**
 * 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_head within the struct.
 */
#define list_entry(ptr, type, member) \
	container_of(ptr, type, member)

container_of也是一个基本的宏

// include/linux/container_of.h
/**
 * 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.
 *
 * WARNING: any const qualifier of @ptr is lost.
 */
#define container_of(ptr, type, member) ({				\
	void *__mptr = (void *)(ptr);					\
	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
		      __same_type(*(ptr), void),			\
		      "pointer type mismatch in container_of()");	\
	((type *)(__mptr - offsetof(type, member))); })

其中offsetof()宏是通过把0地址转换为type类型的指针，然后去获取该结构体中member成员的指针，也就是获取了member在type结构体中的偏移量。最后用指针ptr减去offset，就得到type结构体的真实地址了。

哈希链表

哈希链表（Hash List）是指在对需要存储的数据进行hash时，如果产生了冲突，就使用链表的方式将产生冲突的数据进行存储。通常情况下，哈希表中元素的使用顺序是：数据存储–>数据获取–>数据删除。

内核中哈希链表在include/linux/list.h中实现。

Linux内核链表使用struct hlist_head数据结构来描述哈希表，使用struct hlist_node数据结构来描述链表结构。代码位于include/linux/types.h文件中。

struct hlist_head {
	struct hlist_node *first;
};

struct hlist_node {
	struct hlist_node *next, **pprev;
};

pprev二级指针存在的意义是因为效率。向链表中添加节点时可以直接添加到头部，但是当删除节点时，不但要记录即将删除节点的前一个节点，还需要判断删除的节点是否是头结点。如果使用pprev指针，那么在删除节点时直接使用*(del_node->pprev) = del_node->next即可。

哈希链表初始化

哈希链表初始化分为初始化hlist_head的节点和初始化hlist_node节点。

#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 inline void INIT_HLIST_NODE(struct hlist_node *h)
{
	h->next = NULL;
	h->pprev = NULL;
}

添加节点
添加节点函数有三个，分别是添加到指定哈希链表头上hlist_add_head()、添加到指定节点前面hlist_add_before()、添加到指定节点后面hlist_add_behind()。

/**
 * hlist_add_head - add a new entry at the beginning of the hlist
 * @n: new entry to be added
 * @h: hlist head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 */
static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
{
	struct hlist_node *first = h->first;
	WRITE_ONCE(n->next, first);
	if (first)
		WRITE_ONCE(first->pprev, &n->next);
	WRITE_ONCE(h->first, n);
	WRITE_ONCE(n->pprev, &h->first);
}

/**
 * hlist_add_before - add a new entry before the one specified
 * @n: new entry to be added
 * @next: hlist node to add it before, which must be non-NULL
 */
static inline void hlist_add_before(struct hlist_node *n,
				    struct hlist_node *next)
{
	WRITE_ONCE(n->pprev, next->pprev);
	WRITE_ONCE(n->next, next);
	WRITE_ONCE(next->pprev, &n->next);
	WRITE_ONCE(*(n->pprev), n);
}

/**
 * hlist_add_behind - add a new entry after the one specified
 * @n: new entry to be added
 * @prev: hlist node to add it after, which must be non-NULL
 */
static inline void hlist_add_behind(struct hlist_node *n,
				    struct hlist_node *prev)
{
	WRITE_ONCE(n->next, prev->next);
	WRITE_ONCE(prev->next, n);
	WRITE_ONCE(n->pprev, &prev->next);

	if (n->next)
		WRITE_ONCE(n->next->pprev, &n->next);
}

删除节点
删除节点函数有两个，分别是删除节点hlist_del()、删除节点并初始化已删除的节点hlist_del_init()。

static inline void __hlist_del(struct hlist_node *n)
{
	struct hlist_node *next = n->next;
	struct hlist_node **pprev = n->pprev;

	WRITE_ONCE(*pprev, next);
	if (next)
		WRITE_ONCE(next->pprev, pprev);
}

/**
 * hlist_del - Delete the specified hlist_node from its list
 * @n: Node to delete.
 *
 * Note that this function leaves the node in hashed state.  Use
 * hlist_del_init() or similar instead to unhash @n.
 */
static inline void hlist_del(struct hlist_node *n)
{
	__hlist_del(n);
	n->next = LIST_POISON1;
	n->pprev = LIST_POISON2;
}

/**
 * hlist_del_init - Delete the specified hlist_node from its list and initialize
 * @n: Node to delete.
 *
 * Note that this function leaves the node in unhashed state.
 */
static inline void hlist_del_init(struct hlist_node *n)
{
	if (!hlist_unhashed(n)) {
		__hlist_del(n);
		INIT_HLIST_NODE(n);
	}
}

移动节点
将哈希链表从一个移动到另外一个。

/**
 * hlist_move_list - Move an hlist
 * @old: hlist_head for old list.
 * @new: hlist_head for new list.
 *
 * Move a list from one list head to another. Fixup the pprev
 * reference of the first entry if it exists.
 */
static inline void hlist_move_list(struct hlist_head *old,
				   struct hlist_head *new)
{
	new->first = old->first;
	if (new->first)
		new->first->pprev = &new->first;
	old->first = NULL;
}

遍历哈希链表

#define hlist_for_each(pos, head) \
	for (pos = (head)->first; pos ; pos = pos->next)

这个宏只是遍历一个一个节点的当前位置，那么如何获取节点本身的数据结构呢？这里还需要使用hlist_entry()宏。

#define hlist_entry(ptr, type, member) container_of(ptr,type,member)

红黑树

红黑树（Red Black Tree）被广泛应用在内核的内存管理和进程调度中，用于将排序的元素组织到树中。红黑树被广泛应用在计算机科学的各个领域中，它在速度和实现复杂度之间提供一个很好的平衡。

红黑树是具有以下特征的二叉树。

• 每个节点或红或黑。
• 每个叶节点是黑色的。
• 如果结点都是红色，那么两个子结点都是黑色。
• 从一个内部结点到叶结点的简单路径上，对所有叶节点来说，黑色结点的数目都是相同的。

红黑树的一个优点是，所有重要的操作（例如插入、删除、搜索）都可以在O(log n)时间内完成，n为树中元素的数目。更详细的介绍参考：https://en.wikipedia.org/wiki/Red%E2%80%93black_tree

在linux内核中，红黑树在include/linux/rbtree.h和lib/rbtree.c中定义和实现，有关内核中实现文档参考：Documentation/core-api/rbtree.rstm，关于如何使用内核中的红黑树，文档里也给了详细的例子。

无锁环形缓冲区

生产者和消费者模型是计算机编程中最常见的一种模型。生产者产生数据，而消费者消耗数据，如一个网络设备，硬件设备接收网络包，然后应用程序读取网络包。环形缓冲区是实现生产者和消费者模型的经典算法。环形缓冲区通常有一个读指针和一个写指针。读指针指向环形缓冲区中可读的数据，写指针指向环形缓冲区可写的数据。通过移动读指针和写指针实现缓冲区数据的读取和写入。

在Linux内核中，KFIFO是采用无锁环形缓冲区的实现。FIFO的全称是“First In First Out”，即先进先出的数据结构，它采用环形缓冲区的方法来实现，并提供一个无边界的字节流服务。采用环形缓冲区的好处是，当一个数据元素被消耗之后，其余数据元素不需要移动其存储位置，从而减少复制，提高效率。

在linux内核中，无锁环形缓冲区在文件include/linux/kfifo.h和lib/kfifo.c中定义和实现。

1. 创建KFIFO
   在使用KFIFO之前需要进行初始化，这里有静态初始化和动态初始化两种方式。

/**
 * kfifo_alloc - dynamically allocates a new fifo buffer
 * @fifo: pointer to the fifo
 * @size: the number of elements in the fifo, this must be a power of 2
 * @gfp_mask: get_free_pages mask, passed to kmalloc()
 *
 * This macro dynamically allocates a new fifo buffer.
 *
 * The number of elements will be rounded-up to a power of 2.
 * The fifo will be release with kfifo_free().
 * Return 0 if no error, otherwise an error code.
 */
#define kfifo_alloc(fifo, size, gfp_mask) \
__kfifo_int_must_check_helper( \
({ \
	typeof((fifo) + 1) __tmp = (fifo); \
	struct __kfifo *__kfifo = &__tmp->kfifo; \
	__is_kfifo_ptr(__tmp) ? \
	__kfifo_alloc(__kfifo, size, sizeof(*__tmp->type), gfp_mask) : \
	-EINVAL; \
}) \
)

该函数创建并分配一个大小为size的KFIFO环形缓冲区。第一个参数fifo是指向该环形缓冲区的struct kfifo数据结构；第二个参数size是指定缓冲区元素的数量；第三个参数gfp_mask表示分配KFIFO元素使用的分配掩码。

静态分配可以使用如下的宏。

/**
 * INIT_KFIFO - Initialize a fifo declared by DECLARE_KFIFO
 * @fifo: name of the declared fifo datatype
 */
#define INIT_KFIFO(fifo) \
(void)({ \
	typeof(&(fifo)) __tmp = &(fifo); \
	struct __kfifo *__kfifo = &__tmp->kfifo; \
	__kfifo->in = 0; \
	__kfifo->out = 0; \
	__kfifo->mask = __is_kfifo_ptr(__tmp) ? 0 : ARRAY_SIZE(__tmp->buf) - 1;\
	__kfifo->esize = sizeof(*__tmp->buf); \
	__kfifo->data = __is_kfifo_ptr(__tmp) ?  NULL : __tmp->buf; \
})

/**
 * DEFINE_KFIFO - macro to define and initialize a fifo
 * @fifo: name of the declared fifo datatype
 * @type: type of the fifo elements
 * @size: the number of elements in the fifo, this must be a power of 2
 *
 * Note: the macro can be used for global and local fifo data type variables.
 */
#define DEFINE_KFIFO(fifo, type, size) \
	DECLARE_KFIFO(fifo, type, size) = \
	(typeof(fifo)) { \
		{ \
			{ \
			.in	= 0, \
			.out	= 0, \
			.mask	= __is_kfifo_ptr(&(fifo)) ? \
				  0 : \
				  ARRAY_SIZE((fifo).buf) - 1, \
			.esize	= sizeof(*(fifo).buf), \
			.data	= __is_kfifo_ptr(&(fifo)) ? \
				NULL : \
				(fifo).buf, \
			} \
		} \
	}

2. 入列
   把数据写入KFIFO环形缓冲区可以使用kfifo_in()函数接口。

/**
 * kfifo_in - put data into the fifo
 * @fifo: address of the fifo to be used
 * @buf: the data to be added
 * @n: number of elements to be added
 *
 * This macro copies the given buffer into the fifo and returns the
 * number of copied elements.
 *
 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these macro.
 */
#define	kfifo_in(fifo, buf, n) \
({ \
	typeof((fifo) + 1) __tmp = (fifo); \
	typeof(__tmp->ptr_const) __buf = (buf); \
	unsigned long __n = (n); \
	const size_t __recsize = sizeof(*__tmp->rectype); \
	struct __kfifo *__kfifo = &__tmp->kfifo; \
	(__recsize) ?\
	__kfifo_in_r(__kfifo, __buf, __n, __recsize) : \
	__kfifo_in(__kfifo, __buf, __n); \
})

该函数把buf指针指向的n个数据复制到KFIFO环形缓冲区中。第一个参数fifo指的是KFIFO环形缓冲区；第二个参数buf指向要复制的数据的buffer；第三个数据是要复制数据元素的数量。

3. 出列
   从KFIFO环形缓冲区中列出或者摘取数据可以使用kfifo_out()函数接口。

/**
 * kfifo_out - get data from the fifo
 * @fifo: address of the fifo to be used
 * @buf: pointer to the storage buffer
 * @n: max. number of elements to get
 *
 * This macro get some data from the fifo and return the numbers of elements
 * copied.
 *
 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these macro.
 */
#define	kfifo_out(fifo, buf, n) \
__kfifo_uint_must_check_helper( \
({ \
	typeof((fifo) + 1) __tmp = (fifo); \
	typeof(__tmp->ptr) __buf = (buf); \
	unsigned long __n = (n); \
	const size_t __recsize = sizeof(*__tmp->rectype); \
	struct __kfifo *__kfifo = &__tmp->kfifo; \
	(__recsize) ?\
	__kfifo_out_r(__kfifo, __buf, __n, __recsize) : \
	__kfifo_out(__kfifo, __buf, __n); \
}) \
)

该函数是从fifo指向的环形缓冲区中复制n个数据元素到buf指向的缓冲区中。如果KFIFO环形缓冲区的数据元素小于n个，那么复制出去的数据元素小于n个。

4. 获取缓冲区大小
   KFIFO提供了几个接口函数来查询环形缓冲区的状态。

/**
 * kfifo_size - returns the size of the fifo in elements
 * @fifo: address of the fifo to be used
 */
#define kfifo_size(fifo)	((fifo)->kfifo.mask + 1)

/**
 * kfifo_len - returns the number of used elements in the fifo
 * @fifo: address of the fifo to be used
 */
#define kfifo_len(fifo) \
({ \
	typeof((fifo) + 1) __tmpl = (fifo); \
	__tmpl->kfifo.in - __tmpl->kfifo.out; \
})

/**
 * kfifo_is_empty - returns true if the fifo is empty
 * @fifo: address of the fifo to be used
 */
#define	kfifo_is_empty(fifo) \
({ \
	typeof((fifo) + 1) __tmpq = (fifo); \
	__tmpq->kfifo.in == __tmpq->kfifo.out; \
})

/**
 * kfifo_is_full - returns true if the fifo is full
 * @fifo: address of the fifo to be used
 */
#define	kfifo_is_full(fifo) \
({ \
	typeof((fifo) + 1) __tmpq = (fifo); \
	kfifo_len(__tmpq) > __tmpq->kfifo.mask; \
})

kfifo_size()用来获取环形缓冲区的大小，也就是最大可以容纳多少个数据元素。kfifo_len()用来获取当前环形缓冲区中有多少个有效数据元素。kfifo_is_empty()判断环形缓冲区是否为空。kfifo_is_full()判断环形缓冲区是否为满。

5. 与用户空间数据交互
   KFIFO还封装了两个函数与用户空间数据交互。

/**
 * kfifo_from_user - puts some data from user space into the fifo
 * @fifo: address of the fifo to be used
 * @from: pointer to the data to be added
 * @len: the length of the data to be added
 * @copied: pointer to output variable to store the number of copied bytes
 *
 * This macro copies at most @len bytes from the @from into the
 * fifo, depending of the available space and returns -EFAULT/0.
 *
 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these macro.
 */
#define	kfifo_from_user(fifo, from, len, copied) \
__kfifo_uint_must_check_helper( \
({ \
	typeof((fifo) + 1) __tmp = (fifo); \
	const void __user *__from = (from); \
	unsigned int __len = (len); \
	unsigned int *__copied = (copied); \
	const size_t __recsize = sizeof(*__tmp->rectype); \
	struct __kfifo *__kfifo = &__tmp->kfifo; \
	(__recsize) ? \
	__kfifo_from_user_r(__kfifo, __from, __len,  __copied, __recsize) : \
	__kfifo_from_user(__kfifo, __from, __len, __copied); \
}) \
)

/**
 * kfifo_to_user - copies data from the fifo into user space
 * @fifo: address of the fifo to be used
 * @to: where the data must be copied
 * @len: the size of the destination buffer
 * @copied: pointer to output variable to store the number of copied bytes
 *
 * This macro copies at most @len bytes from the fifo into the
 * @to buffer and returns -EFAULT/0.
 *
 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these macro.
 */
#define	kfifo_to_user(fifo, to, len, copied) \
__kfifo_int_must_check_helper( \
({ \
	typeof((fifo) + 1) __tmp = (fifo); \
	void __user *__to = (to); \
	unsigned int __len = (len); \
	unsigned int *__copied = (copied); \
	const size_t __recsize = sizeof(*__tmp->rectype); \
	struct __kfifo *__kfifo = &__tmp->kfifo; \
	(__recsize) ? \
	__kfifo_to_user_r(__kfifo, __to, __len, __copied, __recsize) : \
	__kfifo_to_user(__kfifo, __to, __len, __copied); \
}) \
)

kfifo_from_user()是把from指向的用户空间的len个数据元素复制到KFIFO中，最后一个参数copied表示成功复制了几个数据元素。kfifo_to_user()则相反，把KFIFO的数据元素复制到用户空间。这两个宏结合了copy_to_user()、copy_from_user()以及KFIFO的机制，给驱动开发者提供了方便。

映射

在Linux中，IDR是一个Small id to pointer translation service，用于管理整数ID，将整数和指针映射。使用的时候首先为一个数据结构的指针分配一个整数ID，接下来通过ID可以快速查找对应的指针。

数组和链表也能用于这样的转换，但是数组不能用于查询范围很大的情况，链表的迭代效率很低，因此不能用于映射量很大的情况。某些情况下可以用hash表来替代IDR，但是IDR相比于hash表来说不必预分配一个很大的数组，并且最坏情况要比hash表好。平衡二叉树能更好的控制最坏情况，但是IDR处理的情况比较特殊，只需要管理整数和指针，所以可以实现出比平衡二叉树更优的算法，不论在存储上还是在查询上都表现更好。 IDR也是一种radix tree，每个节点有256个分支，通过一些技巧性的位运算可以得到很高的查询效率。

内核中idr在文件include/linux/idr.h和lib/idr.c中定义和实现。

静态初始化接口

/**
 * IDR_INIT() - Initialise an IDR.
 * @name: Name of IDR.
 *
 * A freshly-initialised IDR contains no IDs.
 */
#define IDR_INIT(name)	IDR_INIT_BASE(name, 0)

/**
 * DEFINE_IDR() - Define a statically-allocated IDR.
 * @name: Name of IDR.
 *
 * An IDR defined using this macro is ready for use with no additional
 * initialisation required.  It contains no IDs.
 */
#define DEFINE_IDR(name)	struct idr name = IDR_INIT(name)

动态初始化接口

/**
 * idr_init_base() - Initialise an IDR.
 * @idr: IDR handle.
 * @base: The base value for the IDR.
 *
 * This variation of idr_init() creates an IDR which will allocate IDs
 * starting at %base.
 */
static inline void idr_init_base(struct idr *idr, int base)
{
	INIT_RADIX_TREE(&idr->idr_rt, IDR_RT_MARKER);
	idr->idr_base = base;
	idr->idr_next = 0;
}

/**
 * idr_init() - Initialise an IDR.
 * @idr: IDR handle.
 *
 * Initialise a dynamically allocated IDR.  To initialise a
 * statically allocated IDR, use DEFINE_IDR().
 */
static inline void idr_init(struct idr *idr)
{
	idr_init_base(idr, 0);
}

释放接口

/**
 * idr_remove() - Remove an ID from the IDR.
 * @idr: IDR handle.
 * @id: Pointer ID.
 *
 * Removes this ID from the IDR.  If the ID was not previously in the IDR,
 * this function returns %NULL.
 *
 * Since this function modifies the IDR, the caller should provide their
 * own locking to ensure that concurrent modification of the same IDR is
 * not possible.
 *
 * Return: The pointer formerly associated with this ID.
 */
void *idr_remove(struct idr *idr, unsigned long id)
{
	return radix_tree_delete_item(&idr->idr_rt, id - idr->idr_base, NULL);
}
EXPORT_SYMBOL_GPL(idr_remove);

// lib/radix-tree.c
/**
 * idr_destroy - release all internal memory from an IDR
 * @idr: idr handle
 *
 * After this function is called, the IDR is empty, and may be reused or
 * the data structure containing it may be freed.
 *
 * A typical clean-up sequence for objects stored in an idr tree will use
 * idr_for_each() to free all objects, if necessary, then idr_destroy() to
 * free the memory used to keep track of those objects.
 */
void idr_destroy(struct idr *idr)
{
	struct radix_tree_node *node = rcu_dereference_raw(idr->idr_rt.xa_head);
	if (radix_tree_is_internal_node(node))
		radix_tree_free_nodes(node);
	idr->idr_rt.xa_head = NULL;
	root_tag_set(&idr->idr_rt, IDR_FREE);
}
EXPORT_SYMBOL(idr_destroy);

分配ID

// lib/radix-tree.c
/**
 * idr_preload - preload for idr_alloc()
 * @gfp_mask: allocation mask to use for preloading
 *
 * Preallocate memory to use for the next call to idr_alloc().  This function
 * returns with preemption disabled.  It will be enabled by idr_preload_end().
 */
void idr_preload(gfp_t gfp_mask)
{
	if (__radix_tree_preload(gfp_mask, IDR_PRELOAD_SIZE))
		local_lock(&radix_tree_preloads.lock);
}
EXPORT_SYMBOL(idr_preload);

// lib/idr.c
/**
 * idr_alloc() - Allocate an ID.
 * @idr: IDR handle.
 * @ptr: Pointer to be associated with the new ID.
 * @start: The minimum ID (inclusive).
 * @end: The maximum ID (exclusive).
 * @gfp: Memory allocation flags.
 *
 * Allocates an unused ID in the range specified by @start and @end.  If
 * @end is <= 0, it is treated as one larger than %INT_MAX.  This allows
 * callers to use @start + N as @end as long as N is within integer range.
 *
 * The caller should provide their own locking to ensure that two
 * concurrent modifications to the IDR are not possible.  Read-only
 * accesses to the IDR may be done under the RCU read lock or may
 * exclude simultaneous writers.
 *
 * Return: The newly allocated ID, -ENOMEM if memory allocation failed,
 * or -ENOSPC if no free IDs could be found.
 */
int idr_alloc(struct idr *idr, void *ptr, int start, int end, gfp_t gfp)
{
	u32 id = start;
	int ret;

	if (WARN_ON_ONCE(start < 0))
		return -EINVAL;

	ret = idr_alloc_u32(idr, ptr, &id, end > 0 ? end - 1 : INT_MAX, gfp);
	if (ret)
		return ret;

	return id;
}
EXPORT_SYMBOL_GPL(idr_alloc);

// include/linux/idr.h
/**
 * idr_preload_end - end preload section started with idr_preload()
 *
 * Each idr_preload() should be matched with an invocation of this
 * function.  See idr_preload() for details.
 */
static inline void idr_preload_end(void)
{
	local_unlock(&radix_tree_preloads.lock);
}

查询迭代

// lib/idr.c
/**
 * idr_find() - Return pointer for given ID.
 * @idr: IDR handle.
 * @id: Pointer ID.
 *
 * Looks up the pointer associated with this ID.  A %NULL pointer may
 * indicate that @id is not allocated or that the %NULL pointer was
 * associated with this ID.
 *
 * This function can be called under rcu_read_lock(), given that the leaf
 * pointers lifetimes are correctly managed.
 *
 * Return: The pointer associated with this ID.
 */
void *idr_find(const struct idr *idr, unsigned long id)
{
	return radix_tree_lookup(&idr->idr_rt, id - idr->idr_base);
}
EXPORT_SYMBOL_GPL(idr_find);

/**
 * idr_replace() - replace pointer for given ID.
 * @idr: IDR handle.
 * @ptr: New pointer to associate with the ID.
 * @id: ID to change.
 *
 * Replace the pointer registered with an ID and return the old value.
 * This function can be called under the RCU read lock concurrently with
 * idr_alloc() and idr_remove() (as long as the ID being removed is not
 * the one being replaced!).
 *
 * Returns: the old value on success.  %-ENOENT indicates that @id was not
 * found.  %-EINVAL indicates that @ptr was not valid.
 */
void *idr_replace(struct idr *idr, void *ptr, unsigned long id)
{
	struct radix_tree_node *node;
	void __rcu **slot = NULL;
	void *entry;

	id -= idr->idr_base;

	entry = __radix_tree_lookup(&idr->idr_rt, id, &node, &slot);
	if (!slot || radix_tree_tag_get(&idr->idr_rt, id, IDR_FREE))
		return ERR_PTR(-ENOENT);

	__radix_tree_replace(&idr->idr_rt, node, slot, ptr);

	return entry;
}
EXPORT_SYMBOL(idr_replace);

/**
 * idr_for_each() - Iterate through all stored pointers.
 * @idr: IDR handle.
 * @fn: Function to be called for each pointer.
 * @data: Data passed to callback function.
 *
 * The callback function will be called for each entry in @idr, passing
 * the ID, the entry and @data.
 *
 * If @fn returns anything other than %0, the iteration stops and that
 * value is returned from this function.
 *
 * idr_for_each() can be called concurrently with idr_alloc() and
 * idr_remove() if protected by RCU.  Newly added entries may not be
 * seen and deleted entries may be seen, but adding and removing entries
 * will not cause other entries to be skipped, nor spurious ones to be seen.
 */
int idr_for_each(const struct idr *idr,
		int (*fn)(int id, void *p, void *data), void *data)
{
	struct radix_tree_iter iter;
	void __rcu **slot;
	int base = idr->idr_base;

	radix_tree_for_each_slot(slot, &idr->idr_rt, &iter, 0) {
		int ret;
		unsigned long id = iter.index + base;

		if (WARN_ON_ONCE(id > INT_MAX))
			break;
		ret = fn(id, rcu_dereference_raw(*slot), data);
		if (ret)
			return ret;
	}

	return 0;
}
EXPORT_SYMBOL(idr_for_each);

// include/linux/idr.h
/**
 * idr_is_empty() - Are there any IDs allocated?
 * @idr: IDR handle.
 *
 * Return: %true if any IDs have been allocated from this IDR.
 */
static inline bool idr_is_empty(const struct idr *idr)
{
	return radix_tree_empty(&idr->idr_rt) &&
		radix_tree_tagged(&idr->idr_rt, IDR_FREE);
}

参考

Linux内核数据结构
linux
Trees I: Radix trees
Trees II: red-black trees
巧夺天工的kfifo(修订版）