Linux(内核剖析):15---内核数据结构之队列（struct kfifo）

一、队列概述

• 任何操作系统内核都少不了一种编程模型：生产者和消费者。在该模式中，生产者创建数据（比如说需要读取的错误信息或者需要处理的网络包），而消费者则反过来，读取消息和处理包，或者以其他方式消费这些数据。实现该模型的最简单的方式无非是使用队列。生产者将数据推进 队列，然后消费者从队列中摘取数据。消费者获取数据的順序和推入队列的顺序一致。也就是说，第一个进队列的数据一定是第一个离开队列的。也正是这个原因，队列也称为FIFO。顾名思义，FIFO就是先进先出的缩写
• 下图是一个标准队列的例子

二、Linux内核队列概述（kfifo）

• Linux内核通用队列实现称为kfifo
• 相关声明在文件<include/kfifo.h>中，相关定义在<kernel/kfifo.c>中

struct kfifo

• 以下代码来自：Linux2.6.22/include/kfifo.h

struct kfifo {
	unsigned char *buffer;	/* the buffer holding the data */
	unsigned int size;	/* the size of the allocated buffer */
	unsigned int in;	/* data is added at offset (in % size) */
	unsigned int out;	/* data is extracted from off. (out % size) */
	spinlock_t *lock;	/* protects concurrent modifications */
};

enqueue、dequeue

• Linux提供了两个主要操作：enqueue（入队列）、dequeue（出队列）

入口偏移、出口偏移

• kfifo对象维护了两个偏移量：入口偏移和出口偏移
  • 入口偏移是指下一次入队列时的位置
  • 出口偏移是指下一次出队列时的位置
• 出口偏移总是小于等于入口偏移，否则无意义，因为那样说明要出队列的元素根本还没有入队列

三、创建队列

• 备注：kfifo_init和kfifo_alloc的size必须是2的幂

创建队列并分配缓冲区（kfifo_init）

• 该函数创建并初始化一个kfifo对象，其使用buffer指向的size字节大小的内存。在kfifo_alloc中会被调用

/**
 * kfifo_init - allocates a new FIFO using a preallocated buffer
 * @buffer: the preallocated buffer to be used.
 * @size: the size of the internal buffer, this have to be a power of 2.
 * @gfp_mask: get_free_pages mask, passed to kmalloc()
 * @lock: the lock to be used to protect the fifo buffer
 *
 * Do NOT pass the kfifo to kfifo_free() after use! Simply free the
 * &struct kfifo with kfree().
 */
struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
			 gfp_t gfp_mask, spinlock_t *lock)
{
	struct kfifo *fifo;

	/* size must be a power of 2 */
	BUG_ON(size & (size - 1));

	fifo = kmalloc(sizeof(struct kfifo), gfp_mask);
	if (!fifo)
		return ERR_PTR(-ENOMEM);

	fifo->buffer = buffer;
	fifo->size = size;
	fifo->in = fifo->out = 0;
	fifo->lock = lock;

	return fifo;
}
EXPORT_SYMBOL(kfifo_init);

动态创建（kfifo_alloc）

• 参数：
  • size：创建的大小
  • gfp_mask：使用此参数标识分配队列（在后面讨论内存分配时会介绍）
• 返回值：返回创建的队列
• kfifo_alooc()动态创建的队列需要使用kfifo_free来释放（kfifo_free见下）

/**
 * kfifo_alloc - allocates a new FIFO and its internal buffer
 * @size: the size of the internal buffer to be allocated.
 * @gfp_mask: get_free_pages mask, passed to kmalloc()
 * @lock: the lock to be used to protect the fifo buffer
 *
 * The size will be rounded-up to a power of 2.
 */
struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock)
{
	unsigned char *buffer;
	struct kfifo *ret;

	/*
	 * round up to the next power of 2, since our 'let the indices
	 * wrap' tachnique works only in this case.
	 */
	if (size & (size - 1)) {
		BUG_ON(size > 0x80000000);
		size = roundup_pow_of_two(size);
	}

	buffer = kmalloc(size, gfp_mask);
	if (!buffer)
		return ERR_PTR(-ENOMEM);

	ret = kfifo_init(buffer, size, gfp_mask, lock);

	if (IS_ERR(ret))
		kfree(buffer);

	return ret;
}
EXPORT_SYMBOL(kfifo_alloc);

静态声明kfifo

• 静态声明kfifo比较简单，但不太常用
• 例如下面会创建一个名称为name，大小为size的kfifo对象。和前面一样，size必须是2的幂

DECLARE_KFIFO(name,size);
INIT_KFIFO(name);

四、入队列（kfifo_put）

• 将数据推入队列使用这个函数

kfifo_put

• 参数：把buffer所指的len字节数据拷贝到fifo所指的队列中
• 返回值：成功返回推入的数据长度。如果队列中的空闲字节小于len，则该函数值最多可拷贝队列可用空间那么多的数据，这样的话，返回值可能小于len，甚至会返回0，这时意味着没有任何数据被推入

/**
 * kfifo_put - puts some data into the FIFO
 * @fifo: the fifo to be used.
 * @buffer: the data to be added.
 * @len: the length of the data to be added.
 *
 * This function copies at most @len bytes from the @buffer into
 * the FIFO depending on the free space, and returns the number of
 * bytes copied.
 */
static inline unsigned int kfifo_put(struct kfifo *fifo,
				     unsigned char *buffer, unsigned int len)
{
	unsigned long flags;
	unsigned int ret;

	spin_lock_irqsave(fifo->lock, flags);

	ret = __kfifo_put(fifo, buffer, len);

	spin_unlock_irqrestore(fifo->lock, flags);

	return ret;
}

/**
 * __kfifo_put - puts some data into the FIFO, no locking version
 * @fifo: the fifo to be used.
 * @buffer: the data to be added.
 * @len: the length of the data to be added.
 *
 * This function copies at most @len bytes from the @buffer into
 * the FIFO depending on the free space, and returns the number of
 * bytes copied.
 *
 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these functions.
 */
unsigned int __kfifo_put(struct kfifo *fifo,
			 unsigned char *buffer, unsigned int len)
{
	unsigned int l;

	len = min(len, fifo->size - fifo->in + fifo->out);

	/*
	 * Ensure that we sample the fifo->out index -before- we
	 * start putting bytes into the kfifo.
	 */

	smp_mb();

	/* first put the data starting from fifo->in to buffer end */
	l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));
	memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);

	/* then put the rest (if any) at the beginning of the buffer */
	memcpy(fifo->buffer, buffer + l, len - l);

	/*
	 * Ensure that we add the bytes to the kfifo -before-
	 * we update the fifo->in index.
	 */

	smp_wmb();

	fifo->in += len;

	return len;
}
EXPORT_SYMBOL(__kfifo_put);

五、出队列（kfifo_get）

• 将数据推出队列使用这个函数

kfifo_get

• 参数：从fifo所指的队列中拷贝出长度为len字节的数据到buffer所指的缓冲中
• 返回值：成功返回拷贝的数据长度。如果队列中数据大小小于len，则该函数拷贝出的数据必然小于需要的数据大小

/**
 * kfifo_get - gets some data from the FIFO
 * @fifo: the fifo to be used.
 * @buffer: where the data must be copied.
 * @len: the size of the destination buffer.
 *
 * This function copies at most @len bytes from the FIFO into the
 * @buffer and returns the number of copied bytes.
 */
static inline unsigned int kfifo_get(struct kfifo *fifo,
				     unsigned char *buffer, unsigned int len)
{
	unsigned long flags;
	unsigned int ret;

	spin_lock_irqsave(fifo->lock, flags);

	ret = __kfifo_get(fifo, buffer, len);

	/*
	 * optimization: if the FIFO is empty, set the indices to 0
	 * so we don't wrap the next time
	 */
	if (fifo->in == fifo->out)
		fifo->in = fifo->out = 0;

	spin_unlock_irqrestore(fifo->lock, flags);

	return ret;
}

/**
 * __kfifo_get - gets some data from the FIFO, no locking version
 * @fifo: the fifo to be used.
 * @buffer: where the data must be copied.
 * @len: the size of the destination buffer.
 *
 * This function copies at most @len bytes from the FIFO into the
 * @buffer and returns the number of copied bytes.
 *
 * Note that with only one concurrent reader and one concurrent
 * writer, you don't need extra locking to use these functions.
 */
unsigned int __kfifo_get(struct kfifo *fifo,
			 unsigned char *buffer, unsigned int len)
{
	unsigned int l;

	len = min(len, fifo->in - fifo->out);

	/*
	 * Ensure that we sample the fifo->in index -before- we
	 * start removing bytes from the kfifo.
	 */

	smp_rmb();

	/* first get the data from fifo->out until the end of the buffer */
	l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));
	memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);

	/* then get the rest (if any) from the beginning of the buffer */
	memcpy(buffer + l, fifo->buffer, len - l);

	/*
	 * Ensure that we remove the bytes from the kfifo -before-
	 * we update the fifo->out index.
	 */

	smp_mb();

	fifo->out += len;

	return len;
}
EXPORT_SYMBOL(__kfifo_get);

六、重置队列（kfifo_reset）

• 重置kfifo，排期所有队列的内容

/**
 * kfifo_reset - removes the entire FIFO contents
 * @fifo: the fifo to be emptied.
 */
static inline void kfifo_reset(struct kfifo *fifo)
{
	unsigned long flags;

	spin_lock_irqsave(fifo->lock, flags);

	__kfifo_reset(fifo);

	spin_unlock_irqrestore(fifo->lock, flags);
}

/**
 * __kfifo_reset - removes the entire FIFO contents, no locking version
 * @fifo: the fifo to be emptied.
 */
static inline void __kfifo_reset(struct kfifo *fifo)
{
	fifo->in = fifo->out = 0;
}

七、撤销队列（kfifo_free）

• 撤销一个使用kfifo_alloc()分配的队列，可以调研kfifo_free

/**
 * kfifo_free - frees the FIFO
 * @fifo: the fifo to be freed.
 */
void kfifo_free(struct kfifo *fifo)
{
	kfree(fifo->buffer);
	kfree(fifo);
}
EXPORT_SYMBOL(kfifo_free);

• 如果你是使用kfifo_init()方法创建的队列，那么需要负责相关的缓冲。具体方法取决于你是如何创建它的