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)
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)
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)
/**
* 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