线程通信之无锁队列

什么是生产者消费者模式

生产者消费者模式是通过一个容器来解决生产者和消费者的强耦合问题。生产者和消费者彼此之间不直接通讯，而通过阻塞队列来进行通讯，所以生产者生产完数据之后不用等待消费者处理，直接扔给阻塞队列，消费者不找生产者要数据，而是直接从阻塞队列里取，阻塞队列就相当于一个缓冲区，平衡了生产者和消费者的处理能力。
如图：

单生产者单消费者模型的队列操作过程是不需要进行加锁的。生产者通过写索引控制入队操作，消费者通过读索引控制出队列操作。二者相互之间对索引是独享，不存在竞争关系。如下图所示：

Linux内核中有无锁队列的实现，非常简洁，代码如下：
头文件：

#ifndef KFIFO_H#define KFIFO_H#define MAX_KFIFO_SIZE 0x1000#ifdef __cplusplusextern "C"{#endif#include <stdio.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) */};struct kfifo *kfifo_alloc(unsigned int size);void kfifo_free(struct kfifo *fifo);unsigned int __kfifo_put(struct kfifo *fifo,                        const unsigned char *buffer, unsigned int len);unsigned int __kfifo_get(struct kfifo *fifo,                         unsigned char *buffer, unsigned int len);#ifdef __cplusplus}#endif#endif // KFIFO_H

源文件：

#include <stdio.h>#include <stdlib.h>#include <string.h>#include "kfifo.h"#define __u32 unsigned long#define __u64 unsigned long long#if defined(__GNUC__)#define min(x,y) ({         \        typeof(x) _x = (x); \        typeof(y) _y = (y); \        (void) (&_x == &_y);\        _x < _y ? _x : _y; })#define max(x,y) ({         \        typeof(x) _x = (x); \        typeof(y) _y = (y); \        (void) (&_x == &_y);\        _x > _y ? _x : _y; })#else#define max(a,b)            (((a) > (b)) ? (a) : (b))#define min(a,b)            (((a) < (b)) ? (a) : (b))#endif/** * fls - find last bit set * @x: the word to search * * This is defined the same way as ffs: * - return 32..1 to indicate bit 31..0 most significant bit set * - return 0 to indicate no bits set */#if defined(__GNUC__)static inline int fls(int x){    int r;    __asm__("bsrl %1,%0\n\t"            "jnz 1f\n\t"            "movl $-1,%0\n"            "1:" : "=r" (r) : "rm" (x));    return r+1;}#elsestatic inline int fls(int x){    int position;    int i;    if(0 != x)    {        for (i = (x >> 1), position = 0; i != 0; ++position)            i >>= 1;    }    else    {        position = -1;    }    return position+1;}#endif/** * fls64 - find last bit set in a 64-bit value * @n: the value to search * * This is defined the same way as ffs: * - return 64..1 to indicate bit 63..0 most significant bit set * - return 0 to indicate no bits set */static inline int fls64(__u64 x){    __u32 h = x >> 32;    if (h)        return fls(h) + 32;    return fls(x);}static inline unsigned fls_long(unsigned long l){    if (sizeof(l) == 4)        return fls(l);    return fls64(l);}static inline unsigned long roundup_pow_of_two(unsigned long x){    return 1UL << fls_long(x - 1);}/** * * 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){    unsigned char *buffer;    struct kfifo *fifo;    /*     *       * round up to the next power of 2, since our 'let the indices     *            * wrap' tachnique works only in this case.     *                 */    if (size & (size - 1)) {            if(size > 0x80000000);                return NULL;            size = roundup_pow_of_two(size);        }    buffer = (unsigned char *)malloc(size);    if (!buffer)        return NULL;    fifo = (struct kfifo*)malloc(sizeof(struct kfifo));    if (!fifo)    {         free(buffer);         return NULL;    }    fifo->buffer = buffer;    fifo->size = size;    fifo->in = fifo->out = 0;    return fifo;}/** * * kfifo_free - frees the FIFO * * @fifo: the fifo to be freed. * */void kfifo_free(struct kfifo *fifo){    free(fifo->buffer);    free(fifo);}/*** __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,                        const unsigned char *buffer, unsigned int len){        unsigned int l;        len = min(len, fifo->size - fifo->in + fifo->out);        /* 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);        fifo->in += len;        return len;}/*** __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);        /* 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);        fifo->out += len;        return len;}/*** __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;}/*** __kfifo_len - returns the number of bytes available in the FIFO, no locking version* @fifo: the fifo to be used.*/static inline unsigned int __kfifo_len(struct kfifo *fifo){        return fifo->in - fifo->out;}

用c++封装使用类：

#ifndef MSG_QUEUE_H#define MSG_QUEUE_H#include "kfifo.h"template <typename T>class msg_queue{public:    msg_queue()    {        _kfifo = kfifo_alloc(MAX_KFIFO_SIZE);    }    ~msg_queue()    {        if(NULL != _kfifo)        {            kfifo_free(_kfifo);        }    }    bool push(T data)    {        int len = 0;        len = __kfifo_put(_kfifo, (const unsigned char *)&data, sizeof(T));        if(len > 0)            return true;        else            return false;    }    T pop()    {        T data;        int len = __kfifo_get(_kfifo, (unsigned char *)&data, sizeof(T));        if(len > 0)            return data;        else            return NULL;    }private:       kfifo* _kfifo;};#endif // MSG_QUEUE_H

使用示例：

#include <iostream>#include <string>#include "msg_queue.h"using namespace std;int main(void){    msg_queue<int> test_queue;    int a = 1111;    test_queue.push(a);    printf("a=%d\n",a);    int b = 0;    b = test_queue.pop();    printf("b=%d\n", b);    return 0;}

运行结果：