linux内核数据结构之kfifo【转】-阿里云开发者社区

转自：http://www.cnblogs.com/Anker/p/3481373.html

1、前言

　　最近项目中用到一个环形缓冲区（ring buffer），代码是由linux内核的kfifo改过来的。缓冲区在文件系统中经常用到，通过缓冲区缓解cpu读写内存和读写磁盘的速度。例如一个进程A产生数据发给另外一个进程B，进程B需要对进程A传的数据进行处理并写入文件，如果B没有处理完，则A要延迟发送。为了保证进程A减少等待时间，可以在A和B之间采用一个缓冲区，A每次将数据存放在缓冲区中，B每次冲缓冲区中取。这是典型的生产者和消费者模型，缓冲区中数据满足FIFO特性，因此可以采用队列进行实现。Linux内核的kfifo正好是一个环形队列，可以用来当作环形缓冲区。生产者与消费者使用缓冲区如下图所示：

　　环形缓冲区的详细介绍及实现方法可以参考http://en.wikipedia.org/wiki/Circular_buffer，介绍的非常详细，列举了实现环形队列的几种方法。环形队列的不便之处在于如何判断队列是空还是满。维基百科上给三种实现方法。

2、linux 内核kfifo

　　kfifo设计的非常巧妙，代码很精简，对于入队和出对处理的出人意料。首先看一下kfifo的数据结构：

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 */ };

kfifo提供的方法有：

 1 //根据给定buffer创建一个kfifo
 2 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,  3 gfp_t gfp_mask, spinlock_t *lock);  4 //给定size分配buffer和kfifo  5 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask,  6 spinlock_t *lock);  7 //释放kfifo空间  8 void kfifo_free(struct kfifo *fifo)  9 //向kfifo中添加数据 10 unsigned int kfifo_put(struct kfifo *fifo, 11 const unsigned char *buffer, unsigned int len) 12 //从kfifo中取数据 13 unsigned int kfifo_put(struct kfifo *fifo, 14 const unsigned char *buffer, unsigned int len) 15 //获取kfifo中有数据的buffer大小 16 unsigned int kfifo_len(struct kfifo *fifo)

定义自旋锁的目的为了防止多进程/线程并发使用kfifo。因为in和out在每次get和out时，发生改变。初始化和创建kfifo的源代码如下：

 1 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,  2 gfp_t gfp_mask, spinlock_t *lock)  3 {  4 struct kfifo *fifo;  6 /* size must be a power of 2 */  7 BUG_ON(!is_power_of_2(size));  9 fifo = kmalloc(sizeof(struct kfifo), gfp_mask); 10 if (!fifo) 11 return ERR_PTR(-ENOMEM); 13 fifo->buffer = buffer; 14 fifo->size = size; 15 fifo->in = fifo->out = 0; 16 fifo->lock = lock; 17 18 return fifo; 19 } 20 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock) 21 { 22 unsigned char *buffer; 23 struct kfifo *ret; 29 if (!is_power_of_2(size)) { 30 BUG_ON(size > 0x80000000); 31 size = roundup_pow_of_two(size); 32  } 34 buffer = kmalloc(size, gfp_mask); 35 if (!buffer) 36 return ERR_PTR(-ENOMEM); 38 ret = kfifo_init(buffer, size, gfp_mask, lock); 39 40 if (IS_ERR(ret)) 41  kfree(buffer); 43 return ret; 44 }

　　在kfifo_init和kfifo_calloc中，kfifo->size的值总是在调用者传进来的size参数的基础上向2的幂扩展，这是内核一贯的做法。这样的好处不言而喻--对kfifo->size取模运算可以转化为与运算，如：kfifo->in % kfifo->size 可以转化为 kfifo->in & (kfifo->size – 1)

kfifo的巧妙之处在于in和out定义为无符号类型，在put和get时，in和out都是增加，当达到最大值时，产生溢出，使得从0开始，进行循环使用。put和get代码如下所示：

 1 static inline unsigned int kfifo_put(struct kfifo *fifo,  2 const unsigned char *buffer, unsigned int len)  3 {  4 unsigned long flags;  5 unsigned int ret;  6 spin_lock_irqsave(fifo->lock, flags);  7 ret = __kfifo_put(fifo, buffer, len);  8 spin_unlock_irqrestore(fifo->lock, flags);  9 return ret; 10 } 11 12 static inline unsigned int kfifo_get(struct kfifo *fifo, 13 unsigned char *buffer, unsigned int len) 14 { 15 unsigned long flags; 16 unsigned int ret; 17 spin_lock_irqsave(fifo->lock, flags); 18 ret = __kfifo_get(fifo, buffer, len); 19 //当fifo->in == fifo->out时，buufer为空 20 if (fifo->in == fifo->out) 21 fifo->in = fifo->out = 0; 22 spin_unlock_irqrestore(fifo->lock, flags); 23 return ret; 24 } 25 26 27 unsigned int __kfifo_put(struct kfifo *fifo, 28 const unsigned char *buffer, unsigned int len) 29 { 30 unsigned int l; 31 //buffer中空的长度 32 len = min(len, fifo->size - fifo->in + fifo->out); 34 /* 35  * Ensure that we sample the fifo->out index -before- we 36  * start putting bytes into the kfifo. 37 */ 39  smp_mb(); 41 /* first put the data starting from fifo->in to buffer end */ 42 l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); 43 memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l); 45 /* then put the rest (if any) at the beginning of the buffer */ 46 memcpy(fifo->buffer, buffer + l, len - l); 47 48 /* 49  * Ensure that we add the bytes to the kfifo -before- 50  * we update the fifo->in index. 51 */ 53  smp_wmb(); 55 fifo->in += len; //每次累加，到达最大值后溢出，自动转为0 57 return len; 58 } 59 60 unsigned int __kfifo_get(struct kfifo *fifo, 61 unsigned char *buffer, unsigned int len) 62 { 63 unsigned int l; 64 //有数据的缓冲区的长度 65 len = min(len, fifo->in - fifo->out); 67 /* 68  * Ensure that we sample the fifo->in index -before- we 69  * start removing bytes from the kfifo. 70 */ 72  smp_rmb(); 74 /* first get the data from fifo->out until the end of the buffer */ 75 l = min(len, fifo->size - (fifo->out & (fifo->size - 1))); 76 memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l); 78 /* then get the rest (if any) from the beginning of the buffer */ 79 memcpy(buffer + l, fifo->buffer, len - l); 81 /* 82 * Ensure that we remove the bytes from the kfifo -before- 83 * we update the fifo->out index. 84 */ 86 smp_mb(); 88 fifo->out += len; //每次累加，到达最大值后溢出，自动转为0 90 return len; 91 }

　　put和get在调用__put和__get过程都进行加锁，防止并发。从代码中可以看出put和get都调用两次memcpy，这针对的是边界条件。例如下图：蓝色表示空闲，红色表示占用。

（1）空的kfifo，

（2）put一个buffer后

（3）get一个buffer后

（4）当此时put的buffer长度超出in到末尾长度时，则将剩下的移到头部去

3、测试程序

仿照kfifo编写一个ring_buffer，现有线程互斥量进行并发控制。设计的ring_buffer如下所示：

  1 /**@brief 仿照linux kfifo写的ring buffer
  2  *@atuher Anker date:2013-12-18  3 * ring_buffer.h  4  * */  5  6 #ifndef KFIFO_HEADER_H  7 #define KFIFO_HEADER_H  8  9 #include <inttypes.h>  10 #include <string.h>  11 #include <stdlib.h>  12 #include <stdio.h>  13 #include <errno.h>  14 #include <assert.h>  15  16 //判断x是否是2的次方  17 #define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))  18 //取a和b中最小值  19 #define min(a, b) (((a) < (b)) ? (a) : (b))  20  21 struct ring_buffer  22 {  23 void *buffer; //缓冲区  24 uint32_t size; //大小  25 uint32_t in; //入口位置  26 uint32_t out; //出口位置  27 pthread_mutex_t *f_lock; //互斥锁  28 };  29 //初始化缓冲区  30 struct ring_buffer* ring_buffer_init(void *buffer, uint32_t size, pthread_mutex_t *f_lock)  31 {  32  assert(buffer);  33 struct ring_buffer *ring_buf = NULL;  34 if (!is_power_of_2(size))  35  {  36 fprintf(stderr,"size must be power of 2.\n");  37 return ring_buf;  38  }  39 ring_buf = (struct ring_buffer *)malloc(sizeof(struct ring_buffer));  40 if (!ring_buf)  41  {  42 fprintf(stderr,"Failed to malloc memory,errno:%u,reason:%s",  43  errno, strerror(errno));  44 return ring_buf;  45  }  46 memset(ring_buf, 0, sizeof(struct ring_buffer));  47 ring_buf->buffer = buffer;  48 ring_buf->size = size;  49 ring_buf->in = 0;  50 ring_buf->out = 0;  51 ring_buf->f_lock = f_lock;  52 return ring_buf;  53 }  54 //释放缓冲区  55 void ring_buffer_free(struct ring_buffer *ring_buf)  56 {  57 if (ring_buf)  58  {  59 if (ring_buf->buffer)  60  {  61 free(ring_buf->buffer);  62 ring_buf->buffer = NULL;  63  }  64  free(ring_buf);  65 ring_buf = NULL;  66  }  67 }  68  69 //缓冲区的长度  70 uint32_t __ring_buffer_len(const struct ring_buffer *ring_buf)  71 {  72 return (ring_buf->in - ring_buf->out);  73 }  74  75 //从缓冲区中取数据  76 uint32_t __ring_buffer_get(struct ring_buffer *ring_buf, void * buffer, uint32_t size)  77 {  78 assert(ring_buf || buffer); 79 uint32_t len = 0; 80 size = min(size, ring_buf->in - ring_buf->out); 81 /* first get the data from fifo->out until the end of the buffer */ 82 len = min(size, ring_buf->size - (ring_buf->out & (ring_buf->size - 1))); 83 memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - 1)), len); 84 /* then get the rest (if any) from the beginning of the buffer */ 85 memcpy(buffer + len, ring_buf->buffer, size - len); 86 ring_buf->out += size; 87 return size; 88 } 89 //向缓冲区中存放数据 90 uint32_t __ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size) 91 { 92 assert(ring_buf || buffer); 93 uint32_t len = 0; 94 size = min(size, ring_buf->size - ring_buf->in + ring_buf->out); 95 /* first put the data starting from fifo->in to buffer end */ 96 len = min(size, ring_buf->size - (ring_buf->in & (ring_buf->size - 1))); 97 memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - 1)), buffer, len); 98 /* then put the rest (if any) at the beginning of the buffer */ 99 memcpy(ring_buf->buffer, buffer + len, size - len); 100 ring_buf->in += size; 101 return size; 102 } 103 104 uint32_t ring_buffer_len(const struct ring_buffer *ring_buf) 105 { 106 uint32_t len = 0; 107 pthread_mutex_lock(ring_buf->f_lock); 108 len = __ring_buffer_len(ring_buf); 109 pthread_mutex_unlock(ring_buf->f_lock); 110 return len; 111 } 112 113 uint32_t ring_buffer_get(struct ring_buffer *ring_buf, void *buffer, uint32_t size) 114 { 115 uint32_t ret; 116 pthread_mutex_lock(ring_buf->f_lock); 117 ret = __ring_buffer_get(ring_buf, buffer, size); 118 //buffer中没有数据 119 if (ring_buf->in == ring_buf->out) 120 ring_buf->in = ring_buf->out = 0; 121 pthread_mutex_unlock(ring_buf->f_lock); 122 return ret; 123 } 124 125 uint32_t ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size) 126 { 127 uint32_t ret; 128 pthread_mutex_lock(ring_buf->f_lock); 129 ret = __ring_buffer_put(ring_buf, buffer, size); 130 pthread_mutex_unlock(ring_buf->f_lock); 131 return ret; 132 } 133 #endif

采用多线程模拟生产者和消费者编写测试程序，如下所示：

  1 /**@brief ring buffer测试程序，创建两个线程，一个生产者，一个消费者。
  2  * 生产者每隔1秒向buffer中投入数据，消费者每隔2秒去取数据。  3  *@atuher Anker date:2013-12-18  4  * */  5 #include "ring_buffer.h"  6 #include <pthread.h>  7 #include <time.h>  8  9 #define BUFFER_SIZE 1024 * 1024  10  11 typedef struct student_info  12 {  13  uint64_t stu_id;  14  uint32_t age;  15  uint32_t score;  16 }student_info;  17  18  19 void print_student_info(const student_info *stu_info)  20 {  21  assert(stu_info);  22 printf("id:%lu\t",stu_info->stu_id);  23 printf("age:%u\t",stu_info->age);  24 printf("score:%u\n",stu_info->score);  25 }  26  27 student_info * get_student_info(time_t timer)  28 {  29 student_info *stu_info = (student_info *)malloc(sizeof(student_info));  30 if (!stu_info)  31  {  32 fprintf(stderr, "Failed to malloc memory.\n");  33 return NULL;  34  }  35  srand(timer);  36 stu_info->stu_id = 10000 + rand() % 9999;  37 stu_info->age = rand() % 30;  38 stu_info->score = rand() % 101;  39  print_student_info(stu_info);  40 return stu_info;  41 }  42  43 void * consumer_proc(void *arg)  44 {  45 struct ring_buffer *ring_buf = (struct ring_buffer *)arg;  46  student_info stu_info;  47 while(1)  48  {  49 sleep(2);  50 printf("------------------------------------------\n");  51 printf("get a student info from ring buffer.\n");  52 ring_buffer_get(ring_buf, (void *)&stu_info, sizeof(student_info));  53 printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));  54 print_student_info(&stu_info);  55 printf("------------------------------------------\n");  56  }  57 return (void *)ring_buf;  58 }  59  60 void * producer_proc(void *arg)  61 {  62  time_t cur_time;  63 struct ring_buffer *ring_buf = (struct ring_buffer *)arg;  64 while(1)  65  {  66 time(&cur_time);  67  srand(cur_time);  68 int seed = rand() % 11111;  69 printf("******************************************\n");  70 student_info *stu_info = get_student_info(cur_time + seed);  71 printf("put a student info to ring buffer.\n"); 72 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info)); 73 printf("ring buffer length: %u\n", ring_buffer_len(ring_buf)); 74 printf("******************************************\n"); 75 sleep(1); 76 } 77 return (void *)ring_buf; 78 } 79 80 int consumer_thread(void *arg) 81 { 82 int err; 83 pthread_t tid; 84 err = pthread_create(&tid, NULL, consumer_proc, arg); 85 if (err != 0) 86 { 87 fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n", 88 errno, strerror(errno)); 89 return -1; 90 } 91 return tid; 92 } 93 int producer_thread(void *arg) 94 { 95 int err; 96 pthread_t tid; 97 err = pthread_create(&tid, NULL, producer_proc, arg); 98 if (err != 0) 99 { 100 fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n", 101 errno, strerror(errno)); 102 return -1; 103 } 104 return tid; 105 } 106 107 108 int main() 109 { 110 void * buffer = NULL; 111 uint32_t size = 0; 112 struct ring_buffer *ring_buf = NULL; 113 pthread_t consume_pid, produce_pid; 114 115 pthread_mutex_t *f_lock = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t)); 116 if (pthread_mutex_init(f_lock, NULL) != 0) 117 { 118 fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n", 119 errno, strerror(errno)); 120 return -1; 121 } 122 buffer = (void *)malloc(BUFFER_SIZE); 123 if (!buffer) 124 { 125 fprintf(stderr, "Failed to malloc memory.\n"); 126 return -1; 127 } 128 size = BUFFER_SIZE; 129 ring_buf = ring_buffer_init(buffer, size, f_lock); 130 if (!ring_buf) 131 { 132 fprintf(stderr, "Failed to init ring buffer.\n"); 133 return -1; 134 } 135 #if 0 136 student_info *stu_info = get_student_info(638946124); 137 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info)); 138 stu_info = get_student_info(976686464); 139 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info)); 140 ring_buffer_get(ring_buf, (void *)stu_info, sizeof(student_info)); 141 print_student_info(stu_info); 142 #endif 143 printf("multi thread test.......\n"); 144 produce_pid = producer_thread((void*)ring_buf); 145 consume_pid = consumer_thread((void*)ring_buf); 146 pthread_join(produce_pid, NULL); 147 pthread_join(consume_pid, NULL); 148 ring_buffer_free(ring_buf); 149 free(f_lock); 150 return 0; 151 }