内核无锁第四层级 — 免锁
环形缓冲区是生产者和消费者模型中常用的数据结构。生产者将数据放入数组的尾端，而消费者从数组的另一端移走数据，当达到数组的尾部时，生产者绕回到数组的头部。
如果只有一个生产者和一个消费者，那么就可以做到免锁访问环形缓冲区（Ring Buffer）。写入索引只允许生产者访问并修改，只要写入者在更新索引之前将新的值保存到缓冲区中，则读者将始终看到一致的数据结构。同理，读取索引也只允许消费者访问并修改。
图 2. 环形缓冲区实现原理图
如图所示，当读者和写者指针相等时，表明缓冲区是空的，而只要写入指针在读取指针后面时，表明缓冲区已满。
清单 9. 2.6.10 环形缓冲区实现代码
/* * __kfifo_put - puts some data into the FIFO, no locking version * 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); /* 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 * 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; }
以上代码摘自 2.6.10 内核，通过代码的注释（斜体部分）可以看出，当只有一个消费者和一个生产者时，可以不用添加任何额外的锁，就能达到对共享数据的访问。