Linux驱动中completion接口浅析(wait_for_complete例子，很好)

completion是一种轻量级的机制，它允许一个线程告诉另一个线程工作已经完成。可以利用下面的宏静态创建completion： DECLARE_COMPLETION(my_completion);

如果运行时创建completion，则必须采用以下方法动态创建和初始化： struct compltion my_completion; init_completion(&my_completion);

completion的相关定义包含在kernel/include/Linux/completion.h中：

struct completion { unsigned int done; wait_queue_head_t wait; };

#define COMPLETION_INITIALIZER(work) / { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }

#define DECLARE_COMPLETION(work) / struct completion work = COMPLETION_INITIALIZER(work)

static inline void init_completion(struct completion *x) { x->done = 0; init_waitqueue_head(&x->wait); }

要等待completion，可进行如下调用： void wait_for_completion(struct completion *c);

触发completion事件，调用： void complete(struct completion *c); //唤醒一个等待线程 void complete_all(struct completion *c);//唤醒所有的等待线程

为说明completion的使用方法，将《Linux设备驱动程序》一书中的complete模块的代码摘抄如下： /* * complete.c -- the writers awake the readers * * Copyright (C) 2003 Alessandro Rubini and Jonathan Corbet * Copyright (C) 2003 O'Reilly & Associates * * The source code in this file can be freely used, adapted, * and redistributed in source or binary form, so long as an * acknowledgment appears in derived source files. The citation * should list that the code comes from the book "Linux Device * Drivers" by Alessandro Rubini and Jonathan Corbet, published * by O'Reilly & Associates. No warranty is attached; * we cannot take responsibility for errors or fitness for use. * * $Id: complete.c,v 1.2 2004/09/26 07:02:43 gregkh Exp $ */

#include <linux/module.h> #include <linux/init.h>

#include <linux/sched.h> /* current and everything */ #include <linux/kernel.h> /* printk() */ #include <linux/fs.h> /* everything... */ #include <linux/types.h> /* size_t */ #include <linux/completion.h>

MODULE_LICENSE("Dual BSD/GPL");

static int complete_major = 253;//指定主设备号

DECLARE_COMPLETION(comp);

ssize_t complete_read (struct file *filp, char __user *buf, size_t count, loff_t *pos) { printk(KERN_DEBUG "process %i (%s) going to sleep/n", current->pid, current->comm); wait_for_completion(&comp); printk(KERN_DEBUG "awoken %i (%s)/n", current->pid, current->comm); return 0; /* EOF */ }

ssize_t complete_write (struct file *filp, const char __user *buf, size_t count, loff_t *pos) { printk(KERN_DEBUG "process %i (%s) awakening the readers.../n", current->pid, current->comm); complete(&comp); return count; /* succeed, to avoid retrial */ }

struct file_operations complete_fops = { .owner = THIS_MODULE, .read = complete_read, .write = complete_write, };

int complete_init(void) { int result;

/* * Register your major, and accept a dynamic number */ result = register_chrdev(complete_major, "complete", &complete_fops); if (result < 0) return result; if (complete_major == 0) complete_major = result; /* dynamic */ return 0; }

void complete_cleanup(void) { unregister_chrdev(complete_major, "complete"); }

module_init(complete_init); module_exit(complete_cleanup);

该模块定义了一个简单的completion设备：任何试图从该设备中读取的进程都将等待，直到其他设备写入该设备为止。编译此模块的Makefile如下： obj-m := complete.o KDIR := /lib/modules/$(Shell uname -r)/build PWD := $(shell pwd) default: $(MAKE) -C $(KDIR) M=$(PWD) modules clean: rm -f *.ko *.o *.mod.c

在linux终端中执行以下命令，编译生成模块，并进行动态加载。 #make #mknod completion c 253 0 #insmod complete.ko 再打开三个终端，一个用于读进程： #cat completion 一个用于写进程： #echo >completion 另一个查看系统日志： #tail -f /var/log/messages

值得注意的是，当我们使用的complete_all接口时，如果要重复使用一个completion结构，则必须执行 INIT_COMPLETION(struct completion c)来重新初始化它。可以在kernel/include/linux/completion.h中找到这个宏的定义： #define INIT_COMPLETION(x) ((x).done = 0)

以下代码对书中原有的代码进行了一番变动，将唤醒接口由原来的complete换成了complete_all，并且为了重复利用completion结构，所有读进程都结束后就重新初始化completion结构，具体代码如下： #include <linux/module.h> #include <linux/init.h>

#include <linux/sched.h> #include <linux/kernel.h> #include <linux/fs.h> #include <linux/types.h> #include <linux/completion.h>

MODULE_LICENSE("Dual BSD/GPL");

#undef KERN_DEBUG #define KERN_DEBUG "<1>"

static int complete_major=253; static int reader_count = 0;

DECLARE_COMPLETION(comp);

ssize_t complete_read (struct file *filp,char __user *buf,size_t count,loff_t *pos) { printk(KERN_DEBUG "process %i (%s) going to sleep,waiting for writer/n",current->pid,current->comm); reader_count++; printk(KERN_DEBUG "In read ,before comletion: reader count = %d /n",reader_count); wait_for_completion(&comp); reader_count--; printk(KERN_DEBUG "awoken %s (%i) /n",current->comm,current->pid); printk(KERN_DEBUG "In read,after completion : reader count = %d /n",reader_count);

/*如果使用complete_all，则completion结构只能用一次，再次使用它时必须调用此宏进行重新初始化*/ if(reader_count == 0) INIT_COMPLETION(comp);
return 0; }

ssize_t complete_write(struct file *filp,const char __user *buf,size_t count,loff_t *pos) { printk(KERN_DEBUG "process %i (%s) awoking the readers.../n",current->pid,current->comm); printk(KERN_DEBUG "In write ,before do complete_all : reader count = %d /n",reader_count);
if(reader_count != 0) complete_all(&comp);
printk(KERN_DEBUG "In write ,after do complete_all : reader count = %d /n",reader_count);
return count; }

struct file_operations complete_fops={ .owner = THIS_MODULE, .read = complete_read, .write = complete_write, };

int complete_init(void) { int result;

result=register_chrdev(complete_major,"complete",&complete_fops); if(result<0) return result; if(complete_major==0) complete_major =result;

printk(KERN_DEBUG "complete driver test init! complete_major=%d/n",complete_major); printk(KERN_DEBUG "静态初始化completion/n");
return 0; }

void complete_exit(void) { unregister_chrdev(complete_major,"complete"); printk(KERN_DEBUG "complete driver is removed/n"); }

module_init(complete_init); module_exit(complete_exit);

这里测试步骤和上述一样，只不过需要多打开几个终端来执行多个进程同时读操作。