LDD3源码分析之简单休眠

作者：刘昊昱 

博客：http://blog.csdn.net/liuhaoyutz

编译环境：Ubuntu 10.10

内核版本：2.6.32-38-generic-pae

LDD3源码路径：examples/misc-modules/sleepy.c

 

本文分析LDD3第六章中关于简单休眠的示例代码sleepy.c。

首先列出sleepy.c的完整代码：

 1/* 2 * sleepy.c -- the writers awake the readers 3 * 4 * Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet 5 * Copyright (C) 2001 O'Reilly & Associates 6 * 7 * The source code in this file can be freely used, adapted, 8 * and redistributed in source or binary form, so long as an 9 * acknowledgment appears in derived source files.  The citation10 * should list that the code comes from the book "Linux Device11 * Drivers" by Alessandro Rubini and Jonathan Corbet, published12 * by O'Reilly & Associates.   No warranty is attached;13 * we cannot take responsibility for errors or fitness for use.14 *15 * $Id: sleepy.c,v 1.7 2004/09/26 07:02:43 gregkh Exp $16 */1718#include <linux/module.h>19#include <linux/init.h>2021#include <linux/sched.h>  /* current and everything */22#include <linux/kernel.h> /* printk() */23#include <linux/fs.h>     /* everything... */24#include <linux/types.h>  /* size_t */25#include <linux/wait.h>2627MODULE_LICENSE("Dual BSD/GPL");2829static int sleepy_major = 0;3031static DECLARE_WAIT_QUEUE_HEAD(wq);32static int flag = 0;3334ssize_t sleepy_read (struct file *filp, char __user *buf, size_t count, loff_t *pos)35{36    printk(KERN_DEBUG "process %i (%s) going to sleep\n",37            current->pid, current->comm);38    wait_event_interruptible(wq, flag != 0);39    flag = 0;40    printk(KERN_DEBUG "awoken %i (%s)\n", current->pid, current->comm);41    return 0; /* EOF */42}4344ssize_t sleepy_write (struct file *filp, const char __user *buf, size_t count,45        loff_t *pos)46{47    printk(KERN_DEBUG "process %i (%s) awakening the readers...\n",48            current->pid, current->comm);49    flag = 1;50    wake_up_interruptible(&wq);51    return count; /* succeed, to avoid retrial */52}535455struct file_operations sleepy_fops = {56    .owner = THIS_MODULE,57    .read =  sleepy_read,58    .write = sleepy_write,59};606162int sleepy_init(void)63{64    int result;6566    /*67     * Register your major, and accept a dynamic number68     */69    result = register_chrdev(sleepy_major, "sleepy", &sleepy_fops);70    if (result < 0)71        return result;72    if (sleepy_major == 0)73        sleepy_major = result; /* dynamic */74    return 0;75}7677void sleepy_cleanup(void)78{79    unregister_chrdev(sleepy_major, "sleepy");80}8182module_init(sleepy_init);83module_exit(sleepy_cleanup);

在模块初始化函数中，注册字符设备”sleepy”时，指定了该设备的读写函数分别是sleepy_read和sleepy_write。当某个进程对sleepy执行读操作时，会进入休眠。当某个进程对sleepy执行写操作时，会唤醒相应等待队列中的所有休眠进程。

为了管理休眠进程，需要建立等待队列，等待队列就是一个进程链表，其中包含等待某个特定事件的所有进程。等待队列通过“等待队列头”来管理，等待队列头是一个类型为wait_queue_head_t的结构体。可以静态初始化一个等待队列头：

DECLARE_WAIT_QUEUE_HEAD(name);

也可以动态初始化一个等待队列头：

wait_queue_head_t my_queue;

init_waitqueue_head(&my_queue);

一个进程要进入休眠，最常用的函数是：

wait_event_interruptible(queue, condition);

queue是等待队列头，condition是一个条件表达式，进程进入休眠前和被唤醒后，都会检查condition的值是否为真，如果不为真，则进程会进入休眠。

对应wait_event_interruptible的唤醒函数是：

wake_up_interruptible(wait_queue_head_t *queue)

sleepy.c第31行定义了等待队列头wq：

31static DECLARE_WAIT_QUEUE_HEAD(wq);

在sleepy_read函数中，38行调用wait_event_interruptible(wq, flag != 0)进入休眠。所以只要有进程对sleepy执行读操作，就会进入休眠。

在sleepy_write函数中，49行将flag设置为1，然后调用wake_up_interruptible(&wq)将等待在wq上的进程唤醒。

注意，因为在sleepy_read函数中，休眠进程被唤醒后，会把flag重新设置为0，所以虽然全部休眠进程都会被唤醒，但一次只有一个进程能真正继续执行，其它进程会重新休眠。但是为简单起见，这里没考虑并发处理等问题。

要测试sleepy模块，我们先创建sleepy_load和sleepy_unload脚本。

sleepy_load脚本的内容如下：

#!/bin/sh# $Id: complete_load,v 1.4 2004/11/03 06:19:49 rubini Exp $module="sleepy"device="sleepy"mode="666"# Group: since distributions do it differently, look for wheel or use staffif grep -q '^staff:' /etc/group; then    group="staff"else    group="wheel"fi# invoke insmod with all arguments we got# and use a pathname, as insmod doesn't look in . by default/sbin/insmod ./$module.ko $* || exit 1# retrieve major numbermajor=$(awk "\$2==\"$module\" {print \$1}" /proc/devices)# Remove stale nodes and replace them, then give gid and perms# Usually the script is shorter, it's scull that has several devices in it.rm -f /dev/${device}mknod /dev/${device} c $major 0chgrp $group /dev/${device} chmod $mode  /dev/${device}

sleepy_unload脚本的内容如下：

#!/bin/shmodule="sleepy"device="sleepy"# invoke rmmod with all arguments we got/sbin/rmmod $module $* || exit 1# Remove stale nodesrm -f /dev/${device}

sleepy模块的测试过程如下图所示：