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>The Linux KerneLinux内核驱动模块编程指南 (内核版本2.2, 2.4)</TH
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>第九章.阻塞进程</TD
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>继续</A
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><H1
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><A
NAME="AEN1008"
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>9.1. 阻塞进程</H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1015"
></A
>9.1.1. 阻塞进程</H2
><P
>当别人让你做一件你不能马上去做的事时，你会如何反映？如果你是人类的话，而且对方也是人类的话，你只会说：“现在不行，我忙着在。闪开！”但是如果你是一个内核模块而且你被一个进程以同样的问题困扰，你会有另外一个选择。你可以让该进程休眠直到你可以为它服务时。毕竟，这样的情况在内核中时时刻刻都在发生（这就是系统让多进程在单CPU上同时运行的方法）。</P
><P
>这个内核模块就是一个这样的例子。文件（名叫 <TT
CLASS="FILENAME"
>/proc/sleep</TT
> <TT
CLASS="FILENAME"
> </TT
>）只可以在同一时刻被一个进程打开。如果该文件已经被打开，内核模块将调用函数
<TT
CLASS="FUNCTION"
>module_interruptible_sleep_on</TT
><A
NAME="AEN1048"
HREF="#FTN.AEN1048"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
><SPAN
CLASS="footnote"
>。该函数修改task的状态（task是一个内核中的结构体数据结构，其中保存着对应进程的信息和该进程正在调用的系统调用，如果有的话）为</SPAN
><TT
CLASS="PARAMETER"
><I
>TASK_INTERRUPTIBLE</I
></TT
>，意味着改进程将不会继续运行直到被唤醒，然后被添加到系统的进程等待队列中，一个等待打开该文件的队列中。然后，该函数调用系统调度器去切换到另一个不同的但有CPU运算请求的进程。</P
><P
>当一个进程处理完该文件并且关闭了该文件， <TT
CLASS="FUNCTION"
>module_close</TT
>
酒杯调用执行了。该函数唤醒所有在等待队列中的进程（还没有只唤醒特定进程的机制）。然后该函数返回，那个刚刚关闭文件的进程得以继续运行。及时的，进程调度器会判定该进程执行已执行完毕，将CPU转让给别的进程。被提供CPU使用权的那个进程就恰好从先前系统调用
<TT
CLASS="FUNCTION"
>module_interruptible_sleep_on</TT
><A
NAME="AEN1056"
HREF="#FTN.AEN1056"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
><SPAN
CLASS="footnote"
>
后的地方开始继续执行。它可以设置一个全局变量去通知别的进程该文件已被打开占用了。当别的请求该文件的进程获得CPU时间片时，它们将检测该变量然后返回休眠。</SPAN
></P
><P
>更有趣的是， <TT
CLASS="FUNCTION"
>module_close</TT
>
并不垄断唤醒等待中的请求文件的进程的权力。一个信号，像
<B
CLASS="KEYCAP"
>Ctrl</B
>+<B
CLASS="KEYCAP"
>c</B
> (<TT
CLASS="PARAMETER"
><I
>SIGINT</I
></TT
>) 也能够唤醒别的进程<A
NAME="AEN1077"
HREF="#FTN.AEN1077"
><SPAN
CLASS="footnote"
>[3]</SPAN
></A
> 。在这种情况下，我们想立即返回 <TT
CLASS="PARAMETER"
><I
>-EINTR</I
></TT
>
。这对用户很重要，举个例子来说，用户可以在某个进程接受到文件前终止该进程。</P
><P
>还有一点值得注意。有些时候进程并不愿意休眠，它们要么立即执行它们想做的，要么被告知任务无法进行。这样的进程在打开文件时会使用标志 <TT
CLASS="PARAMETER"
><I
>O_NONBLOCK</I
></TT
>
。在别的进程被阻塞时内核应该做出的响应是返回错误代码
<TT
CLASS="PARAMETER"
><I
>-EAGAIN</I
></TT
> ，像在本例中对该文件的请求的进程。程序 <B
CLASS="COMMAND"
>cat_noblock</B
>，在本章的源代码目录下可以找到，就能够使用标志位 <TT
CLASS="PARAMETER"
><I
>O_NONBLOCK </I
></TT
>打开文件。</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN1106"
></A
><P
><B
>Example 9-1. sleep.c</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>/* sleep.c - create a /proc file, and if several processes try to open it at
* the same time, put all but one to sleep
*/
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary because we use proc fs */
#include <linux/proc_fs.h>
/* For putting processes to sleep and waking them up */
#include <linux/sched.h>
#include <linux/wrapper.h>
/* In 2.2.3 /usr/include/linux/version.h includes a macro for this, but 2.0.35
* doesn't - so I add it here if necessary.
*/
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h> /* for get_user and put_user */
#endif
/* The module's file functions */
/* Here we keep the last message received, to prove that we can process our
* input
*/
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];
/* Since we use the file operations struct, we can't use the special proc
* output provisions - we have to use a standard read function, which is this
* function
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_output (
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the user segment) */
size_t len, /* The length of the buffer */
loff_t *offset) /* Offset in the file - ignore */
#else
static int module_output (
struct inode *inode, /* The inode read */
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the user segment) */
int len) /* The length of the buffer */
#endif
{
static int finished = 0;
int i;
char message[MESSAGE_LENGTH+30];
/* Return 0 to signify end of file - that we have nothing more to say at this
* point.
*/
if (finished) {
finished = 0;
return 0;
}
/* If you don't understand this by now, you're hopeless as a kernel
* programmer.
*/
sprintf(message, "Last input:%s\n", Message);
for (i = 0; i < len && message[i]; i++)
put_user(message[i], buf+i);
finished = 1;
return i; /* Return the number of bytes "read" */
}
/* This function receives input from the user when the user writes to the /proc
* file.
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_input (
struct file *file, /* The file itself */
const char *buf, /* The buffer with input */
size_t length, /* The buffer's length */
loff_t *offset) /* offset to file - ignore */
#else
static int module_input (
struct inode *inode, /* The file's inode */
struct file *file, /* The file itself */
const char *buf, /* The buffer with the input */
int length) /* The buffer's length */
#endif
{
int i;
/* Put the input into Message, where module_output will later be able to use
* it
*/
for(i = 0; i < MESSAGE_LENGTH-1 && i < length; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(Message[i], buf+i);
#else
Message[i] = get_user(buf+i);
#endif
/* we want a standard, zero terminated string */
Message[i] = '\0';
/* We need to return the number of input characters used */
return i;
}
/* 1 if the file is currently open by somebody */
int Already_Open = 0;
/* Queue of processes who want our file */
static struct wait_queue *WaitQ = NULL;
/* Called when the /proc file is opened */
static int module_open(struct inode *inode, struct file *file)
{
/* If the file's flags include O_NONBLOCK, it means the process doesn't want
* to wait for the file. In this case, if the file is already open, we
* should fail with -EAGAIN, meaning "you'll have to try again", instead of
* blocking a process which would rather stay awake.
*/
if ((file->f_flags & O_NONBLOCK) && Already_Open)
return -EAGAIN;
/* This is the correct place for MOD_INC_USE_COUNT because if a process is
* in the loop, which is within the kernel module, the kernel module must
* not be removed.
*/
MOD_INC_USE_COUNT;
/* If the file is already open, wait until it isn't */
while (Already_Open)
{
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int i, is_sig = 0;
#endif
/* This function puts the current process, including any system calls,
* such as us, to sleep. Execution will be resumed right after the
* function call, either because somebody called wake_up(&WaitQ) (only
* module_close does that, when the file is closed) or when a signal,
* such as Ctrl-C, is sent to the process
*/
module_interruptible_sleep_on(&WaitQ);
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