著者：Ori Pomerantz
翻译：徐辉
8．阻塞进程
如果有人让你做你一时做不到的事情你会怎么办呢？如果你是个人被另一个人打扰，你唯一可以做的就是对他说：“现在不行，我很忙，走开！”但是如果你是内核模块，被进程打扰，你就有另一种选择。你可以让这个进程去挂起直到你可以为之提供服务。毕竟，进程是在不停的被内核挂起或唤醒（这就是多个进程看上去同时在一个处理器上运行的方法）。
这个内核模块就是一个这样的例子。这个文件（称作/proc/sleep）在一个时刻只能被一个进程打开。如果这个文件已经被打开，内核模块就调用module_interruptible_sleep_on（注8.1）。这个函数把任务（一个任务是一个内核数据结构，它包含进程以及它所在系统调用的信息）的状态改变成TASK_INTERRUPTIBLE，表示直到被唤醒任务不会运行，并且把它加入到WaitQ——等待访问文件的任务队列。那么，这个函数调用调度器进行上下文切换到其他要使用CPU的进程。
当进程完成对文件的处理后，关闭该文件并且调用module_ close。这个函数唤醒所有队列中的进程（还没有一个机制唤醒其中一个）。然后返回，刚才关闭文件的进程可以继续运行。调度器及时决定哪个进程已经完成，并且把CPU的控制给另一个进程。同时，队列中的某个进程将会从调度器那里得到对CPU的控制。它正在对module_interruptible_sleep_on的调用后开始。然后它可以设置一个全局变量告诉别的进程这个文件还打开着，正在继续它的生命。当别的进程有得到CPU的机会时，它们会看到这个全局变量，然后就重新挂起。
为使生命更加精彩，module_close并没有对唤醒等待访问文件的进程进行垄断。一个象Ctrl-C（SIGINT）之类的信号同样可以唤醒进程。在这种情况下，我们希望立即返回-EINTR。这是很重要的，比如用户可以在进程接到文件前杀死进程。
还有一点需要记住。有些时候进程不希望被挂起，它们希望立刻得到它们要的东西，或者被告知不能做到。这样的进程在打开文件时使用O_NONBLOCK标志。内核在遇到其他方面的挂起进程的操作（比如本例中的打开文件）时要返回一个错误码-ERROR作为回应。程序cat_noblock就可以用来使用标志O_NONBLOCK打开文件，它可以在本章源程序目录中找到。
ex sleep.c
/* sleep.c - create a /proc file, and if several
* processes try to open it at the same time, put all
* but one to sleep */
/* Copyright (C) 1998-99 by Ori Pomerantz */
/* The necessary header files */
/* Standard in kernel modules */
#include /* Were doing kernel work */
#include /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include
#endif
/* Necessary because we use proc fs */
#include
/* For putting processes to sleep and waking them up */
#include
#include
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesnt - 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 /* for get_user and put_user */
#endif
/* The modules 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 cant 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 dont understand this by now, youre
* hopeless as a kernel programmer. */
sprintf(message, "Last input:%s\n", Message);
for(i=0; 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 buffers length */
loff_t *offset) /* offset to file - ignore */
#else
static int module_input(
struct inode *inode, /* The files inode */
struct file *file, /* The file itself */
const char *buf, /* The buffer with the input */
int length) /* The buffers length */
#endif
{
int i;
/* Put the input into Message, where module_output
* will later be able to use it */
for(i=0; 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] = \;
/* 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 files flags include O_NONBLOCK, it means
* the process doesnt want to wait for the file.
* In this case, if the file is already open, we
* should fail with -EAGAIN, meaning "youll 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 isnt */
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);
/* If we woke up because we got a signal were not
* blocking, return -EINTR (fail the system call).
* This allows processes to be killed or stopped. */
/*
* Emmanuel Papirakis:
*
* This is a little update to work with 2.2.*. Signals
* now are contained in two words (64 bits) and are
* stored in a structure that contains an array of two
* unsigned longs. We now have to make 2 checks in our if.
*
* Ori Pomerantz:
*
* Nobody promised me theyll never use more than 64
* bits, or that this book wont be used for a version
* of Linux with a word size of 16 bits. This code
* would work in any case.
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
for(i=0; i<_NSIG_WORDS && !is_sig; i++)
is_sig = current->signal.sig[i] &
~current->blocked.sig[i];
if (is_sig) {
#else
if (current->signal & ~current->blocked) {
#endif
/* Its important to put MOD_DEC_USE_COUNT here,
* because for processes where the open is
* interrupted there will never be a corresponding
* close. If we dont decrement the usage count
* here, we will be left with a positive usage
* count which well have no way to bring down to
* zero, giving us an immortal module, which can
* only be killed by rebooting the machine. */
MOD_DEC_USE_COUNT;
return -EINTR;
}
}
/* If we got here, Already_Open must be zero */
/* Open the file */
Already_Open = 1;
return 0; /* Allow the access */
}
/* Called when the /proc file is closed */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int module_close(struct inode *inode, struct file *file)
#else
void module_close(struct inode *inode, struct file *file)
#endif
{
/* Set Already_Open to zero, so one of the processes
* in the WaitQ will be able to set Already_Open back
* to one and to open the file. All the other processes
* will be called when Already_Open is back to one, so
* theyll go back to sleep. */
Already_Open = 0;
/* Wake up all the processes in WaitQ, so if anybody
* is waiting for the file, they can have it. */
module_wake_up(&WaitQ);
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0; /* success */
#endif
}
/* This function decides whether to allow an operation
* (return zero) or not allow it (return a non-zero
* which indicates why it is not allowed).
*
* The operation can be one of the following values:
* 0 - Execute (run the "file" - meaningless in our case)
* 2 - Write (input to the kernel module)
* 4 - Read (output from the kernel module)
*
* This is the real function that checks file
* permissions. The permissions returned by ls -l are
* for referece only, and can be overridden here.
*/
static int module_permission(struct inode *inode, int op)
{
/* We allow everybody to read from our module, but
* only root (uid 0) may write to it */
if (op == 4 || (op == 2 && current->euid == 0))
return 0;
/* If its anything else, access is denied */
return -EACCES;
}
/* Structures to register as the /proc file, with
* pointers to all the relevant functions. *********** */
/* File operations for our proc file. This is where
* we place pointers to all the functions called when
* somebody tries to do something to our file. NULL
* means we dont want to deal with something. */
static struct file_operations File_Ops_
