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）。
这个内核模块就是一个这样的例子。文件（名叫 /proc/sleep ）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 module_interruptible_sleep_on[1]。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TASK_INTERRUPTIBLE，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。
当一个进程处理完该文件并且关闭了该文件， module_close 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 module_interruptible_sleep_on[2] 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。
更有趣的是， module_close 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 Ctrl+c (SIGINT) 也能够唤醒别的进程[3] 。在这种情况下，我们想立即返回 -EINTR 。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。
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 O_NONBLOCK 。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 -EAGAIN ，像在本例中对该文件的请求的进程。程序 cat_noblock，在本章的源代码目录下可以找到，就能够使用标志位 O_NONBLOCK 打开文件。
Example 9-1. 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
*/
#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);
/* If we woke up because we got a signal we're 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 they'll never use more than 64 bits, or that this book
* won't 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
/* It's 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 don't decrement the usage count here, we will be left
* with a positive usage count which we'll 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 they'll 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 it's 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 don't want to deal with something.
*/
static struct file_operations File_Ops_4_Our_Proc_File = {
NULL, /* lseek */
module_output, /* "read" from the file */
module_input, /* "write" to the file */
NULL, /* readdir */
NULL, /* select */
NULL, /* ioctl */
NULL, /* mmap */
module_open, /* called when the /proc file is opened */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush */
#endif
module_close}; /* called when it's classed */
/* Inode operations for our proc file. We need it so we'll have somewhere to
* specify the file operations structure we want to use, and the function we
* use for permissions. It's also possible to specify functions to be called
* for anything else which could be done to an inode (although we don't bother,
* we just put NULL).
*/
static struct inode_operations Inode_Ops_4_Our_Proc_File = {
&File_Ops_4_Our_Proc_File,
NULL, /* create */
NULL, /* lookup */
NULL, /* link */
NULL, /* unlink */
NULL, /* symlink */
NULL, /* mkdir */
NULL, /* rmdir */
NULL, /* mknod */
NULL, /* rename */
NULL, /* readlink */
NULL, /* follow_link */
NULL, /* readpage */
NULL, /* writepage */
NULL, /* bmap */
NULL, /* truncate */
module_permission}; /* check for permissions */
/* Directory entry */
static struct proc_dir_entry Our_Proc_File = {
0, /* Inode number - ignore, it will be filled by
* proc_register[_dynamic]
*/
5, /* Length of the file name */
"sleep", /* The file name */
/* File mode - this is a regular file which can be read by its owner, its
* group, and everybody else. Also, its owner can write to it.
*
* Actually, this field is just for reference, it's module_permission that
* does the actual check. It could use this field, but in our
* implementation it doesn't, for simplicity.
*/
S_IFREG | S_IRUGO | S_IWUSR,
1, /* Number of links (directories where the file is referenced) */
0, 0, /* The uid and gid for the file - we give it to root */
80, /* The size of the file reported by ls. */
/* A pointer to the inode structure for the file, if we need it. In our
* case we do, because we need a write function.
*/
&Inode_Ops_4_Our_Proc_File,
/* The read function for the file. Irrelevant, because we put it in the
* inode structure above
*/
NULL};
/* Module initialization and cleanup */
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register_dynamic is a success, failure otherwise */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
/* proc_root is the root directory for the proc fs (/proc). This is where
* we want our file to be located.
*/
}
/* Cleanup - unregister our file from /proc. This could get dangerous if
* there are still processes waiting in WaitQ, because they are inside our
* open function, which will get unloaded. I'll explain how to avoid removal
* of a kernel module in such a case in chapter 10.
*/
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
} |
| [1] | 最方便的保持某个文件被打开的方法是使用命令 tail -f 打开该文件。 |
| [2] | 这就意味着该进程仍然在内核态中——该进程已经调用了 open 的系统调用，但系统调用却没有返回。在这段时间内该进程将不会得知别人正在使用CPU。 |
| [3] | 这是因为我们使用的是 module_interruptible_sleep_on. 我们也可以使用 module_sleep_on ，但这样会导致一些十分愤怒的用户，因为他们的 Ctrl+c 将不起任何作用。 |