Chapter 9. Blocking Processes

阻塞进程

Enter Sandman

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(/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 wait_event_interruptible[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 WaitQ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就被调用执行了。 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 */
#include <linux/proc_fs.h>	/* Necessary because we use proc fs */
#include <linux/sched.h>	/* For putting processes to sleep and 
				   waking them up */
#include <asm/uaccess.h>	/* for get_user and put_user */

/* 
 * 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];

static struct proc_dir_entry *Our_Proc_File;
#define PROC_ENTRY_FILENAME "sleep"

/* 
 * 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
 */
static ssize_t module_output(struct file *file,	/* see include/linux/fs.h   */
			     char *buf,	/* The buffer to put data to 
					   (in the user segment)    */
			     size_t len,	/* The length of the buffer */
			     loff_t * offset)
{
	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.
 */
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 */
	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++)
		get_user(Message[i], buf + i);
	/* 
	 * 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 
 */
DECLARE_WAIT_QUEUE_HEAD(WaitQ);
/* 
 * 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 try_module_get(THIS_MODULE) because 
	 * if a process is in the loop, which is within the kernel module,
	 * the kernel module must not be removed.
	 */
	try_module_get(THIS_MODULE);

	/* 
	 * If the file is already open, wait until it isn't 
	 */

	while (Already_Open) {
		int i, is_sig = 0;

		/* 
		 * 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
		 */
		wait_event_interruptible(WaitQ, !Already_Open);

		/* 
		 * 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.
 */
		for (i = 0; i < _NSIG_WORDS && !is_sig; i++)
			is_sig =
			    current->pending.signal.sig[i] & ~current->
			    blocked.sig[i];

		if (is_sig) {
			/* 
			 * It's important to put module_put(THIS_MODULE) 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.
			 */
			module_put(THIS_MODULE);
			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 
 */
int module_close(struct inode *inode, struct file *file)
{
	/* 
	 * 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.
	 */
	wake_up(&WaitQ);

	module_put(THIS_MODULE);

	return 0;		/* success */
}

/*
 * 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 reference only, and can be overridden here.
 */
static int module_permission(struct inode *inode, int op, struct nameidata *nd)
{
	/* 
	 * 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 = {
	.read = module_output,	/* "read" from the file */
	.write = module_input,	/* "write" to the file */
	.open = module_open,	/* called when the /proc file is opened */
	.release = module_close,	/* called when it's closed */
};

/* 
 * 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 = {
	.permission = module_permission,	/* check for permissions */
};

/* 
 * Module initialization and cleanup 
 */

/* 
 * Initialize the module - register the proc file 
 */

int init_module()
{
	int rv = 0;
	Our_Proc_File = create_proc_entry(PROC_ENTRY_FILENAME, 0644, NULL);
	Our_Proc_File->owner = THIS_MODULE;
	Our_Proc_File->proc_iops = &Inode_Ops_4_Our_Proc_File;
	Our_Proc_File->proc_fops = &File_Ops_4_Our_Proc_File;
	Our_Proc_File->mode = S_IFREG | S_IRUGO | S_IWUSR;
	Our_Proc_File->uid = 0;
	Our_Proc_File->gid = 0;
	Our_Proc_File->size = 80;

	if (Our_Proc_File == NULL) {
		rv = -ENOMEM;
		remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
		printk(KERN_INFO "Error: Could not initialize /proc/test\n");
	}

	return rv;
}

/* 
 * 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()
{
	remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
}

Notes

[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。