From: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Date: Thu, 20 May 2010 20:14:13 +0000 (-0400)
Subject: Use spinlock to busy wait.
X-Git-Url: http://git.cascardo.eti.br/?p=cascardo%2Fkernel%2Fsamples%2Fchar2%2F.git;a=commitdiff_plain;h=37d2e76fba58bb46fc3578ef150681619a4adbe9

Use spinlock to busy wait.
---

diff --git a/hellochar.c b/hellochar.c
index 1b0c9f2..206b6d1 100644
--- a/hellochar.c
+++ b/hellochar.c
@@ -20,71 +20,36 @@
 #include <linux/fs.h>
 #include <linux/cdev.h>
 #include <linux/slab.h>
-#include <linux/mutex.h>
-#include <asm/uaccess.h>
+#include <linux/spinlock.h>
 
 MODULE_LICENSE("GPL");
 
-#define MAXLEN 4000
 static dev_t devnum;
 static struct cdev *dev;
 static const char default_greeting[] = "Hello, World!\n";
 
-struct hello_buffer {
-	size_t len;
-	char buffer[0];
-};
-static struct hello_buffer *hello;
-static DEFINE_MUTEX(hello_mtx);
+static DEFINE_SPINLOCK(hello_lock);
 
 static int hello_open(struct inode *ino, struct file *fp)
 {
-	if (mutex_lock_interruptible(&hello_mtx))
-		return -ERESTARTSYS;
-	if (fp->f_flags & O_TRUNC) {
-		memset(hello->buffer, 0, MAXLEN);
-		hello->len = 0;
-	}
-	if (fp->f_flags & O_APPEND)
-		fp->f_pos = hello->len;
-	mutex_unlock(&hello_mtx);
 	return 0;
 }
 
 static ssize_t hello_read(struct file *fp, char __user *buf, size_t sz,
 	loff_t *pos)
 {
-	int r;
-	if (mutex_lock_interruptible(&hello_mtx))
-		return -ERESTARTSYS;
-	if (sz + *pos > hello->len)
-		sz = hello->len - *pos;
-	r = copy_to_user(buf, hello->buffer + *pos, sz);
-	mutex_unlock(&hello_mtx);
-	if (r)
-		return -EFAULT;
-	*pos += sz;
-	return sz;
+	int i = 1 << 28;
+	spin_lock(&hello_lock);
+	while (i--)
+		cpu_relax();
+	spin_unlock(&hello_lock);
+	return 0;
 }
 
 static ssize_t hello_write(struct file *fp, const char __user *buf, size_t sz,
 	loff_t *pos)
 {
-	int r;
-	if (mutex_lock_interruptible(&hello_mtx))
-		return -ERESTARTSYS;
-	if (sz + *pos > MAXLEN)
-		sz = MAXLEN - *pos;
-	r = copy_from_user(hello->buffer + *pos, buf, sz);
-	if (r) {
-		mutex_unlock(&hello_mtx);
-		return -EFAULT;
-	}
-	*pos += sz;
-	if (hello->len < *pos)
-		hello->len = *pos;
-	mutex_unlock(&hello_mtx);
-	return sz;
+	return 0;
 }
 
 static int hello_release(struct inode *ino, struct file *fp)
@@ -103,13 +68,6 @@ static const struct file_operations hello_fops = {
 static int __init ch_init(void)
 {
 	int r = 0;
-	hello = kzalloc(sizeof(*hello) + MAXLEN, GFP_KERNEL);
-	if (!hello) {
-		r = -ENOMEM;
-		goto out;
-	}
-	memcpy(hello->buffer, default_greeting, sizeof(default_greeting));
-	hello->len = sizeof(default_greeting);
 	r = alloc_chrdev_region(&devnum, 0, 256, "hello");
 	if (r)
 		goto reg_out;
@@ -129,8 +87,6 @@ add_out:
 cdev_out:
 	unregister_chrdev_region(devnum, 256);
 reg_out:
-	kfree(hello);
-out:
 	return r;
 }
 
@@ -138,7 +94,6 @@ static void __exit ch_exit(void)
 {
 	cdev_del(dev);
 	unregister_chrdev_region(devnum, 256);
-	kfree(hello);
 }
 
 module_init(ch_init);
diff --git a/usr.c b/usr.c
index 0829125..d4d2aba 100644
--- a/usr.c
+++ b/usr.c
@@ -19,19 +19,18 @@
 
 #include <fcntl.h>
 #include <unistd.h>
+#include <stdlib.h>
 
 int main(void)
 {
 	int fd;
 	char buffer[16];
-	int r;
 	fd = open("/dev/hello", O_RDONLY);
 	if (fd < 0)
 		return 1;
-	while (1) {
-		lseek(fd, 0, SEEK_SET);
-		r = read(fd, buffer, sizeof(buffer));
-		write(1, buffer, r);
-	}
+	read(fd, buffer, sizeof(buffer));
+	sleep(5);
+	write(fd, buffer, sizeof(buffer));
+	close(fd);
 	return 0;
 }