Use spinlock to busy wait.

diff --git a/hellochar.c b/hellochar.c
index 1b0c9f2..206b6d1 100644 (file)
--- 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 (file)
--- 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;
 }