Serialize access to global structure using semaphore.

diff --git a/hellochar.c b/hellochar.c
index 71605f6..85510f7 100644 (file)
--- a/hellochar.c
+++ b/hellochar.c
@@ -20,6 +20,7 @@
 #include <linux/fs.h>
 #include <linux/cdev.h>
 #include <linux/slab.h>
+#include <linux/semaphore.h>
 #include <asm/uaccess.h>
 
 MODULE_LICENSE("GPL");
@@ -33,16 +34,19 @@ struct hello_buffer {
        size_t len;
        char buffer[0];
 };
-struct hello_buffer *hello;
+static struct hello_buffer *hello;
+static DECLARE_MUTEX(hello_mtx);
 
 static int hello_open(struct inode *ino, struct file *fp)
 {
+       down(&hello_mtx);
        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;
+       up(&hello_mtx);
        return 0;
 }
 
@@ -50,9 +54,11 @@ static ssize_t hello_read(struct file *fp, char __user *buf, size_t sz,
        loff_t *pos)
 {
        int r;
+       down(&hello_mtx);
        if (sz + *pos > hello->len)
                sz = hello->len - *pos;
        r = copy_to_user(buf, hello->buffer + *pos, sz);
+       up(&hello_mtx);
        if (r)
                return -EFAULT;
        *pos += sz;
@@ -63,14 +69,18 @@ static ssize_t hello_write(struct file *fp, const char __user *buf, size_t sz,
        loff_t *pos)
 {
        int r;
+       down(&hello_mtx);
        if (sz + *pos > MAXLEN)
                sz = MAXLEN - *pos;
        r = copy_from_user(hello->buffer + *pos, buf, sz);
-       if (r)
+       if (r) {
+               up(&hello_mtx);
                return -EFAULT;
+       }
        *pos += sz;
        if (hello->len < *pos)
                hello->len = *pos;
+       up(&hello_mtx);
        return sz;
 }