Fix potential concurrent problems in chardev2.c

After forking, Each file descriptor in the child refers to the same
open file description as the parent. So when calling open() before
fork(), the child can access the device file without checking by
exclusive access in device_open(). It may cause race conditions
in device_ioctl().

Because of that, it is unnecessary to check the multiple access
in device_open(). It just needs check in device_ioctl(), since
read(), write(), seek() system call are atomic [1][2].

Related discussion:
- https://github.com/sysprog21/lkmpg/issues/148

[1] https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/
[2] https://www.kernel.org/doc/html/latest/filesystems/files.html

Close #148

examples/chardev2.c

@@ -37,10 +37,6 @@ static int device_open(struct inode *inode, struct file *file)
 {
     pr_info("device_open(%p)\n", file);
 
-    /* We don't want to talk to two processes at the same time. */
-    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN))
-        return -EBUSY;
-
     try_module_get(THIS_MODULE);
     return SUCCESS;
 }
@@ -49,9 +45,6 @@ static int device_release(struct inode *inode, struct file *file)
 {
     pr_info("device_release(%p,%p)\n", inode, file);
 
-    /* We're now ready for our next caller */
-    atomic_set(&already_open, CDEV_NOT_USED);
-
     module_put(THIS_MODULE);
     return SUCCESS;
 }
@@ -129,6 +122,11 @@ device_ioctl(struct file *file, /* ditto */
              unsigned long ioctl_param)
 {
     int i;
+    long ret = SUCCESS;
+
+    /* We don't want to talk to two processes at the same time. */
+    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN))
+        return -EBUSY;
 
     /* Switch according to the ioctl called */
     switch (ioctl_num) {
@@ -166,11 +164,14 @@ device_ioctl(struct file *file, /* ditto */
         /* This ioctl is both input (ioctl_param) and output (the return
          * value of this function).
          */
-        return (long)message[ioctl_param];
+        ret = (long)message[ioctl_param];
         break;
     }
 
-    return SUCCESS;
+    /* We're now ready for our next caller */
+    atomic_set(&already_open, CDEV_NOT_USED);
+
+    return ret;
 }
 
 /* Module Declarations */