vt_kdsetmode: extend console locking

As per the long-suffering comment.
Reported-by: Minh Yuan <yuanmingbuaa@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/tty/vt/vt_ioctl.c

@@ -246,6 +246,8 @@ int vt_waitactive(int n)
  *
  * XXX It should at least call into the driver, fbdev's definitely need to
  * restore their engine state. --BenH
+ *
+ * Called with the console lock held.
  */
 static int vt_kdsetmode(struct vc_data *vc, unsigned long mode)
 {
@@ -262,7 +264,6 @@ static int vt_kdsetmode(struct vc_data *vc, unsigned long mode)
 		return -EINVAL;
 	}
 
-	/* FIXME: this needs the console lock extending */
 	if (vc->vc_mode == mode)
 		return 0;
 
@@ -271,12 +272,10 @@ static int vt_kdsetmode(struct vc_data *vc, unsigned long mode)
 		return 0;
 
 	/* explicitly blank/unblank the screen if switching modes */
-	console_lock();
 	if (mode == KD_TEXT)
 		do_unblank_screen(1);
 	else
 		do_blank_screen(1);
-	console_unlock();
 
 	return 0;
 }
@@ -378,7 +377,10 @@ static int vt_k_ioctl(struct tty_struct *tty, unsigned int cmd,
 		if (!perm)
 			return -EPERM;
 
-		return vt_kdsetmode(vc, arg);
+		console_lock();
+		ret = vt_kdsetmode(vc, arg);
+		console_unlock();
+		return ret;
 
 	case KDGETMODE:
 		return put_user(vc->vc_mode, (int __user *)arg);