vt: More locking checks

I honestly have no idea what the subtle differences between
con_is_visible, con_is_fg (internal to vt.c) and con_is_bound are. But
it looks like both vc->vc_display_fg and con_driver_map are protected
by the console_lock, so probably better if we hold that when checking
this.

To do that I had to deinline the con_is_visible function.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Martin Hostettler <textshell@uchuujin.de>
Cc: Adam Borowski <kilobyte@angband.pl>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-5-daniel.vetter@ffwll.ch

drivers/tty/vt/vt.c

@@ -3822,6 +3822,8 @@ int con_is_bound(const struct consw *csw)
 {
 	int i, bound = 0;
 
+	WARN_CONSOLE_UNLOCKED();
+
 	for (i = 0; i < MAX_NR_CONSOLES; i++) {
 		if (con_driver_map[i] == csw) {
 			bound = 1;
@@ -3833,6 +3835,20 @@ int con_is_bound(const struct consw *csw)
 }
 EXPORT_SYMBOL(con_is_bound);
 
+/**
+ * con_is_visible - checks whether the current console is visible
+ * @vc: virtual console
+ *
+ * RETURNS: zero if not visible, nonzero if visible
+ */
+bool con_is_visible(const struct vc_data *vc)
+{
+	WARN_CONSOLE_UNLOCKED();
+
+	return *vc->vc_display_fg == vc;
+}
+EXPORT_SYMBOL(con_is_visible);
+
 /**
  * con_debug_enter - prepare the console for the kernel debugger
  * @sw: console driver

include/linux/console_struct.h

@@ -168,9 +168,6 @@ extern void vc_SAK(struct work_struct *work);
 
 #define CUR_DEFAULT CUR_UNDERLINE
 
-static inline bool con_is_visible(const struct vc_data *vc)
-{
-	return *vc->vc_display_fg == vc;
-}
+bool con_is_visible(const struct vc_data *vc);
 
 #endif /* _LINUX_CONSOLE_STRUCT_H */