tty: tty_io: use console_list_lock for list synchronization

show_cons_active() uses the console_lock to gather information
on registered consoles. It requires that no consoles are unregistered
until it is finished. The console_list_lock should be used because
list synchronization responsibility will be removed from the
console_lock in a later change.

Note, the console_lock is still needed to serialize the device()
callback with other console operations.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-34-john.ogness@linutronix.de

Author: jogness

drivers/tty/tty_io.c

@@ -3527,16 +3527,13 @@ static ssize_t show_cons_active(struct device *dev,
 	ssize_t count = 0;
 
 	/*
-	 * Hold the console_lock to guarantee that no consoles are
+	 * Hold the console_list_lock to guarantee that no consoles are
 	 * unregistered until all console processing is complete.
 	 * This also allows safe traversal of the console list and
 	 * race-free reading of @flags.
-	 *
-	 * Take console_lock to serialize device() callback with
-	 * other console operations. For example, fg_console is
-	 * modified under console_lock when switching vt.
 	 */
-	console_lock();
+	console_list_lock();
+
 	for_each_console(c) {
 		if (!c->device)
 			continue;
@@ -3548,6 +3545,13 @@ static ssize_t show_cons_active(struct device *dev,
 		if (i >= ARRAY_SIZE(cs))
 			break;
 	}
+
+	/*
+	 * Take console_lock to serialize device() callback with
+	 * other console operations. For example, fg_console is
+	 * modified under console_lock when switching vt.
+	 */
+	console_lock();
 	while (i--) {
 		int index = cs[i]->index;
 		struct tty_driver *drv = cs[i]->device(cs[i], &index);
@@ -3563,6 +3567,8 @@ static ssize_t show_cons_active(struct device *dev,
 	}
 	console_unlock();
 
+	console_list_unlock();
+
 	return count;
 }
 static DEVICE_ATTR(active, S_IRUGO, show_cons_active, NULL);