proc: consoles: use console_list_lock for list iteration

The console_lock is used in part to guarantee safe list iteration.
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: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-35-john.ogness@linutronix.de

fs/proc/consoles.c

@@ -33,7 +33,16 @@ static int show_console_dev(struct seq_file *m, void *v)
 	if (con->device) {
 		const struct tty_driver *driver;
 		int index;
+
+		/*
+		 * 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();
 		driver = con->device(con, &index);
+		console_unlock();
+
 		if (driver) {
 			dev = MKDEV(driver->major, driver->minor_start);
 			dev += index;
@@ -64,15 +73,11 @@ static void *c_start(struct seq_file *m, loff_t *pos)
 	loff_t off = 0;
 
 	/*
-	 * Take console_lock to serialize device() callback with
-	 * other console operations. For example, fg_console is
-	 * modified under console_lock when switching vt.
-	 *
-	 * Hold the console_lock to guarantee safe traversal of the
+	 * Hold the console_list_lock to guarantee safe traversal of the
 	 * console list. SRCU cannot be used because there is no
 	 * place to store the SRCU cookie.
 	 */
-	console_lock();
+	console_list_lock();
 	for_each_console(con)
 		if (off++ == *pos)
 			break;
@@ -90,7 +95,7 @@ static void *c_next(struct seq_file *m, void *v, loff_t *pos)
 
 static void c_stop(struct seq_file *m, void *v)
 {
-	console_unlock();
+	console_list_unlock();
 }
 
 static const struct seq_operations consoles_op = {