linux-kernel - [PATCH printk v5 09/40] tty: tty_io: document console

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Date:   Wed, 16 Nov 2022 17:27:21 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Subject: [PATCH printk v5 09/40] tty: tty_io: document console_lock usage

show_cons_active() uses the console_lock to gather information
on registered consoles. Since the console_lock is being used for
multiple reasons, explicitly document these reasons. This will
be useful when the console_lock is split into fine-grained
locking.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
 drivers/tty/tty_io.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index de06c3c2ff70..ee4da2fec328 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3526,6 +3526,16 @@ static ssize_t show_cons_active(struct device *dev,
 	struct console *c;
 	ssize_t count = 0;
 
+	/*
+	 * Hold the console_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();
 	for_each_console(c) {
 		if (!c->device)
-- 
2.30.2