linux-kernel - [PATCH printk v2 21/38] serial: kgdboc: use srcu console list iterator

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Message-Id: <20221019145600.1282823-22-john.ogness@linutronix.de>
Date:   Wed, 19 Oct 2022 17:01:43 +0206
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,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org
Subject: [PATCH printk v2 21/38] serial: kgdboc: use srcu console list iterator

Use srcu console list iteration for safe console list traversal.

Note that configure_kgdboc() still requires the console_lock in
order to ensure that no console is in its write() callback when
its direct() callback is called. Add comments to clarify this.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 drivers/tty/serial/kgdboc.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index b17aa7e49894..e9d3f8c6e3dc 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -171,6 +171,7 @@ static int configure_kgdboc(void)
 	int err = -ENODEV;
 	char *cptr = config;
 	struct console *cons;
+	int cookie;
 
 	if (!strlen(config) || isspace(config[0])) {
 		err = 0;
@@ -193,8 +194,14 @@ static int configure_kgdboc(void)
 	if (!p)
 		goto noconfig;
 
+	/*
+	 * Stop console printing because the device() callback may
+	 * assume the console is not within its write() callback.
+	 */
 	console_lock();
-	for_each_console(cons) {
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(cons) {
 		int idx;
 		if (cons->device && cons->device(cons, &idx) == p &&
 		    idx == tty_line) {
@@ -202,6 +209,8 @@ static int configure_kgdboc(void)
 			break;
 		}
 	}
+	console_srcu_read_unlock(cookie);
+
 	console_unlock();
 
 	kgdb_tty_driver = p;
@@ -451,6 +460,7 @@ static void kgdboc_earlycon_pre_exp_handler(void)
 {
 	struct console *con;
 	static bool already_warned;
+	int cookie;
 
 	if (already_warned)
 		return;
@@ -463,9 +473,14 @@ static void kgdboc_earlycon_pre_exp_handler(void)
 	 * serial drivers might be OK with this, print a warning once per
 	 * boot if we detect this case.
 	 */
-	for_each_console(con)
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(con) {
 		if (con == kgdboc_earlycon_io_ops.cons)
-			return;
+			break;
+	}
+	console_srcu_read_unlock(cookie);
+	if (con)
+		return;
 
 	already_warned = true;
 	pr_warn("kgdboc_earlycon is still using bootconsole\n");
-- 
2.30.2