linux-kernel - [PATCH printk 10/18] kgbd: Pretend that console list walk is safe

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Date:   Sat, 24 Sep 2022 02:10:46 +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>,
        Aaron Tomlin <atomlin@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org
Subject: [PATCH printk 10/18] kgbd: Pretend that console list walk is safe

From: Thomas Gleixner <tglx@...utronix.de>

Provide a special list iterator macro for KGDB to allow unprotected list
walks and add a few comments to explain the hope based approach.

Preperatory change for changing the console list to hlist and adding
lockdep asserts to regular list walks.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
---
 drivers/tty/serial/kgdboc.c |  5 ++++-
 include/linux/console.h     | 10 ++++++++++
 kernel/debug/kdb/kdb_io.c   |  7 ++++++-
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index af2aa76bae15..57a5fd27dffe 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -462,10 +462,13 @@ static void kgdboc_earlycon_pre_exp_handler(void)
 	 * we have no other choice so we keep using it.  Since not all
 	 * serial drivers might be OK with this, print a warning once per
 	 * boot if we detect this case.
+	 *
+	 * Pretend that walking the console list is safe...
 	 */
-	for_each_console(con)
+	for_each_console_kgdb(con) {
 		if (con == kgdboc_earlycon_io_ops.cons)
 			return;
+	}
 
 	already_warned = true;
 	pr_warn("kgdboc_earlycon is still using bootconsole\n");
diff --git a/include/linux/console.h b/include/linux/console.h
index 24344f9b0bc1..86a6125512b9 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -187,6 +187,16 @@ extern void console_list_unlock(void) __releases(console_mutex);
 #define for_each_console(con)						\
 	for (con = console_drivers; con != NULL; con = con->next)
 
+/**
+ * for_each_console_kgdb() - Iterator over registered consoles for KGDB
+ * @con:	struct console pointer used as loop cursor
+ *
+ * Has no serialization requirements and KGDB pretends that this is safe.
+ * Don't use outside of the KGDB fairy tale land!
+ */
+#define for_each_console_kgdb(con)					\
+	for (con = console_drivers; con != NULL; con = con->next)
+
 extern int console_set_on_cmdline;
 extern struct console *early_console;
 
diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 67d3c48a1522..fb3775e61a3b 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -558,7 +558,12 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		cp++;
 	}
 
-	for_each_console(c) {
+	/*
+	 * This is a completely unprotected list walk designed by the
+	 * wishful thinking department. See the oops_in_progress comment
+	 * below - especially the encourage section...
+	 */
+	for_each_console_kgdb(c) {
 		if (!(c->flags & CON_ENABLED))
 			continue;
 		if (c == dbg_io_ops->cons)
-- 
2.30.2