linux-kernel - [patch RFC 10/29] kgbd: Pretend that console list walk is safe

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Date:   Sun, 11 Sep 2022 00:27:47 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Helge Deller <deller@....de>,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: [patch RFC 10/29] kgbd: Pretend that console list walk is safe

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>
---
 drivers/tty/serial/kgdboc.c |    5 ++++-
 include/linux/console.h     |   12 +++++++++++-
 kernel/debug/kdb/kdb_io.c   |    7 ++++++-
 3 files changed, 21 insertions(+), 3 deletions(-)

--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -462,10 +462,13 @@ static void kgdboc_earlycon_pre_exp_hand
 	 * 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");
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -184,7 +184,17 @@ extern void console_list_unlock(void);
  * Requires console_lock to be held which guarantees that the
  * list is immutable.
  */
-#define for_each_console(con)						\
+#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;
--- 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 *ms
 		cp++;
 	}
 
-	for_each_console(c) {
+	/*
+	 * This is a completely unprotected list walk designed by the
+	 * wishful thinking departement. See the oops_inprogress comment
+	 * below - especially the encourage section...
+	 */
+	for_each_console_kgdb(c) {
 		if (!(c->flags & CON_ENABLED))
 			continue;
 		if (c == dbg_io_ops->cons)