linux-kernel - [PATCH printk v1 09/10] kdb: if available, only use atomic consoles for output mirroring

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Date:   Tue,  3 Aug 2021 15:19:00 +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>,
        Sumit Garg <sumit.garg@...aro.org>,
        Cengiz Can <cengiz@...nel.wtf>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: [PATCH printk v1 09/10] kdb: if available, only use atomic consoles for output mirroring

Currently kdb uses the @oops_in_progress hack to mirror kdb output
to all active consoles from NMI context. Ignoring locks is unsafe.

Now that an NMI-safe atomic interface is available for consoles,
use only that interface to mirror kdb output if such a console is
available.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/debug/kdb/kdb_io.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 6735ac36b718..871b89d6294b 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -544,6 +544,7 @@ static int kdb_search_string(char *searched, char *searchfor)
 
 static void kdb_msg_write(const char *msg, int msg_len)
 {
+	bool atomic_console_available;
 	struct console *c;
 	const char *cp;
 	int len;
@@ -559,11 +560,26 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		cp++;
 	}
 
+	atomic_console_available = have_atomic_console();
+
 	for_each_console(c) {
 		if (!(c->flags & CON_ENABLED))
 			continue;
 		if (c == dbg_io_ops->cons)
 			continue;
+
+		/*
+		 * If any atomic consoles are available, only atomic
+		 * consoles are used.
+		 */
+		if (atomic_console_available) {
+			if (c->write_atomic) {
+				c->write_atomic(c, msg, msg_len);
+				touch_nmi_watchdog();
+			}
+			continue;
+		}
+
 		/*
 		 * Set oops_in_progress to encourage the console drivers to
 		 * disregard their internal spin locks: in the current calling
-- 
2.20.1