linux-kernel - [PATCH v5 2/2] tty/sysrq: Replay kernel log messages on consoles via sysrq

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Date: Thu, 7 Mar 2024 18:29:11 +0530
From: Sreenath Vijayan <sreenath.vijayan@...y.com>
To: pmladek@...e.com, john.ogness@...utronix.de, corbet@....net,
        gregkh@...uxfoundation.org, jirislaby@...nel.org
Cc: rdunlap@...radead.org, rostedt@...dmis.org, senozhatsky@...omium.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org, taichi.shimoyashiki@...y.com,
        daniel.palmer@...y.com, anandakumar.balasubramaniam@...y.com,
        sreenath.vijayan@...y.com
Subject: [PATCH v5 2/2] tty/sysrq: Replay kernel log messages on consoles via
 sysrq

When terminal is unresponsive, one cannot use dmesg to view
the printk ring buffer messages. Also, syslog services may be
disabled, especially on embedded systems, to check the messages
after a reboot. In this scenario, replay the messages in printk
ring buffer on consoles via sysrq by pressing sysrq+R.

The console loglevel will determine which all kernel log messages
are displayed. The messages will be displayed only when
console_trylock() succeeds. Users could repeat the sysrq key when
it fails. If the owner of console subsystem lock is stuck,
repeating the key won't work.

Suggested-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Sreenath Vijayan <sreenath.vijayan@...y.com>
Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@...y.com>
---
 Documentation/admin-guide/sysrq.rst |  8 ++++++++
 drivers/tty/sysrq.c                 | 13 ++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
index 2f2e5bd440f9..8098a1b4908d 100644
--- a/Documentation/admin-guide/sysrq.rst
+++ b/Documentation/admin-guide/sysrq.rst
@@ -161,6 +161,8 @@ Command	    Function
             will be printed to your console. (``0``, for example would make
             it so that only emergency messages like PANICs or OOPSes would
             make it to your console.)
+
+``R``	    Replay the kernel log messages on consoles.
 =========== ===================================================================
 
 Okay, so what can I use them for?
@@ -211,6 +213,12 @@ processes.
 "just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
 frozen (probably root) filesystem via the FIFREEZE ioctl.
 
+``Replay logs(R)`` is useful to view the kernel log messages when system is hung
+or you are not able to use dmesg command to view the messages in printk buffer.
+User may have to press the key combination multiple times if console system is
+busy. If it is completely locked up, then messages won't be printed. Output
+messages depend on current console loglevel.
+
 Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 02217e3c916b..c4deea41a8b5 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -450,6 +450,17 @@ static const struct sysrq_key_op sysrq_unrt_op = {
 	.enable_mask	= SYSRQ_ENABLE_RTNICE,
 };
 
+static void sysrq_handle_replay_logs(u8 key)
+{
+	console_replay_all();
+}
+static struct sysrq_key_op sysrq_replay_logs_op = {
+	.handler        = sysrq_handle_replay_logs,
+	.help_msg       = "replay-kernel-logs(R)",
+	.action_msg     = "replay kernel logs on consoles",
+	.enable_mask    = SYSRQ_ENABLE_DUMP,
+};
+
 /* Key Operations table and lock */
 static DEFINE_SPINLOCK(sysrq_key_table_lock);
 
@@ -519,7 +530,7 @@ static const struct sysrq_key_op *sysrq_key_table[62] = {
 	NULL,				/* O */
 	NULL,				/* P */
 	NULL,				/* Q */
-	NULL,				/* R */
+	&sysrq_replay_logs_op,		/* R */
 	NULL,				/* S */
 	NULL,				/* T */
 	NULL,				/* U */
-- 
2.25.1