From 1b743485e27f3d874695434cc8103f557dfdf4b9 Mon Sep 17 00:00:00 2001 From: Sreenath Vijayan Date: Wed, 13 Mar 2024 15:52:52 +0530 Subject: [PATCH] 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 Signed-off-by: Shimoyashiki Taichi Reviewed-by: John Ogness Signed-off-by: Sreenath Vijayan Link: https://lore.kernel.org/r/cc3b9b1aae60a236c6aed1dc7b0ffa2c7cd1f183.1710220326.git.sreenath.vijayan@sony.com Signed-off-by: Greg Kroah-Hartman --- Documentation/admin-guide/sysrq.rst | 9 +++++++++ drivers/tty/sysrq.c | 13 ++++++++++++- 2 files changed, 21 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst index 2f2e5bd440f9..a85b3384d1e7 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 +213,13 @@ 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, which can be modified using +sysrq[0-9] (see above). + 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..e5974b8239c9 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 */