Commit 1b743485 authored by Sreenath Vijayan's avatar Sreenath Vijayan Committed by Greg Kroah-Hartman
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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: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarShimoyashiki Taichi <taichi.shimoyashiki@sony.com>
Reviewed-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarSreenath Vijayan <sreenath.vijayan@sony.com>
Link: https://lore.kernel.org/r/cc3b9b1aae60a236c6aed1dc7b0ffa2c7cd1f183.1710220326.git.sreenath.vijayan@sony.com


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 693f75b9

Documentation/admin-guide/sysrq.rst

+9 −0
Original line number Diff line number Diff line
@@ -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?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

drivers/tty/sysrq.c

+12 −1
Original line number Diff line number Diff line
@@ -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 */