Linux Magic System Request Key Hacks
+====================================
+
Documentation for sysrq.c
-* What is the magic SysRq key?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+What is the magic SysRq key?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
It is a 'magical' key combo you can hit which the kernel will respond to
regardless of whatever else it is doing, unless it is completely locked up.
-* How do I enable the magic SysRq key?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+How do I enable the magic SysRq key?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
configuring the kernel. When running a kernel with SysRq compiled in,
/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
the SysRq key. The default value in this file is set by the
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
- 0 - disable sysrq completely
- 1 - enable all functions of sysrq
- >1 - bitmask of allowed sysrq functions (see below for detailed function
- description):
+
+ - 0 - disable sysrq completely
+ - 1 - enable all functions of sysrq
+ - >1 - bitmask of allowed sysrq functions (see below for detailed function
+ description)::
+
2 = 0x2 - enable control of console logging level
4 = 0x4 - enable control of keyboard (SAK, unraw)
8 = 0x8 - enable debugging dumps of processes etc.
128 = 0x80 - allow reboot/poweroff
256 = 0x100 - allow nicing of all RT tasks
-You can set the value in the file by the following command:
+You can set the value in the file by the following command::
+
echo "number" >/proc/sys/kernel/sysrq
The number may be written here either as decimal or as hexadecimal
with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
written in hexadecimal.
-Note that the value of /proc/sys/kernel/sysrq influences only the invocation
-via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
-allowed (by a user with admin privileges).
+Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation
+via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is
+always allowed (by a user with admin privileges).
-* How do I use the magic SysRq key?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
+How do I use the magic SysRq key?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+On x86 - You press the key combo :kbd:`ALT-SysRq-<command key>`.
+
+.. note::
+ Some
keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
also known as the 'Print Screen' key. Also some keyboards cannot
handle so many keys being pressed at the same time, so you might
- have better luck with "press Alt", "press SysRq", "release SysRq",
- "press <command key>", release everything.
+ have better luck with press :kbd:`Alt`, press :kbd:`SysRq`,
+ release :kbd:`SysRq`, press :kbd:`<command key>`, release everything.
-On SPARC - You press 'ALT-STOP-<command key>', I believe.
+On SPARC - You press :kbd:`ALT-STOP-<command key>`, I believe.
-On the serial console (PC style standard serial ports only) -
- You send a BREAK, then within 5 seconds a command key. Sending
- BREAK twice is interpreted as a normal BREAK.
+On the serial console (PC style standard serial ports only)
+ You send a ``BREAK``, then within 5 seconds a command key. Sending
+ ``BREAK`` twice is interpreted as a normal BREAK.
-On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
- Print Screen (or F13) - <command key> may suffice.
+On PowerPC
+ Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`,
+ :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice.
-On other - If you know of the key combos for other architectures, please
- let me know so I can add them to this section.
+On other
+ If you know of the key combos for other architectures, please
+ let me know so I can add them to this section.
-On all - write a character to /proc/sysrq-trigger. e.g.:
+On all
+ write a character to /proc/sysrq-trigger. e.g.::
echo t > /proc/sysrq-trigger
-* What are the 'command' keys?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-'b' - Will immediately reboot the system without syncing or unmounting
- your disks.
+What are the 'command' keys?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-'c' - Will perform a system crash by a NULL pointer dereference.
- A crashdump will be taken if configured.
+=========== ===================================================================
+Command Function
+=========== ===================================================================
+``b`` Will immediately reboot the system without syncing or unmounting
+ your disks.
-'d' - Shows all locks that are held.
+``c`` Will perform a system crash by a NULL pointer dereference.
+ A crashdump will be taken if configured.
-'e' - Send a SIGTERM to all processes, except for init.
+``d`` Shows all locks that are held.
-'f' - Will call the oom killer to kill a memory hog process, but do not
- panic if nothing can be killed.
+``e`` Send a SIGTERM to all processes, except for init.
-'g' - Used by kgdb (kernel debugger)
+``f`` Will call the oom killer to kill a memory hog process, but do not
+ panic if nothing can be killed.
-'h' - Will display help (actually any other key than those listed
- here will display help. but 'h' is easy to remember :-)
+``g`` Used by kgdb (kernel debugger)
-'i' - Send a SIGKILL to all processes, except for init.
+``h`` Will display help (actually any other key than those listed
+ here will display help. but ``h`` is easy to remember :-)
-'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
+``i`` Send a SIGKILL to all processes, except for init.
-'k' - Secure Access Key (SAK) Kills all programs on the current virtual
- console. NOTE: See important comments below in SAK section.
+``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
-'l' - Shows a stack backtrace for all active CPUs.
+``k`` Secure Access Key (SAK) Kills all programs on the current virtual
+ console. NOTE: See important comments below in SAK section.
-'m' - Will dump current memory info to your console.
+``l`` Shows a stack backtrace for all active CPUs.
-'n' - Used to make RT tasks nice-able
+``m`` Will dump current memory info to your console.
-'o' - Will shut your system off (if configured and supported).
+``n`` Used to make RT tasks nice-able
-'p' - Will dump the current registers and flags to your console.
+``o`` Will shut your system off (if configured and supported).
-'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
- timer_list timers) and detailed information about all
- clockevent devices.
+``p`` Will dump the current registers and flags to your console.
-'r' - Turns off keyboard raw mode and sets it to XLATE.
+``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular
+ timer_list timers) and detailed information about all
+ clockevent devices.
-'s' - Will attempt to sync all mounted filesystems.
+``r`` Turns off keyboard raw mode and sets it to XLATE.
-'t' - Will dump a list of current tasks and their information to your
- console.
+``s`` Will attempt to sync all mounted filesystems.
-'u' - Will attempt to remount all mounted filesystems read-only.
+``t`` Will dump a list of current tasks and their information to your
+ console.
-'v' - Forcefully restores framebuffer console
-'v' - Causes ETM buffer dump [ARM-specific]
+``u`` Will attempt to remount all mounted filesystems read-only.
-'w' - Dumps tasks that are in uninterruptable (blocked) state.
+``v`` Forcefully restores framebuffer console
+``v`` Causes ETM buffer dump [ARM-specific]
-'x' - Used by xmon interface on ppc/powerpc platforms.
- Show global PMU Registers on sparc64.
- Dump all TLB entries on MIPS.
+``w`` Dumps tasks that are in uninterruptable (blocked) state.
-'y' - Show global CPU Registers [SPARC-64 specific]
+``x`` Used by xmon interface on ppc/powerpc platforms.
+ Show global PMU Registers on sparc64.
+ Dump all TLB entries on MIPS.
-'z' - Dump the ftrace buffer
+``y`` Show global CPU Registers [SPARC-64 specific]
-'0'-'9' - Sets the console log level, controlling which kernel messages
- 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.)
+``z`` Dump the ftrace buffer
+
+``0``-``9`` Sets the console log level, controlling which kernel messages
+ 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.)
+=========== ===================================================================
+
+Okay, so what can I use them for?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-* Okay, so what can I use them for?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Well, unraw(r) is very handy when your X server or a svgalib program crashes.
sak(k) (Secure Access Key) is useful when you want to be sure there is no
when you would try to login. It will kill all programs on given console,
thus letting you make sure that the login prompt you see is actually
the one from init, not some trojan program.
-IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
-IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
-IMPORTANT: such. :IMPORTANT
- It seems others find it useful as (System Attention Key) which is
+
+.. important::
+
+ In its true form it is not a true SAK like the one in a
+ c2 compliant system, and it should not be mistaken as
+ such.
+
+It seems others find it useful as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles.
(For example, X or a svgalib program.)
-reboot(b) is good when you're unable to shut down. But you should also
-sync(s) and umount(u) first.
+``reboot(b)`` is good when you're unable to shut down. But you should also
+``sync(s)`` and ``umount(u)`` first.
-crash(c) can be used to manually trigger a crashdump when the system is hung.
+``crash(c)`` can be used to manually trigger a crashdump when the system is hung.
Note that this just triggers a crash if there is no dump mechanism available.
-sync(s) is great when your system is locked up, it allows you to sync your
+``sync(s)`` is great when your system is locked up, it allows you to sync your
disks and will certainly lessen the chance of data loss and fscking. Note
that the sync hasn't taken place until you see the "OK" and "Done" appear
on the screen. (If the kernel is really in strife, you may not ever get the
OK or Done message...)
-umount(u) is basically useful in the same ways as sync(s). I generally sync(s),
-umount(u), then reboot(b) when my system locks. It's saved me many a fsck.
-Again, the unmount (remount read-only) hasn't taken place until you see the
-"OK" and "Done" message appear on the screen.
+``umount(u)`` is basically useful in the same ways as ``sync(s)``. I generally
+``sync(s)``, ``umount(u)``, then ``reboot(b)`` when my system locks. It's saved
+me many a fsck. Again, the unmount (remount read-only) hasn't taken place until
+you see the "OK" and "Done" message appear on the screen.
-The loglevels '0'-'9' are useful when your console is being flooded with
-kernel messages you do not want to see. Selecting '0' will prevent all but
+The loglevels ``0``-``9`` are useful when your console is being flooded with
+kernel messages you do not want to see. Selecting ``0`` will prevent all but
the most urgent kernel messages from reaching your console. (They will
still be logged if syslogd/klogd are alive, though.)
-term(e) and kill(i) are useful if you have some sort of runaway process you
-are unable to kill any other way, especially if it's spawning other
+``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process
+you are unable to kill any other way, especially if it's spawning other
processes.
-"just thaw it(j)" is useful if your system becomes unresponsive due to a frozen
-(probably root) filesystem via the FIFREEZE ioctl.
+"just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
+frozen (probably root) filesystem via the FIFREEZE ioctl.
+
+Sometimes SysRq seems to get 'stuck' after using it, what can I do?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
That happens to me, also. I've found that tapping shift, alt, and control
on both sides of the keyboard, and hitting an invalid sysrq sequence again
-will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
-virtual console (ALT+Fn) and then back again should also help.
+will fix the problem. (i.e., something like :kbd:`alt-sysrq-z`). Switching to
+another virtual console (:kbd:`ALT+Fn`) and then back again should also help.
+
+I hit SysRq, but nothing seems to happen, what's wrong?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-* I hit SysRq, but nothing seems to happen, what's wrong?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are some keyboards that produce a different keycode for SysRq than the
-pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
-don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
-appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
-this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
+pre-defined value of 99 (see ``KEY_SYSRQ`` in ``include/linux/input.h``), or
+which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find
+an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map
+this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's
probably best to put this command in a boot script. Oh, and by the way, you
-exit 'showkey' by not typing anything for ten seconds.
+exit ``showkey`` by not typing anything for ten seconds.
+
+I want to add SysRQ key events to a module, how does it work?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-* I want to add SysRQ key events to a module, how does it work?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to register a basic function with the table, you must first include
-the header 'include/linux/sysrq.h', this will define everything else you need.
-Next, you must create a sysrq_key_op struct, and populate it with A) the key
+the header ``include/linux/sysrq.h``, this will define everything else you need.
+Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key
handler function you will use, B) a help_msg string, that will print when SysRQ
prints help, and C) an action_msg string, that will print right before your
handler is called. Your handler must conform to the prototype in 'sysrq.h'.
-After the sysrq_key_op is created, you can call the kernel function
-register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
-register the operation pointed to by 'op_p' at table key 'key',
+After the ``sysrq_key_op`` is created, you can call the kernel function
+``register_sysrq_key(int key, struct sysrq_key_op *op_p);`` this will
+register the operation pointed to by ``op_p`` at table key 'key',
if that slot in the table is blank. At module unload time, you must call
-the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
+the function ``unregister_sysrq_key(int key, struct sysrq_key_op *op_p)``, which
will remove the key op pointed to by 'op_p' from the key 'key', if and only if
it is currently registered in that slot. This is in case the slot has been
overwritten since you registered it.
The Magic SysRQ system works by registering key operations against a key op
lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has
a number of operations registered into it at compile time, but is mutable,
-and 2 functions are exported for interface to it:
+and 2 functions are exported for interface to it::
+
register_sysrq_key and unregister_sysrq_key.
+
Of course, never ever leave an invalid pointer in the table. I.e., when
your module that called register_sysrq_key() exits, it must call
unregister_sysrq_key() to clean up the sysrq key table entry that it used.
If for some reason you feel the need to call the handle_sysrq function from
within a function called by handle_sysrq, you must be aware that you are in
a lock (you are also in an interrupt handler, which means don't sleep!), so
-you must call __handle_sysrq_nolock instead.
+you must call ``__handle_sysrq_nolock`` instead.
+
+When I hit a SysRq key combination only the header appears on the console?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-* When I hit a SysRq key combination only the header appears on the console?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Sysrq output is subject to the same console loglevel control as all
other console output. This means that if the kernel was booted 'quiet'
as is common on distro kernels the output may not appear on the actual
console, even though it will appear in the dmesg buffer, and be accessible
-via the dmesg command and to the consumers of /proc/kmsg. As a specific
+via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific
exception the header line from the sysrq command is passed to all console
consumers as if the current loglevel was maximum. If only the header
is emitted it is almost certain that the kernel loglevel is too low.
Should you require the output on the console channel then you will need
-to temporarily up the console loglevel using alt-sysrq-8 or:
+to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or::
echo 8 > /proc/sysrq-trigger
Remember to return the loglevel to normal after triggering the sysrq
command you are interested in.
-* I have more questions, who can I ask?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+I have more questions, who can I ask?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
Just ask them on the linux-kernel mailing list:
linux-kernel@vger.kernel.org
-* Credits
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Credits
+~~~~~~~
+
Written by Mydraal <vulpyne@vulpyne.net>
Updated by Adam Sulmicki <adam@cfar.umd.edu>
Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
projects / GitHub / moto-9609 / android_kernel_motorola_exynos9610.git / commitdiff