Linux Magic System Request Key Hacks
-Documentation for sysrq.c version 1.15
-Last update: $Date: 2001/01/28 10:15:59 $
+Documentation for sysrq.c
* What is the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.
+allowed (by a user with admin privileges).
* How do I use the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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 Alt",
+ have better luck with "press Alt", "press SysRq", "release SysRq",
"press <command key>", release everything.
On SPARC - You press 'ALT-STOP-<command key>', I believe.
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. eg:
+On all - write a character to /proc/sysrq-trigger. e.g.:
echo t > /proc/sysrq-trigger
* What are the 'command' keys?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-'r' - Turns off keyboard raw mode and sets it to XLATE.
+'b' - Will immediately reboot the system without syncing or unmounting
+ your disks.
+
+'c' - Will perform a system crash by a NULL pointer dereference.
+ A crashdump will be taken if configured.
+
+'d' - Shows all locks that are held.
+
+'e' - Send a SIGTERM to all processes, except for init.
+
+'f' - Will call oom_kill to kill a memory hog process.
+
+'g' - Used by kgdb on ppc and sh platforms.
+
+'h' - Will display help (actually any other key than those listed
+ here will display help. but 'h' is easy to remember :-)
+
+'i' - Send a SIGKILL to all processes, except for init.
+
+'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
'k' - Secure Access Key (SAK) Kills all programs on the current virtual
console. NOTE: See important comments below in SAK section.
-'b' - Will immediately reboot the system without syncing or unmounting
- your disks.
+'l' - Shows a stack backtrace for all active CPUs.
-'c' - Will perform a kexec reboot in order to take a crashdump.
+'m' - Will dump current memory info to your console.
+
+'n' - Used to make RT tasks nice-able
'o' - Will shut your system off (if configured and supported).
-'s' - Will attempt to sync all mounted filesystems.
+'p' - Will dump the current registers and flags to your console.
-'u' - Will attempt to remount all mounted filesystems read-only.
+'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.
+
+'s' - Will attempt to sync all mounted filesystems.
't' - Will dump a list of current tasks and their information to your
console.
-'m' - Will dump current memory info to your console.
+'u' - Will attempt to remount all mounted filesystems read-only.
'v' - Dumps Voyager SMP processor info to your console.
+'w' - Dumps tasks that are in uninterruptable (blocked) state.
+
+'x' - Used by xmon interface on ppc/powerpc platforms.
+
+'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.)
-'f' - Will call oom_kill to kill a memory hog process
-
-'e' - Send a SIGTERM to all processes, except for init.
-
-'i' - Send a SIGKILL to all processes, except for init.
-
-'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system
- will be non-functional after this.)
-
-'h' - Will display help ( actually any other key than those listed
- above will display help. but 'h' is easy to remember :-)
-
* Okay, so what can I use them for?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Well, un'R'aw is very handy when your X server or a svgalib program crashes.
-sa'K' (Secure Access Key) is useful when you want to be sure there are no
-trojan program is running at console and which could grab your password
-when you would try to login. It will kill all programs on given console
-and thus letting you make sure that the login prompt you see is actually
+sa'K' (Secure Access Key) is useful when you want to be sure there is no
+trojan program running at console which could grab your password
+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 other find it useful as (System Attention Key) which is
+ 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.)
re'B'oot is good when you're unable to shut down. But you should also 'S'ync
and 'U'mount first.
-'C'rashdump can be used to manually trigger a crashdump when the system is hung.
-The kernel needs to have been built with CONFIG_KEXEC enabled.
+'C'rash 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.
'S'ync 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
Again, the unmount (remount read-only) hasn't taken place until you see the
"OK" and "Done" message appear on the screen.
-The loglevel'0'-'9' is useful when your console is being flooded with
-kernel messages you do not want to see. Setting '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.)
are unable to kill any other way, especially if it's spawning other
processes.
+"'J'ust thaw it" 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?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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. (ie, something like alt-sysrq-z). Switching to another
+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.
* I hit SysRq, but nothing seems to happen, what's wrong?
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 macro
-register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
-sysrq.h, 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 macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
+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
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/char/sysrq.c'. This key table has
a number of operations registered into it at compile time, but is mutable,
-and 4 functions are exported for interface to it: __sysrq_lock_table,
-__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The
-functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined
-in the header itself, and the REGISTER and UNREGISTER macros are built from
-these. More complex (and dangerous!) manipulations of the table are possible
-using these functions, but you must be careful to always lock the table before
-you read or write from it, and to unlock it again when you are done. (And of
-course, to never ever leave an invalid pointer in the table). Null pointers in
-the table are always safe :)
+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.
+Null pointers in the table are always safe. :)
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.
+* 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
+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:
+
+ 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?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
And I'll answer any questions about the registration system you got, also
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff