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21106b0114
Reported-by: Křištof Želechovski <giecrilj@stegny.2a.pl> Signed-off-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
253 lines
12 KiB
Plaintext
253 lines
12 KiB
Plaintext
Linux Magic System Request Key Hacks
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Documentation for sysrq.c
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* What is the magic SysRq key?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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It is a 'magical' key combo you can hit which the kernel will respond to
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regardless of whatever else it is doing, unless it is completely locked up.
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* How do I enable the magic SysRq key?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
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configuring the kernel. When running a kernel with SysRq compiled in,
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/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
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the SysRq key. By default the file contains 1 which means that every
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possible SysRq request is allowed (in older versions SysRq was disabled
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by default, and you were required to specifically enable it at run-time
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but this is not the case any more). Here is the list of possible values
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in /proc/sys/kernel/sysrq:
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0 - disable sysrq completely
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1 - enable all functions of sysrq
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>1 - bitmask of allowed sysrq functions (see below for detailed function
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description):
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2 - enable control of console logging level
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4 - enable control of keyboard (SAK, unraw)
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8 - enable debugging dumps of processes etc.
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16 - enable sync command
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32 - enable remount read-only
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64 - enable signalling of processes (term, kill, oom-kill)
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128 - allow reboot/poweroff
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256 - allow nicing of all RT tasks
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You can set the value in the file by the following command:
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echo "number" >/proc/sys/kernel/sysrq
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Note that the value of /proc/sys/kernel/sysrq influences only the invocation
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via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
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allowed (by a user with admin privileges).
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* How do I use the magic SysRq key?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
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keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
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also known as the 'Print Screen' key. Also some keyboards cannot
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handle so many keys being pressed at the same time, so you might
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have better luck with "press Alt", "press SysRq", "release SysRq",
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"press <command key>", release everything.
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On SPARC - You press 'ALT-STOP-<command key>', I believe.
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On the serial console (PC style standard serial ports only) -
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You send a BREAK, then within 5 seconds a command key. Sending
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BREAK twice is interpreted as a normal BREAK.
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On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
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Print Screen (or F13) - <command key> may suffice.
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On other - If you know of the key combos for other architectures, please
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let me know so I can add them to this section.
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On all - write a character to /proc/sysrq-trigger. e.g.:
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echo t > /proc/sysrq-trigger
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* What are the 'command' keys?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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'b' - Will immediately reboot the system without syncing or unmounting
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your disks.
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'c' - Will perform a system crash by a NULL pointer dereference.
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A crashdump will be taken if configured.
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'd' - Shows all locks that are held.
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'e' - Send a SIGTERM to all processes, except for init.
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'f' - Will call oom_kill to kill a memory hog process.
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'g' - Used by kgdb (kernel debugger)
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'h' - Will display help (actually any other key than those listed
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here will display help. but 'h' is easy to remember :-)
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'i' - Send a SIGKILL to all processes, except for init.
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'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
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'k' - Secure Access Key (SAK) Kills all programs on the current virtual
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console. NOTE: See important comments below in SAK section.
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'l' - Shows a stack backtrace for all active CPUs.
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'm' - Will dump current memory info to your console.
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'n' - Used to make RT tasks nice-able
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'o' - Will shut your system off (if configured and supported).
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'p' - Will dump the current registers and flags to your console.
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'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
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timer_list timers) and detailed information about all
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clockevent devices.
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'r' - Turns off keyboard raw mode and sets it to XLATE.
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's' - Will attempt to sync all mounted filesystems.
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't' - Will dump a list of current tasks and their information to your
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console.
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'u' - Will attempt to remount all mounted filesystems read-only.
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'v' - Forcefully restores framebuffer console
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'v' - Causes ETM buffer dump [ARM-specific]
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'w' - Dumps tasks that are in uninterruptable (blocked) state.
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'x' - Used by xmon interface on ppc/powerpc platforms.
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'y' - Show global CPU Registers [SPARC-64 specific]
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'z' - Dump the ftrace buffer
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'0'-'9' - Sets the console log level, controlling which kernel messages
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will be printed to your console. ('0', for example would make
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it so that only emergency messages like PANICs or OOPSes would
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make it to your console.)
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* Okay, so what can I use them for?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Well, un'R'aw is very handy when your X server or a svgalib program crashes.
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sa'K' (Secure Access Key) is useful when you want to be sure there is no
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trojan program running at console which could grab your password
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when you would try to login. It will kill all programs on given console,
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thus letting you make sure that the login prompt you see is actually
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the one from init, not some trojan program.
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IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
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IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
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IMPORTANT: such. :IMPORTANT
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It seems others find it useful as (System Attention Key) which is
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useful when you want to exit a program that will not let you switch consoles.
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(For example, X or a svgalib program.)
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re'B'oot is good when you're unable to shut down. But you should also 'S'ync
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and 'U'mount first.
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'C'rash can be used to manually trigger a crashdump when the system is hung.
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Note that this just triggers a crash if there is no dump mechanism available.
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'S'ync is great when your system is locked up, it allows you to sync your
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disks and will certainly lessen the chance of data loss and fscking. Note
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that the sync hasn't taken place until you see the "OK" and "Done" appear
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on the screen. (If the kernel is really in strife, you may not ever get the
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OK or Done message...)
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'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
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'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
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Again, the unmount (remount read-only) hasn't taken place until you see the
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"OK" and "Done" message appear on the screen.
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The loglevels '0'-'9' are useful when your console is being flooded with
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kernel messages you do not want to see. Selecting '0' will prevent all but
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the most urgent kernel messages from reaching your console. (They will
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still be logged if syslogd/klogd are alive, though.)
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t'E'rm and k'I'll are useful if you have some sort of runaway process you
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are unable to kill any other way, especially if it's spawning other
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processes.
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"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
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(probably root) filesystem via the FIFREEZE ioctl.
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* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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That happens to me, also. I've found that tapping shift, alt, and control
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on both sides of the keyboard, and hitting an invalid sysrq sequence again
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will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
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virtual console (ALT+Fn) and then back again should also help.
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* I hit SysRq, but nothing seems to happen, what's wrong?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are some keyboards that produce a different keycode for SysRq than the
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pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
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don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
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appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
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this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
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probably best to put this command in a boot script. Oh, and by the way, you
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exit 'showkey' by not typing anything for ten seconds.
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* I want to add SysRQ key events to a module, how does it work?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In order to register a basic function with the table, you must first include
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the header 'include/linux/sysrq.h', this will define everything else you need.
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Next, you must create a sysrq_key_op struct, and populate it with A) the key
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handler function you will use, B) a help_msg string, that will print when SysRQ
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prints help, and C) an action_msg string, that will print right before your
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handler is called. Your handler must conform to the prototype in 'sysrq.h'.
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After the sysrq_key_op is created, you can call the kernel function
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register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
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register the operation pointed to by 'op_p' at table key 'key',
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if that slot in the table is blank. At module unload time, you must call
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the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
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will remove the key op pointed to by 'op_p' from the key 'key', if and only if
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it is currently registered in that slot. This is in case the slot has been
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overwritten since you registered it.
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The Magic SysRQ system works by registering key operations against a key op
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lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
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a number of operations registered into it at compile time, but is mutable,
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and 2 functions are exported for interface to it:
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register_sysrq_key and unregister_sysrq_key.
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Of course, never ever leave an invalid pointer in the table. I.e., when
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your module that called register_sysrq_key() exits, it must call
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unregister_sysrq_key() to clean up the sysrq key table entry that it used.
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Null pointers in the table are always safe. :)
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If for some reason you feel the need to call the handle_sysrq function from
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within a function called by handle_sysrq, you must be aware that you are in
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a lock (you are also in an interrupt handler, which means don't sleep!), so
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you must call __handle_sysrq_nolock instead.
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* When I hit a SysRq key combination only the header appears on the console?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Sysrq output is subject to the same console loglevel control as all
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other console output. This means that if the kernel was booted 'quiet'
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as is common on distro kernels the output may not appear on the actual
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console, even though it will appear in the dmesg buffer, and be accessible
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via the dmesg command and to the consumers of /proc/kmsg. As a specific
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exception the header line from the sysrq command is passed to all console
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consumers as if the current loglevel was maximum. If only the header
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is emitted it is almost certain that the kernel loglevel is too low.
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Should you require the output on the console channel then you will need
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to temporarily up the console loglevel using alt-sysrq-8 or:
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echo 8 > /proc/sysrq-trigger
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Remember to return the loglevel to normal after triggering the sysrq
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command you are interested in.
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* I have more questions, who can I ask?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Just ask them on the linux-kernel mailing list:
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linux-kernel@vger.kernel.org
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* Credits
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Written by Mydraal <vulpyne@vulpyne.net>
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Updated by Adam Sulmicki <adam@cfar.umd.edu>
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Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
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Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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