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| printk is a very useful function similar to printf(). This function works everywhere and at any time (apart from early stage of booting the kernel when video isn't initialized). It uses log levels to tell the console the importance of each message. Full list of levels: |
printk is a very useful function similar to printf(). This function works everywhere and at any time (apart from early stage of booting the kernel when video isn't initialized). It uses log levels to tell the console the importance of each message. Full list of levels: |
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| From: ''Linux/arch/mips/mm/tlbex.c'' | . printk(l'''oglevel''' '''"mesages"'''); |
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| {{{ 185 if (arg & ~RS_MASK) 186 printk(KERN_WARNING "TLB synthesizer field overflow\n");}}} |
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| 37 if (!(errst0 & PI_ERR_ST0_VALID_MASK)) { 38 printk(); 39 return; 40 } }}} |
20 #if 1 321 printk("FIXME: disabling master aborts\n"); 322 csrs->POx_MSK_HEI.csr &= ~(3UL << 14); 323 #endif }}} |
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| ["http://www.urbanmyth.org/linux/oops/slides.html"] | http://www.urbanmyth.org/linux/oops/slides.html <-- useful link |
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| 1. SysRq+e Send '''SIGTERM''' to all tasks ('''with out init !!!''') | 1. SysRq+e Send SIGTERM''' to all tasks ('''with out init !!!''') ''' |
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| 1. SysRq+i Send '''SIGKILL''' to all tasks ('''with out init !!!''') | 1. SysRq+i Send SIGKILL''' to all tasks ('''with out init !!!''') ''' |
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| 1. SysRq+l Send '''SIGTKILL '''to all tasks ('''with init !!!''') | 1. SysRq+l Send SIGTKILL '''to all tasks ('''with init !!!''') ''' |
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| == How to use non-exist debugger? == Linus doesn't want a debugger in the kernel because it can lead to fixing symptoms instead of actually understanding the code and fixing the real problem. |
== How to use debuggers? == Before i start talking about debuggers you must know one thing. Linus discourages the use of debuggers, because debuggers don't always tell the truth. |
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| However, there are some non-official debuggers (Some require patching the kernel). | * gdb . gdb vmlinux /proc/kcore <--> vmlinux is an uncompresed kernel image (find it in the top level kernel source directory) /proc/kcore allows gdb to see Linux's memory |
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| Note: use''' Additional compiling options '''to make gdb more functional.''' ''' | http://sourceware.org/gdb/current/onlinedocs/gdb_toc.html <-- documentation for gdb |
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| 1. '''gdb (it is not a patch it is GNU debugger) ''' + simple to use |
. + simple to use . - you can not change data in running kernel * kgdb . kgdb is a kernel tool that allows you to connect two computers, one with kgdb and second with gdb . + you can modify data and variables . - you must configure connection. |
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| * gdb vmlinux /proc/kcore call gdb with vmlinux(not compressed image of kernel, in the top level kernel source directory) and optional /proc/kcore - memory of kernel(you must be root) * p varaible - show what variable contain * disassemble name_of_function - disassemble function * etc. . - you can not change variables or other data * '''kgdb (connect 2 computers, one with kdb and second with gdb) ''' . +you can change data +all functions of basic gdb are implemented -you must configure connection |
http://kgdb.wiki.kernel.org/ |
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| 1. '''kdb ''' +only one computer needed''' +'''all functions of gdb implemented +simple to use |
* kdb |
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| * type "break" in console * starts on '''oops''' == When everything fail. == |
== When all else fails. == |
What should I have to fix a bug?
- Bug. Well known particular bug.
- A buggy kernel.
- Bit of luck.
Note:
- Having a repeatable bug is more then 50% of success.
- All examples are from 2.6.17.13 (i386)
Function printk().
printk is a very useful function similar to printf(). This function works everywhere and at any time (apart from early stage of booting the kernel when video isn't initialized). It uses log levels to tell the console the importance of each message. Full list of levels:
KERN_EMERG <-- the most important
- KERN_ALERT
- KERN_CRIT
- KERN_ERR
- KERN_WARNING
- KERN_NOTICE
- KERN_INFO
KERN_DEBUG <-- the least important
The console will print messages only with a level higher than console_loglevel. By default printk uses DEFAULT_MESSAGE_LOGLEVEL == KERN_WARNING (but this may be changed in the future).
printk() uses a cyclic buffer to manage the messages. Next klogd reads the messages (using /proc/kmsg) from the buffer and gives them to syslogd which writes them to /var/log/messages. (You can configure syslogd by editing /etc/syslog.conf).
Examples:
printk(loglevel "mesages");
From: Linux/arch/mips/sgi-ip27/ip27-berr.c
20 #if 1
321 printk("FIXME: disabling master aborts\n");
322 csrs->POx_MSK_HEI.csr &= ~(3UL << 14);
323 #endif
Error oops.
An oops is report of a bug in the kernel. When an oops occurs the kernel will print what the registers contain and a "back trace". An oops does not mean the system has crashed, as the system can sometimes recover from the error. If the system can not recover from the error then the kernel will panic and stop running. By default the back trace will contain the addresses of the functions that were called. If you compile your kernel with CONFIG_KALLSYMS=y the oops will be decoded and will display the function names. In the 2.4 kernel you can use ksymoops file_with_oops.txt to see the names of the functions.
http://www.urbanmyth.org/linux/oops/slides.html <-- useful link
Additional compiling options.
These options are very useful when debugging kernel:
CONFIG_PREEMPT=y
CONFIG_DEBUG_KERNEL=y
CONFIG_KALLSYMS=y
CONFIG_SPINLOCK_SLEEP=y
CONFIG_MAGIC_SYSRQ=y
Causing errors and printing extra informations.
- Sometimes you will want to see oops informations about some bug. Use BUG() BUG_ON():
if(bad_thing)
- BUG();
or BUG_ON(bad_thing);
Examples:
From: Linux/arch/arm/plat-omap/dma.c
732 if (omap_dma_in_1510_mode()) {
733 printk(KERN_ERR "DMA linking is not supported in 1510 mode\n");
734 BUG();
735 return;
736 } 1221 BUG_ON(lcd_dma.active);
- Sometimes you will want to see oops informations and then stop system. Use panic(): if(terrible_error)
- panic("var = %ld \n", var);
- Sometimes you will want to see stack. Use dump_stack():
if(debug_check)
- dump_stack();
Examples:
From: Linux/arch/cris/arch-v32/kernel/dma.c
40 if (options & DMA_PANIC_ON_ERROR)
41 panic("request_dma error!"); From: Linux/drivers/scsi/hosts.c
398 if (!sht->detect) {
399 printk(KERN_WARNING "scsi_register() called on new-style "
400 "template for driver %s\n", sht->name);
401 dump_stack();
402 }
Magic SysRq Key.
If you set CONFIG_MAGIC_SYSRQ=y or typed 'echo 1 > /proc/sys/kernel/sysrq', you can use SysRq Key (on PPC or i386) 'Alt+PrintScreen'.
SysRq+b Restart computer
SysRq+e Send SIGTERM to all tasks (with out init !!!)
SysRq+h Help
SysRq+i Send SIGKILL to all tasks (with out init !!!)
SysRq+k Kill all tasks ran from this console
SysRq+l Send SIGTKILL to all tasks (with init !!!)
SysRq+m Dump core and show it on console
SysRq+o Halt system and shutdown it
SysRq+p Print CPU registers on console
SysRq+r Change keyboard from RAW to XLATE
SysRq+s Save dirty buffers on HDD
SysRq+t Show current task info on console
SysRq+u Unmount all filesystems, and remount read only
Note that every user can use SysRq keys, and it can work improperly on an unstable system.
How to use debuggers?
Before i start talking about debuggers you must know one thing. Linus discourages the use of debuggers, because debuggers don't always tell the truth.
- gdb
gdb vmlinux /proc/kcore <--> vmlinux is an uncompresed kernel image (find it in the top level kernel source directory) /proc/kcore allows gdb to see Linux's memory
http://sourceware.org/gdb/current/onlinedocs/gdb_toc.html <-- documentation for gdb
- + simple to use . - you can not change data in running kernel
- kgdb . kgdb is a kernel tool that allows you to connect two computers, one with kgdb and second with gdb . + you can modify data and variables . - you must configure connection.
- kdb
When all else fails.
No one likes bugs. So when you spend hours/days on bug fixing, you may write a short and descriptive email containing your all of the information you have found, and send it to LKML. Good luck with Bug Hunting.