NAME

crash − what happens when the system crashes

DESCRIPTION

This section explains what happens when the system crashes and (very briefly) how to analyze crash dumps. 

When the system crashes voluntarily it prints a message of the form

panic: why i gave up the ghost

on the console, takes a dump on a mass storage peripheral, and then invokes an automatic reboot procedure as described in reboot(8). (If auto-reboot is disabled on the front panel of the machine the system will simply halt at this point.) Unless some unexpected inconsistency is encountered in the state of the file systems due to hardware or software failure, the system will then resume multi-user operations.

The system has a large number of internal consistency checks; if one of these fails, then it will panic with a very short message indicating which one failed.  In many instances, this will be the name of the routine which detected the error, or a two-word description of the inconsistency.  A full understanding of most panic messages requires perusal of the source code for the system. 

The most common cause of system failures is hardware failure, which can reflect itself in different ways.  Here are the messages which are most likely, with some hints as to causes.  Left unstated in all cases is the possibility that hardware or software error produced the message in some unexpected way. 

iinit This cryptic panic message results from a failure to mount the root filesystem during the bootstrap process.  Either the root filesystem has been corrupted, or the system is attempting to use the wrong device as root filesystem.  Usually, an alternate copy of the system binary or an alternate root filesystem can be used to bring up the system to investigate. 

Can’t exec /etc/init
This is not a panic message, as reboots are likely to be futile. Late in the bootstrap procedure, the system was unable to locate and execute the initialization process, init(8). The root filesystem is incorrect or has been corrupted, or the mode or type of /etc/init forbids execution.

IO err in push

hard IO err in swap
The system encountered an error trying to write to the paging device or an error in reading critical information from a disk drive. The offending disk should be fixed if it is broken or unreliable.

realloccg: bad optim

ialloc: dup alloc

alloccgblk: cyl groups corrupted

ialloccg: map corrupted

free: freeing free block

free: freeing free frag

ifree: freeing free inode

alloccg: map corrupted
These panic messages are among those that may be produced when filesystem inconsistencies are detected. The problem generally results from a failure to repair damaged filesystems after a crash, hardware failures, or other condition that should not normally occur. A filesystem check will normally correct the problem.

timeout table overflow
This really shouldn’t be a panic, but until the data structure involved is made to be extensible, running out of entries causes a crash. If this happens, make the timeout table bigger.

KSP not valid

SBI fault

CHM? in kernel
These indicate either a serious bug in the system or, more often, a glitch or failing hardware. If SBI faults recur, check out the hardware or call field service.  If the other faults recur, there is likely a bug somewhere in the system, although these can be caused by a flakey processor. Run processor microdiagnostics.

machine check %x:
description

   machine dependent machine-check information
Machine checks are different on each type of CPU. Most of the internal processor registers are saved at the time of the fault and are printed on the console. For most processors, there is one line that summarizes the type of machine check. Often, the nature of the problem is apparent from this messaage and/or the contents of key registers.

trap type %d, code=%x, pc=%x
A unexpected trap has occurred within the system; the trap types are:

0reserved addressing fault
1privileged instruction fault
2reserved operand fault
3bpt instruction fault
4xfc instruction fault
5system call trap
6arithmetic trap
7ast delivery trap
8segmentation fault
9protection fault
10trace trap
11compatibility mode fault
12page fault
13page table fault

The favorite trap types in system crashes are trap types 8 and 9, indicating a wild reference.  The code is the referenced address, and the pc at the time of the fault is printed.  These problems tend to be easy to track down if they are kernel bugs since the processor stops cold, but random flakiness seems to cause this sometimes. The debugger can be used to locate the instruction and subroutine corresponding to the PC value. If that is insufficient to suggest the nature of the problem, more detailed examination of the system status at the time of the trap usually can produce an explanation.

init died
The system initialization process has exited.  This is bad news, as no new users will then be able to log in.  Rebooting is the only fix, so the system just does it right away.

out of mbufs: map full
The network has exhausted its private page map for network buffers. This usually indicates that buffers are being lost, and rather than allow the system to slowly degrade, it reboots immediately. The map may be made larger if necessary.

That completes the list of panic types you are likely to see. 

When the system crashes it writes (or at least attempts to write) an image of memory into the back end of the dump device, usually the same as the primary swap area.  After the system is rebooted, the program savecore(8) runs and preserves a copy of this core image and the current system in a specified directory for later perusal.  See savecore(8) for details.

To analyze a dump you should begin by running adb(1) with the −k flag on the system load image and core dump.  If the core image is the result of a panic, the panic message is printed.  Normally the command “$c” will provide a stack trace from the point of the crash and this will provide a clue as to what went wrong.  A more complete discussion of system debugging is impossible here.  See, however, “Using ADB to Debug the UNIX Kernel”. 

SEE ALSO

adb(1), reboot(8)
Using ADB to Debug the UNIX Kernel

4th Berkeley Distribution  —  Revision 1.4 of 29/11/88