crash - what happens when the system crashes
This section explains what happens when the system crashes and how you
can 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.
The most common cause of system failures is hardware failure, which can
reflect itself in different ways. Here are the messages which you are
likely to encounter, 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.
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. You should fix your disk if it is broken or unreliable.
timeout table overflow
This really shouldn't be a panic, but until we fix up the data
structure involved, running out of entries causes a crash. If
this happens, you should make the timeout table bigger.
KSP not valid
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 micro-
machine check %x:
machine dependent machine-check information
We should describe machine checks, and will someday. For now,
ask someone who knows (like your friendly field service people).
trap type %d, code=%d, pc=%x
A unexpected trap has occurred within the system; the trap types
0 reserved addressing fault
1 privileged instruction fault
2 reserved operand fault
3 bpt instruction fault
4 xfc instruction fault
5 system call trap
6 arithmetic trap
7 ast delivery trap
8 segmentation fault
9 protection fault
10 trace trap
11 compatibility mode fault
12 page fault
13 page 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 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.
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 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 core dump. Normally the command ``*(intstack-4)$c'' will pro-
vide 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
adb(1), analyze(8), reboot(8)
VAX 11/780 System Maintenance Guide for more information about machine
Using ADB to Debug the UNIX Kernel
4th Berkeley Distribution 1 September 1981 CRASH(8V)