crash
—
UNIX system failures
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 alternative copy of the system binary or an
alternative root filesystem can be used to bring up the system to
investigate.
- Can't exec /sbin/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
/sbin/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 message and/or the contents of key registers. The VAX Hardware
Handbook should be consulted, and, if necessary, your friendly field
service people should be informed of the problem.
- trap type %d, code=%x, pc=%x
- A unexpected trap has occurred within the system; the trap types are:
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 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
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. For more detail see “Using ADB to Debug
the UNIX Kernel”.
gdb(1),
reboot(8)
“VAX 11/780 System Maintenance Guide” and “VAX Hardware
Handbook” for more information about machine checks.
“Using ADB to Debug the UNIX Kernel”