.th CRASH VIII 2/12/75
.tr |
.sh NAME
crash \*- what to do when the system crashes
.sh DESCRIPTION
This section gives at least a few clues about how to proceed if the
system crashes.
It can't pretend to be complete.
.s3
.it "How to bring it back up.||"
If the reason for the crash is not evident
(see below for guidance on `evident')
you may want to try to dump the system if you feel up to
debugging.
At the moment a dump can be taken only on magtape.
With a tape mounted and ready,
stop the machine, load address 44, and start.
This should write a copy of all of core
on the tape with an EOF mark.
Caution:
Any error is taken to mean the end of core has been reached.
This means that you must be sure the ring is in,
the tape is ready, and the tape is clean and new.
If the dump fails, you can try again,
but some of the registers will be lost.
See below for what to do with the tape.
.s3
In restarting after a crash,
always bring up the system single-user.
This is accomplished by following the directions in
.it "boot procedures"
(VIII)
as modified for your particular installation;
a single-user system is indicated by having a particular value
in the switches (173030 unless you've changed
.it init)
as the system starts executing.
When it is running,
perform a
.it dcheck
and
.it icheck
(VIII)
on all file systems which could have been in use at the time
of the crash.
If any serious file system problems are found, they should be repaired.
When you are satisfied with the health of your disks,
check and set the date if necessary,
then come up multi-user.
This is most easily accomplished by changing the
single-user value in the switches to something else,
then logging out
by typing an EOT.
.s3
To even boot \s8UNIX\s10 at all,
three files (and the directories leading to them)
must be intact.
First,
the initialization program
.it /etc/init
must be present and executable.
If it is not,
the CPU will loop in user mode at location 6.
For
.it init
to work correctly,
.it /dev/tty8
and
.it /bin/sh
must be present.
If either does not exist,
the symptom is best described
as thrashing.
.it Init
will go into a
.it fork/exec
loop trying to create a
Shell with proper standard input and output.
.s3
If you cannot get the system to boot,
a runnable system must be obtained from
a backup medium.
The root file system may then be doctored as
a mounted file system as described below.
If there are any problems with the root
file system,
it is probably prudent to go to a
backup system to avoid working on a
mounted file system.
.s3
.it "Repairing disks.||"
The first rule to keep in mind is that an addled disk
should be treated gently;
it shouldn't be mounted unless necessary,
and if it is very valuable yet
in quite bad shape, perhaps it should be dumped before
trying surgery on it.
This is an area where experience and informed courage count for much.
.s3
The problems reported by
.it icheck
typically fall into two kinds.
There can be
problems with the free list:
duplicates in the free list, or free blocks also in files.
These can be cured easily with an
.it "icheck \*-s."
If the same block appears in more than one file
or if a file contains bad blocks,
the files should be deleted, and the free list reconstructed.
The best way to delete such a file is to use
.it clri
(VIII),
then remove its directory entries.
If any of the affected files is really precious,
you can try to copy it to another device
first.
.s3
.it Dcheck
may report files which
have more directory entries than links.
Such situations are potentially dangerous;
.it clri
discusses a special case of the problem.
All the directory entries for the file should be removed.
If on the other hand there are more links than directory entries,
there is no danger of spreading infection, but merely some disk space
that is lost for use.
It is sufficient to copy the file (if it has any entries and is useful)
then use
.it clri
on its inode and remove any directory
entries that do exist.
.s3
Finally,
there may be inodes reported by
.it dcheck
that have 0 links and 0 entries.
These occur on the root device when the system is stopped
with pipes open, and on other file systems when the system
stops with files that have been deleted while still open.
A
.it clri
will free the inode, and an
.it "icheck -s"
will
recover any missing blocks.
.s3
.it "Why did it crash?||"
UNIX types a message
on the console typewriter when it voluntarily crashes.
Here is the current list of such messages,
with enough information to provide
a hope at least of the remedy.
The message has the form `panic: ...',
possibly accompanied by other information.
Left unstated in all cases
is the possibility that hardware or software
error produced the message in some unexpected way.
.s3
.lp +5 5
blkdev
.br
The
.it getblk
routine was called with a nonexistent major device as argument.
Definitely hardware or software error.
.s3
.lp +5 5
devtab
.br
Null device table entry for the major device used as argument to
.it getblk.
Definitely hardware or software error.
.s3
.lp +5 5
iinit
.br
An I/O error reading the super-block for the root file system
during initialization.
.s3
.lp +5 5
out of inodes
.br
A mounted file system has no more i-nodes when creating a file.
Sorry, the device isn't available;
the
.it icheck
should tell you.
.s3
.lp +5 5
no fs
.br
A device has disappeared from the mounted-device table.
Definitely hardware or software error.
.s3
.lp +5 5
no imt
.br
Like `no fs', but produced elsewhere.
.s3
.lp +5 5
no inodes
.br
The in-core inode table is full.
Try increasing NINODE in param.h.
Shouldn't be a panic, just a user error.
.s3
.lp +5 5
no clock
.br
During initialization,
neither the line nor programmable clock was found to exist.
.s3
.lp +5 5
swap error
.br
An unrecoverable I/O error during a swap.
Really shouldn't be a panic,
but it is hard to fix.
.s3
.lp +5 5
unlink \- iget
.br
The directory containing a file being deleted can't be found.
Hardware or software.
.s3
.lp +5 5
out of swap space
.br
A program needs to be swapped out, and there is no more swap space.
It has to be increased.
This really shouldn't be a panic, but there is no easy fix.
.s3
.lp +5 5
out of text
.br
A pure procedure program is being executed,
and the table for such things is full.
This shouldn't be a panic.
.s3
.lp +5 5
trap
.br
An unexpected trap has occurred within the system.
This is accompanied by three numbers:
a `ka6', which is the contents of the segmentation
register for the area in which the system's stack is kept;
`aps', which is the location where the hardware stored
the program status word during the trap;
and a `trap type' which encodes
which trap occurred.
The trap types are:
.s3
.lp +10 5
0	bus error
.lp +10 5
1	illegal instruction
.lp +10 5
2	BPT/trace
.lp +10 5
3	IOT
.lp +10 5
4	power fail
.lp +10 5
5	EMT
.lp +10 5
6	recursive system call (TRAP instruction)
.lp +10 5
7	11/70 cache parity, or programmed interrupt
.lp +10 5
10	floating point trap
.lp +10 5
11	segmentation violation
.i0
.s3
In some of these cases it is
possible for octal 20 to be added into the trap type;
this indicates that the processor was in user mode when the trap occurred.
If you wish to examine the stack after such a trap,
either dump the system, or use the console switches to examine core;
the required address mapping is described below.
.s3
.it "Interpreting dumps.||"
All file system problems
should be taken care of before attempting to look at dumps.
The dump should be read into the file
.it /usr/sys/core;
.it cp
(I) will do.
At this point, you should execute
.it "ps \*-alxk"
and
.it who
to print the process table and the users who were on
at the time of the crash.
You should dump (
.it od
(I))
the first 30 bytes of
.it /usr/sys/core.
Starting at location 4,
the registers R0, R1, R2, R3, R4, R5, SP
and KDSA6 (KISA6 for 11/40s) are stored.
If the dump had to be restarted,
R0 will not be correct.
Next, take the value of KA6 (location 22(8) in the dump)
multiplied by 100(8) and dump 1000(8) bytes starting from there.
This is the per-process data associated with the process running
at the time of the crash.
Relabel
the addresses 140000 to 141776.
R5 is C's frame or display pointer.
Stored at (R5) is the old R5 pointing to the previous
stack frame.
At (R5)+2
is the saved PC of the calling procedure.
Trace
this calling chain until
you obtain an R5 value of 141756, which
is where the user's R5 is stored.
If the chain is broken,
you have to look for a plausible
R5, PC pair and continue from there.
Each PC should be looked up in the system's name list
using
.it db
(I) and its `:' command,
to get a reverse calling order.
In most cases this procedure will give
an idea of what is wrong.
A more complete discussion
of system debugging is impossible here.
.sh "SEE ALSO"
clri, icheck, dcheck, boot procedures (VIII)
.sh BUGS