CRASH(8V)                                                            CRASH(8V)


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 rou‐
       tine which detected the error, or a two-word description of the  incon‐
       sistency.  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 micro‐
              diagnostics.

       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.  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 cur‐
       rent  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)
       VAX  11/780 System Maintenance Guide and VAX Hardware Handbook for more
       information about machine checks.
       Using ADB to Debug the UNIX Kernel


4th Berkeley Distribution        May 20, 1986                        CRASH(8V)
 
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