Strange lockups - 5.4 PRERELEASE / BackupPC/ Samba3

Robert Watson rwatson at
Sat Mar 19 06:19:34 PST 2005

On Thu, 17 Mar 2005, John Pettitt wrote:

> My home server has been freezing at 1AM every night for the last three
> days - this tracks with the upgrade from 5.3 to 5.4 PRE - The only cron
> job running at that time is BackupPC which uses samba3 (smbclient)  to
> backup windows systems. 
> Does anybody know of anything in samba3 or perl 5.8 (BackupPC is a set
> of perl scripts) that might be causing a lockup.  I've got nothing
> unusual in syslog. 
> Right now I'm rebuilding perl and samba under 5.4 in case there is some
> weird library thing between the 5.3 and 5.4 that is causing this - we'll
> see tonight if it works. 

Could you be a bit more specific about the nature of "freeze"?  In
particular, do the following things work:

- Can you create a telnet session to a TCP port on the system?
- Can you ping the system from another system?
- Does the console appear to respond to normal key presses, such as the
  enter key causing a new login prompt?
- Does pressing the caps lock key cause the caps lock light to change

Generally speaking, the first step in debugging system hangs is to compile
the kernel with debugging symbols and the system debugger (KDB+DDB), as
well as the BREAK_TO_DEBUGGER option, and then to try to identify what
kind of hang it is -- are user processes that provide critical services
wedged, or is the kernel wedged?  Does the network stack still operate and
is the scheduler running?  Etc.  Once you have a kernel compiled with the
debugging features turned on, the first thing I'd ask you to do is to see
if, once it's hung, you can break to the debugger using either
Ctrl-Alt-Escape on the console, or a serial break on a serial console.

For somewhat complex reasons, serial consoles can prove to be more
reliable in debugging hangs, as well as more useful when it comes to being
able to copy and paste debugging output on a second computer, so if you're
not currently using a serial console, you might want to switch to one
while debugging this problem.

Robert N M Watson

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