panic during 'cvs -R -d /home/dcvs/ checkout src'

[Chris Pressey]

OK, I updated my sources and rebuilt world yesterday, everything was
fine.  Today I didn't do much except for working on documentation and
scripts, and slapping mount.c around a little - I also tried,
unsuccessfully, to build XFree86 - but very harmless stuff I think...
the system was quite stable...

So I just cvsup'ed twenty minutes ago to grab Rob's RCNG changes, that
went fine.  Then I did the next step, the CVS checkout, and my machine
panicked.  Like so:

  catbus# cvs -R -d /home/dcvs/ checkout src
  cvs: WARNING: Read-only repository access mode selected via 'cvs -R'.
  Using this option to access a repository which some users write to
  may cause intermittent sandbox corruption.
  U src/bin/csh/config.h
  U src/bin/csh/host.defs
  M src/bin/ln/ln.1
  U src/contrib/ntp/ntpd/ntp_loopfilter.c
  panic: lockmgr: non-zero exclusive count
  Debugger("panic")
  Stopped at      Debugger+0x34:  movb    $0,in_Debugger.342
  db>

The machine's still sitting at that prompt, and I can leave it there
pretty much indefinately, so if anyone can make suggestions on how to
get useful information out of DDB, please do!  Meanwhile I'm going to
read up on chapter 10 of the FreeBSD Developer's Handbook because it's
been quite a while since I've used a debugger in any seriousness...

-Chris