Subject: Re: nullfs and panic: locking against myself
To: Jukka Marin <>
From: Todd Vierling <>
List: current-users
Date: 01/04/1998 12:49:46
On Sun, 4 Jan 1998, Jukka Marin wrote:

: > > Personally, I like to use unionfs for NetBSD source tree.. but I'm afraid
: > > I'll run into problems with it too.
: > 
: >   I dunno...I use unionfs for the source tree...I don't have any
: > problems...
: I have used it without problems, too.. I'm just afraid I'll run out of
: luck soon ;-)

I've had problems, but it consists mainly of operations such as the

- accessing the same file for read in both the unionfs and either "real"
  filesystem layer
- removing a directory in a "real" filesystem layer while in use by the
- using nullfs and a union mount, changing a file in both "real" filesystem
  layers shortly after reading that file in the unionfs

Note that all of them center around accessing the unionfs and the real
filesystem layers at the same time.  I've found that union mounting the
source tree works flawlessly, as long as I remember to umount the unionfs
before running a sup update (so the unionfs can't get confused by the source
tree changing).  I think that nullfs has some same problems about changing
the "real" filesystem and then accessing the nullfs.

It seems to me that the unionfs might do a better job if it had a stateless
file access mechanism.  Hrm, who knows.

===== Todd Vierling (Personal; Business =====
== "There's a myth that there is a scarcity of justice to go around, so
== that if we extend justice to 'those people,' it will somehow erode the
== quality of justice everyone else receives."  -- Maria Price