[Unionfs] Kernel panic while reordering branches

[Dave Miller]

We've been seeing a kernel panic under high traffic conditions when
trying to reorder/mode-change the branches on a unionfs mount where the
base branch is NFS.  The error message in the panic seems to indicate
that it's attempting to unmount the NFS partition, but that doesn't make
a lot of sense.

Information about the issue including kernel panic logs and the versions
of kernels and unionfs we've tried is available at
https://bugzilla.mozilla.org/show_bug.cgi?id=402754

Some basic information about what we're trying to pull off and how it's
set up is at
http://www.justdave.net/dave/2007/10/22/stagemo-shift-reloaded/ with the
main difference from that description being that the chroot environment
directories are now bind-mounted on top instead of as a branch within
the unionfs mount, because of issues I'm assuming having to do with the
/dev and /proc directories being buried inside a unionfs layer (I
imagine copyup doesn't play well with /dev nodes :) )

I have a test case set up where I can reliably reproduce this after a
few hours of copying data in and out into the staging area, so if anyone
has patches to try I can find out in a day or so if they help. :)

Thanks in advance for any help!

-- 
Dave Miller                                   http://www.justdave.net/
System Administrator, Mozilla Corporation      http://www.mozilla.com/
Project Leader, Bugzilla Bug Tracking System  http://www.bugzilla.org/