Kernel Panic related to mount/umount

[Bruce Evans]

On Mon, 14 Jan 2008, Ivo Vachkov wrote:

> Last night I discovered by mistake following problem. Commands
>
> mount_cd9660 /dev/acd0 /cdrom
> mount_cd9660 /dev/acd0 /cdrom
> umount /cdrom
> ls -al /cdrom
>
> lead to kernel panic on my laptop. I use FreeBSD-CURRENT, source
> updated for the last time on Dec. 02, 2007. I update it since FreeBSD
> 6.

Unmount frees an active bufobj pointer if the new (in FreeBSD-6?)
feature of multiple read-only mounts is used and some of the mount
instances are removed.  Panics for following the resulting garbage
pointer normally result soon after unmount.  IIRC, it is safe to unmount
in either FIFO or LIFO order but not both.  I forget which is safe.

Bruce