8.0-BETA4 panic: ffs_sync: rofs mod

[Jaakko Heinonen]

On 2009-09-09, Mel Flynn wrote:
> This problem has also been seen on -questions [1] and I forgot to follow
> up after time issues. According to the poster it's easy to reproduce:
> - have a mountpoint in /etc/fstab with ro options
> - unmount the mountpoint
> - remount using -o rw

This has been also reported as PR kern/133614. I have tried to reproduce
the problem earlier but I didn't succeed until now. Here's how to
reproduce it:

1. Have mountd(8) running
2. # mdconfig -a -t vnode -f ufsimg
3. # mount -o ro,rw /dev/md0 /mnt
4. # touch /mnt/foo && sync

This is what's going on:

Initial nmount() is done with "ro" and "rw" options. The mount point
will have "ro", "rw" and "noro" string mount options after nmount().
MNT_RDONLY flag is not set.

Then mountd(8) calls nmount() with "update" and "export" string options.
This results FFS code to re-mount the file system as read-only because
the "ro" string option is active for the mount point. ffs_mount() sets
MNT_RDONLY and FFS fs_ronly flags. MNT_RDONLY gets later cleared in
vfs_domount() because vfs_export() fails with ENOENT but the fs_ronly
flag stays enabled.

The most obvious problem seems to be that both "ro" and "rw" options are
allowed to enter to mount point options concurrently.

I don't have yet a fix to propose.

-- 
Jaakko