unmount(2) returning EROFS

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I have a backup procedure which takes a ffs snapshot, mounts it on a 
temporary mount point, cd's to there, runs the actual backup, cd's back, 
unmounts the snapshot and releases it.

Now, apperantly when the actual backup is very fast (or if it's the fs the 
temporary mount point is on?), umount(8) fails with "read-only file system", 
which I verified (by ktrace) is indeed unmount(2) returning EROFS.
If I immediately (in the sense of "in the same shell command line") try 
another umount, it succeeds.

If I mount the snapshot read-only (-r), the problem disappears.

Apart from unmount(2) not documenting EROFS, why the hell can unmounting 
fail with that error and why only if the fs is NOT mounted r/o?

This all is on -8 in case something got fixed in the meantime.