misc/118160: unable to mount / rw while booting 7.0-BETA3

[Yuri]

Quoting Bruce Evans <brde at optusnet.com.au>:

> Using the 'c' partition is normally an error.  Here it is a very large
> error.  ad12s1b overlaps ad12s1c, so if swap on ad12s1b is actually
> used then it will normally clobber the file system on ad12s1c.  It
> might be possible to make the clobbering harmless, e.g., by putting
> ad12s1b at the end of ad12s1c and not using all of ad12s1c for the
> file system, but this is harder to configure than a normal configuration
> and is supposed to be disallowed.
>
Having root mounted on ad12s1c is probably a result of my typo.

> 
> This misbehaviour seems to be a result of the disallowment not being
> complete.  Mounted file systems normally have exclusive write access
> to their device.  However, for early root mounts, while the mount is
> only r/o, exclusive write access is given up so that fsck can work.
> There seems to be no exclusivity at all, so swapon on a different but
> overlapping device is granted write access.  Then while swapon is on,
> it holds exclusive write access and r/w mounts of root are denied
> (correctly except for the wrong errno and other misleading things in
> the error message).  Also, after the root mount becomes r/w, swapon
> fails (correctly except for the wrong errno).
> 

Should this situation generate system log message, not only EPERM?
If it would have said: 'Attempt to mount already exclusively locked
partition' it would have been clear what the problem is right away.

So I would suggest such message to be added, otherwise this PR can be closed.

Yuri