RFC: root mount enhancement (round 2)

[M. Warner Losh]

In message: <54210.1282752073 at critter.freebsd.dk>
            "Poul-Henning Kamp" <phk at phk.freebsd.dk> writes:
: In message <34EF2360-1B68-4E0C-8CCE-409CE141D0B8 at mac.com>, Marcel Moolenaar wri
: tes:
: 
: >don't want to enhance: A USB disk cannot always be used as a root
: >file system by virtue of the USB stack releasing the root mount
: >lock after creating the umass device, but before CAM has created
: >the corresponding da device. 
: 
: This is a bug which is entirely unrelated to how we find the
: root filesystem:  It should simply be fixed by CAM grabing a
: root mount lock when activated from USB and releasing it
: only when all it's stuff is done.

We already do this...  But it is insufficient since usb discovery is
done asynchronously...

Scott has a similar fix in the pipeline, but I don't know the state of
it.

Warner