How is supposed to be protected the units list?

[Matthew Jacob]

Sorry for being quiet about this. I'm going to look at this again today.

It turns out where I work there is tremendous amount of pain about this 
also.

> So I made this new patch using the bus lock:
> http://www.freebsd.org/~attilio/Sandvine/pdrv/xpt_lock.diff
>
> I would have preferred to have a dedicated lock for the units lists,
> but as long as you seem having strong opionion, I'm fine with it.
> Maybe Matt wants to add his refcounting modifies using this scheme if
> we came to a consensous?
>
> Thanks,
> Attilio
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