Re: raid-on-raid during shutdown

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I wrote

> I tried making raidframe register a shutdown hook which iterates over
> all extant units, attempting to tear down any which have openmask==0.
> This cures it for me.  I actually prefer this to the "tear down on
> last close during shutdown" code, though I haven't tried eliminating
> the latter and letting the shutdown hook do all the teardowns.

On reflection, each is necessary.  Cascading teardowns during shutdown
is necessary to make stacking of dissimilar subsystems work (eg,
raid-on-cgd-on-raid, which would otherwise recall the raid shutdown
hook to be run both before and after cgd's - well, assuming cgd bought
into the same paradigm; I haven't looked at whether it does).  And the
shutdown hook - or something else like it, like dyoung's iterated
detaching - is necessary to deal with cases of something not being open
(and thus not being closed) at shutdown.

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