Re: MP-safe DAD timer destruction with callout_stop

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

> Date: Fri, 12 Jan 2018 04:33:06 +0000 (UTC)
> From: christos%astron.com@localhost (Christos Zoulas)
> 
> Even then (with callout_halt) nothing prevents something from
> calling callout_schedule() or callout_reset() again between
> that time and callout_destroy()... I have code that adds another
> flag that stops further callouts from being reschuled. Does this
> help you?

I don't understand.  With Ozaki-san's patch, nd6_dad_stop prevents new
references to the callout, by removing dp from the dadq.  The only
remaining references should be in the scheduled callout itself, which
will run at most twice more:

(a) once if it was already scheduled and in the process of firing, and
(b) once more immediately after because of the callout_reset(0).

The dp is then freed by (b).  (Ozaki-san: You should also call
callout_destroy before kmem_intr_free.)

We already have way too many states that callouts can be in.  I don't
see why we need a new one, for this or any application -- this is the
standard pattern of prevent new references, wait for existing ones to
drain, and then free.