Re: passive references

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

   Date: Tue, 9 Feb 2016 16:07:32 +0000
   From: Taylor R Campbell <riastradh%NetBSD.org@localhost>

   You shouldn't call psref_target_drain twice, unless you have already
   called psref_target_destroy and psref_target_init again.  You should:

   1. Guarantee that no new references can be made.  For example, remove
   the object in question from a list.

   2. Call psref_target_drain (once) to wait for existing references to
   drain.

   3. Call psref_target_destroy to destroy the psref target.

I can't remember now why I separated the drain and destroy operations,
since there's nothing that needs to happen between them.  Maybe there
should just be one operation.