Re: Simplify bridge(4)

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

   Date: Wed, 10 Feb 2016 18:56:46 +0900
   From: Ryota Ozaki <ozaki-r%netbsd.org@localhost>

   Thanks to introducing softint-based if_input,
   we can simplify bridge(4).

Awesome!  I love patches that have loads more -'s than +'s, and
simplify locking schemes, and remove sketchy cpu_intr_p conditionals,
and things like that.

   Here is a patch:
   http://www.netbsd.org/~ozaki-r/simplify-bridge.diff

Remove cpu_intr_p from BRIDGE_RT_RENTER/REXIT too?

I wonder how much of a difference BRIDGE_MPSAFE really makes on
uniprocessor systems.  If this were new code I wouldn't have done any
conditional compilation of that.  I can't imagine the performance
impact is very high: maybe a few more words of memory are used, but
uniprocessor mutex acquisition should be pretty cheap.  Maybe in a
future patch we can eliminate all that.

Hmm...  Another note, not related to your patch: queue(3) does not
issue the necessary memory barriers for pserialization.  So the use of
LIST_* for rtlist and iflist is not actually safe here -- I imagine it
has worked only by accident before.

Either we need to make a variant of queue(3) that is pserialize-safe
(https://mail-index.netbsd.org/tech-kern/2014/11/21/msg018055.html) or
open-code it here.  (rmind@ objected to a pserialize-safe queue(3) in
favour of just open-coding it in the few places where it's needed.  I
don't think that's a good idea but I didn't care to take on that fight
at the time.)