Re: struct ifnet and ifaddr handling [was: Re: Making global variables of if.c MPSAFE]

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

   Date: Thu, 13 Nov 2014 12:43:26 +0900
   From: Ryota Ozaki <ozaki-r%netbsd.org@localhost>

   Here is a new patch: http://www.netbsd.org/~ozaki-r/psz-ifnet.diff

   I think the patch reflects rmind's suggestions:
   - Use pserialize for IFNET_FOREACH
     - but use a lock for blockable/sleepable critical sections
   - cpu_intr_p workaround for HW interrupt

   Any comments?

Hmm...some quick notes from a non-expert in sys/net:

- You call malloc(M_WAITOK) while the ifnet lock is held, in
  if_alloc_sadl_locked, which is not allowed.

- You call copyout in a pserialize read section, in ifconf, which is
  not allowed because copyout may block.

- I don't know what cpu_intr_p is working around but it's probably not
  a good idea!

Generally, all that you are allowed to do in a pserialize read section
is read a small piece of information, or grab a reference to a data
structure which you are then going to use outside the read section.

I don't think it's going to be easy to scalably parallelize this code
without restructuring it, unless as a stop-gap you use a heaver-weight
reader-writer lock like the prwlock at
<https://www.NetBSD.org/~riastradh/tmp/20140517/rwlock/prwlock.c>.
(No idea how much overhead this might add.)