Eliminating __HAVE_ATOMIC_AS_MEMBAR

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

The attached patch series largely eliminates condiitonals on
__HAVE_ATOMIC_AS_MEMBAR by introducing wrappers
membar_release_preatomic and membar_acquire_postatomic with the
obvious definitions.

I'm on the fence about it because I'm tempted to say that maybe
membar_acquire and membar_release should be defined conditionally in
the first place, and we shouldn't introduce new names:

- If you want a release before a store, or an acquire after a load,
  you should use atomic_store_release or atomic_load_acquire.
  membar_release and membar_acquire are already supposed to be used
  only around atomic r/m/w operations as is.  It's annoying to have a
  proliferation of very-similar membars, a situation I was trying to
  avoid when I deprecated membar_exit/enter.

- On the other hand, it's possible I made a mistake while auditing
  uses of the now-deprecated membar_exit/enter and replacing them by
  membar_release/acquire, and if we redefined membar_release/acquire
  to be weaker, it might invite hard-to-find bugs.

Opinions welcome!


P.S.  There are also still a few uses of membar_enter in critical
locking code that I don't understand and aren't clearly documented,
and a couple uses of membar_producer in subr_pcq.c that seem wrong.
If it weren't such critical central code I would just rip theese out
for being obviously bogus, but someone really needs to go through and
identify -- and very clearly document -- what purpose these serve.

From 53d4e54d5c3926a7de096f1e04686dfd8aabaefe Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
Date: Sun, 22 Jan 2023 18:45:19 +0000
Subject: [PATCH] npf(9): Another comment tweak to match upstream.

No functional change.
---
 sys/net/npf/npf_tableset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sys/net/npf/npf_tableset.c b/sys/net/npf/npf_tableset.c
index 8f2d36c90341..c09a0919009b 100644
--- a/sys/net/npf/npf_tableset.c
+++ b/sys/net/npf/npf_tableset.c
@@ -101,7 +101,7 @@ struct npf_table {
 
 	/*
 	 * Table ID, type and lock.  The ID may change during the
-	 * config reload, it is protected by the npf->config_lock.
+	 * config reload, it is protected by the npf_t::config_lock.
 	 */
 	int			t_type;
 	unsigned		t_id;