[src/trunk]: src/common/lib/libc/atomic libc/atomic: Fix membars in __atomic_...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/d7b4fae333f7
branches:  trunk
changeset: 365151:d7b4fae333f7
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Apr 09 23:38:57 2022 +0000

description:
libc/atomic: Fix membars in __atomic_load/store_* stubs.

- membar_enter/exit ordering was backwards.
- membar_enter doesn't make any sense for load anyway.
- Switch to membar_release for store and membar_acquire for load.

The only sensible orderings for a simple load or store are acquire or
release, respectively, or sequential consistency.  This never
provided correct sequential consistency before -- we should really
make it conditional on memmodel but I don't know offhand what the
values of memmodel might be and this is at least better than before.

diffstat:

 common/lib/libc/atomic/atomic_load.c  |  7 +++----
 common/lib/libc/atomic/atomic_store.c |  7 +++----
 2 files changed, 6 insertions(+), 8 deletions(-)

diffs (58 lines):

diff -r 114adf1cc9f0 -r d7b4fae333f7 common/lib/libc/atomic/atomic_load.c
--- a/common/lib/libc/atomic/atomic_load.c      Sat Apr 09 23:38:31 2022 +0000
+++ b/common/lib/libc/atomic/atomic_load.c      Sat Apr 09 23:38:57 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: atomic_load.c,v 1.3 2020/09/07 00:52:19 mrg Exp $      */
+/*     $NetBSD: atomic_load.c,v 1.4 2022/04/09 23:38:57 riastradh Exp $        */
 
 /*-
  * Copyright (c) 2014 The NetBSD Foundation, Inc.
@@ -27,7 +27,7 @@
  */
 
 #include <sys/cdefs.h>
-__RCSID("$NetBSD: atomic_load.c,v 1.3 2020/09/07 00:52:19 mrg Exp $");
+__RCSID("$NetBSD: atomic_load.c,v 1.4 2022/04/09 23:38:57 riastradh Exp $");
 
 #include "atomic_op_namespace.h"
 
@@ -40,9 +40,8 @@
 __atomic_load_ ## n(const volatile void *ptr, int memmodel) \
 { \
        uint## b ##_t val; \
-       membar_enter(); \
        val = *(const volatile uint ## b ## _t *)ptr; \
-       membar_exit(); \
+       membar_acquire(); \
        return val; \
 }
 
diff -r 114adf1cc9f0 -r d7b4fae333f7 common/lib/libc/atomic/atomic_store.c
--- a/common/lib/libc/atomic/atomic_store.c     Sat Apr 09 23:38:31 2022 +0000
+++ b/common/lib/libc/atomic/atomic_store.c     Sat Apr 09 23:38:57 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: atomic_store.c,v 1.3 2020/09/07 00:52:19 mrg Exp $     */
+/*     $NetBSD: atomic_store.c,v 1.4 2022/04/09 23:38:57 riastradh Exp $       */
 
 /*-
  * Copyright (c) 2014 The NetBSD Foundation, Inc.
@@ -27,7 +27,7 @@
  */
 
 #include <sys/cdefs.h>
-__RCSID("$NetBSD: atomic_store.c,v 1.3 2020/09/07 00:52:19 mrg Exp $");
+__RCSID("$NetBSD: atomic_store.c,v 1.4 2022/04/09 23:38:57 riastradh Exp $");
 
 #include "atomic_op_namespace.h"
 
@@ -40,9 +40,8 @@
 __atomic_store_ ## n(volatile void *ptr, uint ## b ## _t val, \
                      int memmodel) \
 { \
-       membar_enter(); \
+       membar_release(); \
        *(volatile uint ## b ## _t *)ptr = val; \
-       membar_exit(); \
 }
 
 atomic_store_n(1, 8)