[src/trunk]: src/sys/kern rwlock(9): Fix membars.

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

details:   https://anonhg.NetBSD.org/src/rev/b249a07fcd6e
branches:  trunk
changeset: 373677:b249a07fcd6e
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Fri Feb 24 11:01:43 2023 +0000

description:
rwlock(9): Fix membars.

rw_downgrade must be a release operation, and rw_tryupgrade must be
an acquire operation.  membar_producer is not enough -- need to use
membar_release and membar_acquire.

Discussed on tech-kern:
https://mail-index.netbsd.org/tech-kern/2023/02/22/msg028726.html

XXX pullup-8
XXX pullup-9
XXX pullup-10

diffstat:

 sys/kern/kern_rwlock.c |  10 ++++------
 1 files changed, 4 insertions(+), 6 deletions(-)

diffs (48 lines):

diff -r 893a09c791ac -r b249a07fcd6e sys/kern/kern_rwlock.c
--- a/sys/kern/kern_rwlock.c    Thu Feb 23 14:57:29 2023 +0000
+++ b/sys/kern/kern_rwlock.c    Fri Feb 24 11:01:43 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_rwlock.c,v 1.67 2023/01/27 09:28:41 ozaki-r Exp $ */
+/*     $NetBSD: kern_rwlock.c,v 1.68 2023/02/24 11:01:43 riastradh Exp $       */
 
 /*-
  * Copyright (c) 2002, 2006, 2007, 2008, 2009, 2019, 2020
@@ -45,7 +45,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_rwlock.c,v 1.67 2023/01/27 09:28:41 ozaki-r Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_rwlock.c,v 1.68 2023/02/24 11:01:43 riastradh Exp $");
 
 #include "opt_lockdebug.h"
 
@@ -103,11 +103,9 @@
 #ifdef __HAVE_ATOMIC_AS_MEMBAR
 #define        RW_MEMBAR_ACQUIRE()
 #define        RW_MEMBAR_RELEASE()
-#define        RW_MEMBAR_PRODUCER()
 #else
 #define        RW_MEMBAR_ACQUIRE()             membar_acquire()
 #define        RW_MEMBAR_RELEASE()             membar_release()
-#define        RW_MEMBAR_PRODUCER()            membar_producer()
 #endif
 
 /*
@@ -613,7 +611,7 @@
        __USE(curthread);
 #endif
 
-       RW_MEMBAR_PRODUCER();
+       RW_MEMBAR_RELEASE();
 
        for (owner = rw->rw_owner;; owner = next) {
                /*
@@ -712,7 +710,7 @@
                newown = curthread | RW_WRITE_LOCKED | (owner & ~RW_THREAD);
                next = rw_cas(rw, owner, newown);
                if (__predict_true(next == owner)) {
-                       RW_MEMBAR_PRODUCER();
+                       RW_MEMBAR_ACQUIRE();
                        break;
                }
                RW_ASSERT(rw, (next & RW_WRITE_LOCKED) == 0);