CVS commit: src/sys/kern

[Taylor R Campbell]

Module Name:    src
Committed By:   riastradh
Date:           Fri Feb 24 11:01:44 UTC 2023

Modified Files:
        src/sys/kern: kern_rwlock.c

Log Message:
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


To generate a diff of this commit:
cvs rdiff -u -r1.67 -r1.68 src/sys/kern/kern_rwlock.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.


Modified files:

Index: src/sys/kern/kern_rwlock.c
diff -u src/sys/kern/kern_rwlock.c:1.67 src/sys/kern/kern_rwlock.c:1.68
--- src/sys/kern/kern_rwlock.c:1.67	Fri Jan 27 09:28:41 2023
+++ src/sys/kern/kern_rwlock.c	Fri Feb 24 11:01:43 2023
@@ -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 @@ do { \
 #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 @@ rw_downgrade(krwlock_t *rw)
 	__USE(curthread);
 #endif
 
-	RW_MEMBAR_PRODUCER();
+	RW_MEMBAR_RELEASE();
 
 	for (owner = rw->rw_owner;; owner = next) {
 		/*
@@ -712,7 +710,7 @@ rw_tryupgrade(krwlock_t *rw)
 		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);