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authorriastradh <riastradh@NetBSD.org>2022-02-11 21:40:58 +0000
committerriastradh <riastradh@NetBSD.org>2022-02-11 21:40:58 +0000
commitcb5cc3d8ac194632cef3250fd4806d0f7c2e1406 (patch)
tree26fcb95b2a765fe708cf6684d9907bf510e5f053 /lib/libpthread
parentee74c09aab4aef92718eefe7060b28135fb26dec (diff)
libpthread: Fix membars around rwlocks.
1. After loading self->pt_rwlocked, membar_enter() must not be conditional on PTHREAD__ATOMIC_IS_MEMBAR because there is no atomic r/m/w operation here which could imply the acquire barrier. (This should maybe just be a load-acquire operation, but we don't have atomic_load_acquire in userland at the moment -- TBD.) 2. Before storing thread->pt_rwlocked, must issue membar_exit() so that this is a store-release operation -- except if we had just done an atomic r/m/w and PTHREAD__ATOMIC_IS_MEMBAR is set, in which case it can be elided. The second membar_exit() added here might be safely hoisted out of the loop but I'm not sure -- needs more analysis to prove that would be safe.
Diffstat (limited to 'lib/libpthread')
-rw-r--r--lib/libpthread/pthread_rwlock.c12
1 files changed, 6 insertions, 6 deletions
diff --git a/lib/libpthread/pthread_rwlock.c b/lib/libpthread/pthread_rwlock.c
index 79f8d28ae3f..8d452d3dbc6 100644
--- a/lib/libpthread/pthread_rwlock.c
+++ b/lib/libpthread/pthread_rwlock.c
@@ -1,4 +1,4 @@
-/* $NetBSD: pthread_rwlock.c,v 1.42 2020/06/02 00:29:53 joerg Exp $ */
+/* $NetBSD: pthread_rwlock.c,v 1.43 2022/02/11 21:40:58 riastradh Exp $ */
/*-
* Copyright (c) 2002, 2006, 2007, 2008, 2020 The NetBSD Foundation, Inc.
@@ -30,7 +30,7 @@
*/
#include <sys/cdefs.h>
-__RCSID("$NetBSD: pthread_rwlock.c,v 1.42 2020/06/02 00:29:53 joerg Exp $");
+__RCSID("$NetBSD: pthread_rwlock.c,v 1.43 2022/02/11 21:40:58 riastradh Exp $");
#include <sys/types.h>
#include <sys/lwpctl.h>
@@ -228,9 +228,7 @@ pthread__rwlock_rdlock(pthread_rwlock_t *ptr, const struct timespec *ts)
/* Did we get the lock? */
if (self->pt_rwlocked == _RW_LOCKED) {
-#ifndef PTHREAD__ATOMIC_IS_MEMBAR
membar_enter();
-#endif
return 0;
}
if (error != 0)
@@ -352,9 +350,7 @@ pthread__rwlock_wrlock(pthread_rwlock_t *ptr, const struct timespec *ts)
/* Did we get the lock? */
if (self->pt_rwlocked == _RW_LOCKED) {
-#ifndef PTHREAD__ATOMIC_IS_MEMBAR
membar_enter();
-#endif
return 0;
}
if (error != 0)
@@ -526,6 +522,9 @@ pthread_rwlock_unlock(pthread_rwlock_t *ptr)
* by the writer that we are about to wake.
*/
(void)atomic_swap_ptr(&ptr->ptr_owner, (void *)new);
+#ifndef PTHREAD__ATOMIC_IS_MEMBAR
+ membar_exit();
+#endif
/* Wake the writer. */
thread->pt_rwlocked = _RW_LOCKED;
@@ -543,6 +542,7 @@ pthread_rwlock_unlock(pthread_rwlock_t *ptr)
if (thread->pt_sleepobj == NULL)
continue;
new += RW_READ_INCR;
+ membar_exit();
thread->pt_rwlocked = _RW_LOCKED;
}