Module Name: src
Committed By: riastradh
Date: Fri Feb 11 21:40:59 UTC 2022
Modified Files:
src/lib/libpthread: pthread_rwlock.c
Log Message:
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.
To generate a diff of this commit:
cvs rdiff -u -r1.42 -r1.43 src/lib/libpthread/pthread_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/lib/libpthread/pthread_rwlock.c
diff -u src/lib/libpthread/pthread_rwlock.c:1.42 src/lib/libpthread/pthread_rwlock.c:1.43
--- src/lib/libpthread/pthread_rwlock.c:1.42 Tue Jun 2 00:29:53 2020
+++ src/lib/libpthread/pthread_rwlock.c Fri Feb 11 21:40:58 2022
@@ -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
/* 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
/* 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 *
* 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 *
if (thread->pt_sleepobj == NULL)
continue;
new += RW_READ_INCR;
+ membar_exit();
thread->pt_rwlocked = _RW_LOCKED;
}
