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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.
This commit is contained in:
riastradh 2022-02-11 21:40:58 +00:00
parent 6600de342a
commit 63f70f5223
1 changed files with 6 additions and 6 deletions
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12
lib/libpthread/pthread_rwlock.c
View File

@ -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;
}