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/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
*/
/*
* A Zero Reference Lock (ZRL) is a reference count that can lock out new
* references only when the count is zero and only without waiting if the count
* is not already zero. It is similar to a read-write lock in that it allows
* multiple readers and only a single writer, but it does not allow a writer to
* block while waiting for readers to exit, and therefore the question of
* reader/writer priority is moot (no WRWANT bit). Since the equivalent of
* rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
* is perfectly safe for the same reader to acquire the same lock multiple
* times. The fact that a ZRL is reentrant for readers (through multiple calls
* to zrl_add()) makes it convenient for determining whether something is
* actively referenced without the fuss of flagging lock ownership across
* function calls.
*/
#include <sys/zrlock.h>
/*
* A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
* treated as zero references.
*/
#define ZRL_LOCKED ((uint32_t)-1)
#define ZRL_DESTROYED -2
void
zrl_init(zrlock_t *zrl)
{
mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
zrl->zr_refcount = 0;
cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
#ifdef ZFS_DEBUG
zrl->zr_owner = NULL;
zrl->zr_caller = NULL;
#endif
}
void
zrl_destroy(zrlock_t *zrl)
{
ASSERT(zrl->zr_refcount == 0);
mutex_destroy(&zrl->zr_mtx);
zrl->zr_refcount = ZRL_DESTROYED;
cv_destroy(&zrl->zr_cv);
}
void
#ifdef ZFS_DEBUG
zrl_add_debug(zrlock_t *zrl, const char *zc)
#else
zrl_add(zrlock_t *zrl)
#endif
{
uint32_t n = (uint32_t)zrl->zr_refcount;
while (n != ZRL_LOCKED) {
uint32_t cas = atomic_cas_32(
(uint32_t *)&zrl->zr_refcount, n, n + 1);
if (cas == n) {
ASSERT((int32_t)n >= 0);
#ifdef ZFS_DEBUG
if (zrl->zr_owner == curthread) {
DTRACE_PROBE2(zrlock__reentry,
zrlock_t *, zrl, uint32_t, n);
}
zrl->zr_owner = curthread;
zrl->zr_caller = zc;
#endif
return;
}
n = cas;
}
mutex_enter(&zrl->zr_mtx);
while (zrl->zr_refcount == ZRL_LOCKED) {
cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
}
ASSERT(zrl->zr_refcount >= 0);
zrl->zr_refcount++;
#ifdef ZFS_DEBUG
zrl->zr_owner = curthread;
zrl->zr_caller = zc;
#endif
mutex_exit(&zrl->zr_mtx);
}
void
zrl_remove(zrlock_t *zrl)
{
uint32_t n;
n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
ASSERT((int32_t)n >= 0);
#ifdef ZFS_DEBUG
if (zrl->zr_owner == curthread) {
zrl->zr_owner = NULL;
zrl->zr_caller = NULL;
}
#endif
}
int
zrl_tryenter(zrlock_t *zrl)
{
uint32_t n = (uint32_t)zrl->zr_refcount;
if (n == 0) {
uint32_t cas = atomic_cas_32(
(uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
if (cas == 0) {
#ifdef ZFS_DEBUG
ASSERT(zrl->zr_owner == NULL);
zrl->zr_owner = curthread;
#endif
return (1);
}
}
ASSERT((int32_t)n > ZRL_DESTROYED);
return (0);
}
void
zrl_exit(zrlock_t *zrl)
{
ASSERT(zrl->zr_refcount == ZRL_LOCKED);
mutex_enter(&zrl->zr_mtx);
#ifdef ZFS_DEBUG
ASSERT(zrl->zr_owner == curthread);
zrl->zr_owner = NULL;
membar_producer(); /* make sure the owner store happens first */
#endif
zrl->zr_refcount = 0;
cv_broadcast(&zrl->zr_cv);
mutex_exit(&zrl->zr_mtx);
}
int
zrl_refcount(zrlock_t *zrl)
{
ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
int n = (int)zrl->zr_refcount;
return (n <= 0 ? 0 : n);
}
int
zrl_is_zero(zrlock_t *zrl)
{
ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
return (zrl->zr_refcount <= 0);
}
int
zrl_is_locked(zrlock_t *zrl)
{
ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
return (zrl->zr_refcount == ZRL_LOCKED);
}
#ifdef ZFS_DEBUG
kthread_t *
zrl_owner(zrlock_t *zrl)
{
return (zrl->zr_owner);
}
#endif
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