4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
22 * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
23 * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
27 * A Zero Reference Lock (ZRL) is a reference count that can lock out new
28 * references only when the count is zero and only without waiting if the count
29 * is not already zero. It is similar to a read-write lock in that it allows
30 * multiple readers and only a single writer, but it does not allow a writer to
31 * block while waiting for readers to exit, and therefore the question of
32 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
33 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
34 * is perfectly safe for the same reader to acquire the same lock multiple
35 * times. The fact that a ZRL is reentrant for readers (through multiple calls
36 * to zrl_add()) makes it convenient for determining whether something is
37 * actively referenced without the fuss of flagging lock ownership across
40 #include <sys/zrlock.h>
43 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
44 * treated as zero references.
47 #define ZRL_DESTROYED -2
50 zrl_init(zrlock_t *zrl)
52 mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
54 cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
57 zrl->zr_caller = NULL;
62 zrl_destroy(zrlock_t *zrl)
64 ASSERT0(zrl->zr_refcount);
66 mutex_destroy(&zrl->zr_mtx);
67 zrl->zr_refcount = ZRL_DESTROYED;
68 cv_destroy(&zrl->zr_cv);
72 zrl_add_impl(zrlock_t *zrl, const char *zc)
74 uint32_t n = (uint32_t)zrl->zr_refcount;
76 while (n != ZRL_LOCKED) {
77 uint32_t cas = atomic_cas_32(
78 (uint32_t *)&zrl->zr_refcount, n, n + 1);
80 ASSERT3S((int32_t)n, >=, 0);
82 if (zrl->zr_owner == curthread) {
83 DTRACE_PROBE2(zrlock__reentry,
84 zrlock_t *, zrl, uint32_t, n);
86 zrl->zr_owner = curthread;
94 mutex_enter(&zrl->zr_mtx);
95 while (zrl->zr_refcount == ZRL_LOCKED) {
96 cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
98 ASSERT3S(zrl->zr_refcount, >=, 0);
101 zrl->zr_owner = curthread;
104 mutex_exit(&zrl->zr_mtx);
108 zrl_remove(zrlock_t *zrl)
113 if (zrl->zr_owner == curthread) {
114 zrl->zr_owner = NULL;
115 zrl->zr_caller = NULL;
118 n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
119 ASSERT3S((int32_t)n, >=, 0);
123 zrl_tryenter(zrlock_t *zrl)
125 uint32_t n = (uint32_t)zrl->zr_refcount;
128 uint32_t cas = atomic_cas_32(
129 (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
132 ASSERT3P(zrl->zr_owner, ==, NULL);
133 zrl->zr_owner = curthread;
139 ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
145 zrl_exit(zrlock_t *zrl)
147 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
149 mutex_enter(&zrl->zr_mtx);
151 ASSERT3P(zrl->zr_owner, ==, curthread);
152 zrl->zr_owner = NULL;
153 membar_producer(); /* make sure the owner store happens first */
155 zrl->zr_refcount = 0;
156 cv_broadcast(&zrl->zr_cv);
157 mutex_exit(&zrl->zr_mtx);
161 zrl_refcount(zrlock_t *zrl)
163 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
165 int n = (int)zrl->zr_refcount;
166 return (n <= 0 ? 0 : n);
170 zrl_is_zero(zrlock_t *zrl)
172 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
174 return (zrl->zr_refcount <= 0);
178 zrl_is_locked(zrlock_t *zrl)
180 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
182 return (zrl->zr_refcount == ZRL_LOCKED);
187 zrl_owner(zrlock_t *zrl)
189 return (zrl->zr_owner);