1 /*
2 * CDDL HEADER START
3 *
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.
7 *
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.
12 *
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]
18 *
19 * CDDL HEADER END
20 */
21 /*
22 * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
23 * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
24 */
25
26 /*
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
38 * function calls.
39 */
40 #include <sys/zrlock.h>
41
42 /*
43 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
44 * treated as zero references.
45 */
46 #define ZRL_LOCKED -1
47 #define ZRL_DESTROYED -2
48
49 void
50 zrl_init(zrlock_t *zrl)
51 {
52 mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
53 zrl->zr_refcount = 0;
54 cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
55 #ifdef ZFS_DEBUG
56 zrl->zr_owner = NULL;
57 zrl->zr_caller = NULL;
58 #endif
59 }
60
61 void
62 zrl_destroy(zrlock_t *zrl)
63 {
64 ASSERT0(zrl->zr_refcount);
65
66 mutex_destroy(&zrl->zr_mtx);
67 zrl->zr_refcount = ZRL_DESTROYED;
68 cv_destroy(&zrl->zr_cv);
69 }
70
71 void
72 zrl_add_impl(zrlock_t *zrl, const char *zc)
73 {
74 uint32_t n = (uint32_t)zrl->zr_refcount;
75
76 while (n != ZRL_LOCKED) {
77 uint32_t cas = atomic_cas_32(
78 (uint32_t *)&zrl->zr_refcount, n, n + 1);
79 if (cas == n) {
80 ASSERT3S((int32_t)n, >=, 0);
81 #ifdef ZFS_DEBUG
82 if (zrl->zr_owner == curthread) {
83 DTRACE_PROBE2(zrlock__reentry,
84 zrlock_t *, zrl, uint32_t, n);
85 }
86 zrl->zr_owner = curthread;
87 zrl->zr_caller = zc;
88 #endif
89 return;
90 }
91 n = cas;
92 }
93
94 mutex_enter(&zrl->zr_mtx);
95 while (zrl->zr_refcount == ZRL_LOCKED) {
96 cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
97 }
98 ASSERT3S(zrl->zr_refcount, >=, 0);
99 zrl->zr_refcount++;
100 #ifdef ZFS_DEBUG
101 zrl->zr_owner = curthread;
102 zrl->zr_caller = zc;
103 #endif
104 mutex_exit(&zrl->zr_mtx);
105 }
106
107 void
108 zrl_remove(zrlock_t *zrl)
109 {
110 uint32_t n;
111
112 #ifdef ZFS_DEBUG
113 if (zrl->zr_owner == curthread) {
114 zrl->zr_owner = NULL;
115 zrl->zr_caller = NULL;
116 }
117 #endif
118 n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
119 ASSERT3S((int32_t)n, >=, 0);
120 }
121
122 int
123 zrl_tryenter(zrlock_t *zrl)
124 {
125 uint32_t n = (uint32_t)zrl->zr_refcount;
126
127 if (n == 0) {
128 uint32_t cas = atomic_cas_32(
129 (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
130 if (cas == 0) {
131 #ifdef ZFS_DEBUG
132 ASSERT3P(zrl->zr_owner, ==, NULL);
133 zrl->zr_owner = curthread;
134 #endif
135 return (1);
136 }
137 }
138
139 ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
140
141 return (0);
142 }
143
144 void
145 zrl_exit(zrlock_t *zrl)
146 {
147 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
148
149 mutex_enter(&zrl->zr_mtx);
150 #ifdef ZFS_DEBUG
151 ASSERT3P(zrl->zr_owner, ==, curthread);
152 zrl->zr_owner = NULL;
153 membar_producer(); /* make sure the owner store happens first */
154 #endif
155 zrl->zr_refcount = 0;
156 cv_broadcast(&zrl->zr_cv);
157 mutex_exit(&zrl->zr_mtx);
158 }
159
160 int
161 zrl_refcount(zrlock_t *zrl)
162 {
163 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
164
165 int n = (int)zrl->zr_refcount;
166 return (n <= 0 ? 0 : n);
167 }
168
169 int
170 zrl_is_zero(zrlock_t *zrl)
171 {
172 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
173
174 return (zrl->zr_refcount <= 0);
175 }
176
177 int
178 zrl_is_locked(zrlock_t *zrl)
179 {
180 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
181
182 return (zrl->zr_refcount == ZRL_LOCKED);
183 }
184
185 #ifdef ZFS_DEBUG
186 kthread_t *
187 zrl_owner(zrlock_t *zrl)
188 {
189 return (zrl->zr_owner);
190 }
191 #endif