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 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
zrl_init(zrlock_t * zrl)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
zrl_destroy(zrlock_t * zrl)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 #ifdef ZFS_DEBUG
zrl_add_debug(zrlock_t * zrl,const char * zc)73 zrl_add_debug(zrlock_t *zrl, const char *zc)
74 #else
75 zrl_add(zrlock_t *zrl)
76 #endif
77 {
78 uint32_t n = (uint32_t)zrl->zr_refcount;
79
80 while (n != ZRL_LOCKED) {
81 uint32_t cas = atomic_cas_32(
82 (uint32_t *)&zrl->zr_refcount, n, n + 1);
83 if (cas == n) {
84 ASSERT3S((int32_t)n, >=, 0);
85 #ifdef ZFS_DEBUG
86 if (zrl->zr_owner == curthread) {
87 DTRACE_PROBE2(zrlock__reentry,
88 zrlock_t *, zrl, uint32_t, n);
89 }
90 zrl->zr_owner = curthread;
91 zrl->zr_caller = zc;
92 #endif
93 return;
94 }
95 n = cas;
96 }
97
98 mutex_enter(&zrl->zr_mtx);
99 while (zrl->zr_refcount == ZRL_LOCKED) {
100 cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
101 }
102 ASSERT3S(zrl->zr_refcount, >=, 0);
103 zrl->zr_refcount++;
104 #ifdef ZFS_DEBUG
105 zrl->zr_owner = curthread;
106 zrl->zr_caller = zc;
107 #endif
108 mutex_exit(&zrl->zr_mtx);
109 }
110
111 void
zrl_remove(zrlock_t * zrl)112 zrl_remove(zrlock_t *zrl)
113 {
114 uint32_t n;
115
116 #ifdef ZFS_DEBUG
117 if (zrl->zr_owner == curthread) {
118 zrl->zr_owner = NULL;
119 zrl->zr_caller = NULL;
120 }
121 #endif
122 n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
123 ASSERT3S((int32_t)n, >=, 0);
124 }
125
126 int
zrl_tryenter(zrlock_t * zrl)127 zrl_tryenter(zrlock_t *zrl)
128 {
129 uint32_t n = (uint32_t)zrl->zr_refcount;
130
131 if (n == 0) {
132 uint32_t cas = atomic_cas_32(
133 (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
134 if (cas == 0) {
135 #ifdef ZFS_DEBUG
136 ASSERT3P(zrl->zr_owner, ==, NULL);
137 zrl->zr_owner = curthread;
138 #endif
139 return (1);
140 }
141 }
142
143 ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
144
145 return (0);
146 }
147
148 void
zrl_exit(zrlock_t * zrl)149 zrl_exit(zrlock_t *zrl)
150 {
151 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
152
153 mutex_enter(&zrl->zr_mtx);
154 #ifdef ZFS_DEBUG
155 ASSERT3P(zrl->zr_owner, ==, curthread);
156 zrl->zr_owner = NULL;
157 membar_producer(); /* make sure the owner store happens first */
158 #endif
159 zrl->zr_refcount = 0;
160 cv_broadcast(&zrl->zr_cv);
161 mutex_exit(&zrl->zr_mtx);
162 }
163
164 int
zrl_refcount(zrlock_t * zrl)165 zrl_refcount(zrlock_t *zrl)
166 {
167 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
168
169 int n = (int)zrl->zr_refcount;
170 return (n <= 0 ? 0 : n);
171 }
172
173 int
zrl_is_zero(zrlock_t * zrl)174 zrl_is_zero(zrlock_t *zrl)
175 {
176 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
177
178 return (zrl->zr_refcount <= 0);
179 }
180
181 int
zrl_is_locked(zrlock_t * zrl)182 zrl_is_locked(zrlock_t *zrl)
183 {
184 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
185
186 return (zrl->zr_refcount == ZRL_LOCKED);
187 }
188
189 #ifdef ZFS_DEBUG
190 kthread_t *
zrl_owner(zrlock_t * zrl)191 zrl_owner(zrlock_t *zrl)
192 {
193 return (zrl->zr_owner);
194 }
195 #endif
196