xref: /titanic_51/usr/src/uts/common/fs/zfs/zrlock.c (revision c10c16dec587a0662068f6e2991c29ed3a9db943)
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  */
24 
25 /*
26  * A Zero Reference Lock (ZRL) is a reference count that can lock out new
27  * references only when the count is zero and only without waiting if the count
28  * is not already zero. It is similar to a read-write lock in that it allows
29  * multiple readers and only a single writer, but it does not allow a writer to
30  * block while waiting for readers to exit, and therefore the question of
31  * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
32  * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
33  * is perfectly safe for the same reader to acquire the same lock multiple
34  * times. The fact that a ZRL is reentrant for readers (through multiple calls
35  * to zrl_add()) makes it convenient for determining whether something is
36  * actively referenced without the fuss of flagging lock ownership across
37  * function calls.
38  */
39 #include <sys/zrlock.h>
40 
41 /*
42  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
43  * treated as zero references.
44  */
45 #define	ZRL_LOCKED	((uint32_t)-1)
46 #define	ZRL_DESTROYED	-2
47 
48 void
49 zrl_init(zrlock_t *zrl)
50 {
51 	mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
52 	zrl->zr_refcount = 0;
53 	cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
54 #ifdef	ZFS_DEBUG
55 	zrl->zr_owner = NULL;
56 	zrl->zr_caller = NULL;
57 #endif
58 }
59 
60 void
61 zrl_destroy(zrlock_t *zrl)
62 {
63 	ASSERT(zrl->zr_refcount == 0);
64 
65 	mutex_destroy(&zrl->zr_mtx);
66 	zrl->zr_refcount = ZRL_DESTROYED;
67 	cv_destroy(&zrl->zr_cv);
68 }
69 
70 void
71 #ifdef	ZFS_DEBUG
72 zrl_add_debug(zrlock_t *zrl, const char *zc)
73 #else
74 zrl_add(zrlock_t *zrl)
75 #endif
76 {
77 	uint32_t n = (uint32_t)zrl->zr_refcount;
78 
79 	while (n != ZRL_LOCKED) {
80 		uint32_t cas = atomic_cas_32(
81 		    (uint32_t *)&zrl->zr_refcount, n, n + 1);
82 		if (cas == n) {
83 			ASSERT((int32_t)n >= 0);
84 #ifdef	ZFS_DEBUG
85 			if (zrl->zr_owner == curthread) {
86 				DTRACE_PROBE2(zrlock__reentry,
87 				    zrlock_t *, zrl, uint32_t, n);
88 			}
89 			zrl->zr_owner = curthread;
90 			zrl->zr_caller = zc;
91 #endif
92 			return;
93 		}
94 		n = cas;
95 	}
96 
97 	mutex_enter(&zrl->zr_mtx);
98 	while (zrl->zr_refcount == ZRL_LOCKED) {
99 		cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
100 	}
101 	ASSERT(zrl->zr_refcount >= 0);
102 	zrl->zr_refcount++;
103 #ifdef	ZFS_DEBUG
104 	zrl->zr_owner = curthread;
105 	zrl->zr_caller = zc;
106 #endif
107 	mutex_exit(&zrl->zr_mtx);
108 }
109 
110 void
111 zrl_remove(zrlock_t *zrl)
112 {
113 	uint32_t n;
114 
115 	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
116 	ASSERT((int32_t)n >= 0);
117 #ifdef	ZFS_DEBUG
118 	if (zrl->zr_owner == curthread) {
119 		zrl->zr_owner = NULL;
120 		zrl->zr_caller = NULL;
121 	}
122 #endif
123 }
124 
125 int
126 zrl_tryenter(zrlock_t *zrl)
127 {
128 	uint32_t n = (uint32_t)zrl->zr_refcount;
129 
130 	if (n == 0) {
131 		uint32_t cas = atomic_cas_32(
132 		    (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
133 		if (cas == 0) {
134 #ifdef	ZFS_DEBUG
135 			ASSERT(zrl->zr_owner == NULL);
136 			zrl->zr_owner = curthread;
137 #endif
138 			return (1);
139 		}
140 	}
141 
142 	ASSERT((int32_t)n > ZRL_DESTROYED);
143 
144 	return (0);
145 }
146 
147 void
148 zrl_exit(zrlock_t *zrl)
149 {
150 	ASSERT(zrl->zr_refcount == ZRL_LOCKED);
151 
152 	mutex_enter(&zrl->zr_mtx);
153 #ifdef	ZFS_DEBUG
154 	ASSERT(zrl->zr_owner == curthread);
155 	zrl->zr_owner = NULL;
156 	membar_producer();	/* make sure the owner store happens first */
157 #endif
158 	zrl->zr_refcount = 0;
159 	cv_broadcast(&zrl->zr_cv);
160 	mutex_exit(&zrl->zr_mtx);
161 }
162 
163 int
164 zrl_refcount(zrlock_t *zrl)
165 {
166 	ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
167 
168 	int n = (int)zrl->zr_refcount;
169 	return (n <= 0 ? 0 : n);
170 }
171 
172 int
173 zrl_is_zero(zrlock_t *zrl)
174 {
175 	ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
176 
177 	return (zrl->zr_refcount <= 0);
178 }
179 
180 int
181 zrl_is_locked(zrlock_t *zrl)
182 {
183 	ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
184 
185 	return (zrl->zr_refcount == ZRL_LOCKED);
186 }
187 
188 #ifdef	ZFS_DEBUG
189 kthread_t *
190 zrl_owner(zrlock_t *zrl)
191 {
192 	return (zrl->zr_owner);
193 }
194 #endif
195