zrlock.c revision 288560 
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
49void
50zrl_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
61void
62zrl_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
71void
72#ifdef	ZFS_DEBUG
73zrl_add_debug(zrlock_t *zrl, const char *zc)
74#else
75zrl_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
111void
112zrl_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
126int
127zrl_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
148void
149zrl_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
164int
165zrl_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
173int
174zrl_is_zero(zrlock_t *zrl)
175{
176	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
177
178	return (zrl->zr_refcount <= 0);
179}
180
181int
182zrl_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
190kthread_t *
191zrl_owner(zrlock_t *zrl)
192{
193	return (zrl->zr_owner);
194}
195#endif
196