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 * Copyright 2016 The MathWorks, Inc. All rights reserved.
25 */
26
27/*
28 * A Zero Reference Lock (ZRL) is a reference count that can lock out new
29 * references only when the count is zero and only without waiting if the count
30 * is not already zero. It is similar to a read-write lock in that it allows
31 * multiple readers and only a single writer, but it does not allow a writer to
32 * block while waiting for readers to exit, and therefore the question of
33 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
34 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
35 * is perfectly safe for the same reader to acquire the same lock multiple
36 * times. The fact that a ZRL is reentrant for readers (through multiple calls
37 * to zrl_add()) makes it convenient for determining whether something is
38 * actively referenced without the fuss of flagging lock ownership across
39 * function calls.
40 */
41#include <sys/zrlock.h>
42
43/*
44 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
45 * treated as zero references.
46 */
47#define	ZRL_LOCKED	-1
48#define	ZRL_DESTROYED	-2
49
50void
51zrl_init(zrlock_t *zrl)
52{
53	mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
54	zrl->zr_refcount = 0;
55	cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
56#ifdef	ZFS_DEBUG
57	zrl->zr_owner = NULL;
58	zrl->zr_caller = NULL;
59#endif
60}
61
62void
63zrl_destroy(zrlock_t *zrl)
64{
65	ASSERT0(zrl->zr_refcount);
66
67	mutex_destroy(&zrl->zr_mtx);
68	zrl->zr_refcount = ZRL_DESTROYED;
69	cv_destroy(&zrl->zr_cv);
70}
71
72void
73zrl_add_impl(zrlock_t *zrl, const char *zc)
74{
75	for (;;) {
76		uint32_t n = (uint32_t)zrl->zr_refcount;
77		while (n != ZRL_LOCKED) {
78			uint32_t cas = atomic_cas_32(
79			    (uint32_t *)&zrl->zr_refcount, n, n + 1);
80			if (cas == n) {
81				ASSERT3S((int32_t)n, >=, 0);
82#ifdef	ZFS_DEBUG
83				if (zrl->zr_owner == curthread) {
84					DTRACE_PROBE2(zrlock__reentry,
85					    zrlock_t *, zrl, uint32_t, n);
86				}
87				zrl->zr_owner = curthread;
88				zrl->zr_caller = zc;
89#endif
90				return;
91			}
92			n = cas;
93		}
94
95		mutex_enter(&zrl->zr_mtx);
96		while (zrl->zr_refcount == ZRL_LOCKED) {
97			cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
98		}
99		mutex_exit(&zrl->zr_mtx);
100	}
101}
102
103void
104zrl_remove(zrlock_t *zrl)
105{
106	uint32_t n;
107
108#ifdef	ZFS_DEBUG
109	if (zrl->zr_owner == curthread) {
110		zrl->zr_owner = NULL;
111		zrl->zr_caller = NULL;
112	}
113#endif
114	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
115	ASSERT3S((int32_t)n, >=, 0);
116}
117
118int
119zrl_tryenter(zrlock_t *zrl)
120{
121	uint32_t n = (uint32_t)zrl->zr_refcount;
122
123	if (n == 0) {
124		uint32_t cas = atomic_cas_32(
125		    (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
126		if (cas == 0) {
127#ifdef	ZFS_DEBUG
128			ASSERT3P(zrl->zr_owner, ==, NULL);
129			zrl->zr_owner = curthread;
130#endif
131			return (1);
132		}
133	}
134
135	ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
136
137	return (0);
138}
139
140void
141zrl_exit(zrlock_t *zrl)
142{
143	ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
144
145	mutex_enter(&zrl->zr_mtx);
146#ifdef	ZFS_DEBUG
147	ASSERT3P(zrl->zr_owner, ==, curthread);
148	zrl->zr_owner = NULL;
149	membar_producer();	/* make sure the owner store happens first */
150#endif
151	zrl->zr_refcount = 0;
152	cv_broadcast(&zrl->zr_cv);
153	mutex_exit(&zrl->zr_mtx);
154}
155
156int
157zrl_refcount(zrlock_t *zrl)
158{
159	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
160
161	int n = (int)zrl->zr_refcount;
162	return (n <= 0 ? 0 : n);
163}
164
165int
166zrl_is_zero(zrlock_t *zrl)
167{
168	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
169
170	return (zrl->zr_refcount <= 0);
171}
172
173int
174zrl_is_locked(zrlock_t *zrl)
175{
176	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
177
178	return (zrl->zr_refcount == ZRL_LOCKED);
179}
180
181#ifdef	ZFS_DEBUG
182kthread_t *
183zrl_owner(zrlock_t *zrl)
184{
185	return (zrl->zr_owner);
186}
187#endif
188