1/*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2005 John Baldwin <jhb@FreeBSD.org>
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25 * SUCH DAMAGE.
26 */
27
28#ifndef __SYS_REFCOUNT_H__
29#define __SYS_REFCOUNT_H__
30
31#include <sys/types.h>
32#include <sys/kassert.h>
33#if !defined(_KERNEL) && !defined(_STANDALONE)
34#include <stdbool.h>
35#endif
36
37#include <machine/atomic.h>
38
39#define REFCOUNT_SATURATED(val) (((val) & (1U << 31)) != 0)
40#define REFCOUNT_SATURATION_VALUE (3U << 30)
41
42/*
43 * Attempt to handle reference count overflow and underflow. Force the counter
44 * to stay at the saturation value so that a counter overflow cannot trigger
45 * destruction of the containing object and instead leads to a less harmful
46 * memory leak.
47 */
48static __inline void
49_refcount_update_saturated(volatile u_int *count)
50{
51#ifdef INVARIANTS
52 panic("refcount %p wraparound", count);
53#else
54 atomic_store_int(count, REFCOUNT_SATURATION_VALUE);
55#endif
56}
57
58static __inline void
59refcount_init(volatile u_int *count, u_int value)
60{
61 KASSERT(!REFCOUNT_SATURATED(value),
62 ("invalid initial refcount value %u", value));
63 atomic_store_int(count, value);
64}
65
66static __inline u_int
67refcount_load(volatile const u_int *count)
68{
69 return (atomic_load_int(count));
70}
71
72static __inline u_int
73refcount_acquire(volatile u_int *count)
74{
75 u_int old;
76
77 old = atomic_fetchadd_int(count, 1);
78 if (__predict_false(REFCOUNT_SATURATED(old)))
79 _refcount_update_saturated(count);
80
81 return (old);
82}
83
84static __inline u_int
85refcount_acquiren(volatile u_int *count, u_int n)
86{
87 u_int old;
88
89 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
90 ("refcount_acquiren: n=%u too large", n));
91 old = atomic_fetchadd_int(count, n);
92 if (__predict_false(REFCOUNT_SATURATED(old)))
93 _refcount_update_saturated(count);
94
95 return (old);
96}
97
98static __inline __result_use_check bool
99refcount_acquire_checked(volatile u_int *count)
100{
101 u_int old;
102
103 old = atomic_load_int(count);
104 for (;;) {
105 if (__predict_false(REFCOUNT_SATURATED(old + 1)))
106 return (false);
107 if (__predict_true(atomic_fcmpset_int(count, &old,
108 old + 1) == 1))
109 return (true);
110 }
111}
112
113/*
114 * This functions returns non-zero if the refcount was
115 * incremented. Else zero is returned.
116 */
117static __inline __result_use_check bool
118refcount_acquire_if_gt(volatile u_int *count, u_int n)
119{
120 u_int old;
121
122 old = atomic_load_int(count);
123 for (;;) {
124 if (old <= n)
125 return (false);
126 if (__predict_false(REFCOUNT_SATURATED(old)))
127 return (true);
128 if (atomic_fcmpset_int(count, &old, old + 1))
129 return (true);
130 }
131}
132
133static __inline __result_use_check bool
134refcount_acquire_if_not_zero(volatile u_int *count)
135{
136
137 return (refcount_acquire_if_gt(count, 0));
138}
139
140static __inline bool
141refcount_releasen(volatile u_int *count, u_int n)
142{
143 u_int old;
144
145 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
146 ("refcount_releasen: n=%u too large", n));
147
148 atomic_thread_fence_rel();
149 old = atomic_fetchadd_int(count, -n);
150 if (__predict_false(old < n || REFCOUNT_SATURATED(old))) {
151 _refcount_update_saturated(count);
152 return (false);
153 }
154 if (old > n)
155 return (false);
156
157 /*
158 * Last reference. Signal the user to call the destructor.
159 *
160 * Ensure that the destructor sees all updates. This synchronizes with
161 * release fences from all routines which drop the count.
162 */
163 atomic_thread_fence_acq();
164 return (true);
165}
166
167static __inline bool
168refcount_release(volatile u_int *count)
169{
170
171 return (refcount_releasen(count, 1));
172}
173
174#define _refcount_release_if_cond(cond, name) \
175static __inline __result_use_check bool \
176_refcount_release_if_##name(volatile u_int *count, u_int n) \
177{ \
178 u_int old; \
179 \
180 KASSERT(n > 0, ("%s: zero increment", __func__)); \
181 old = atomic_load_int(count); \
182 for (;;) { \
183 if (!(cond)) \
184 return (false); \
185 if (__predict_false(REFCOUNT_SATURATED(old))) \
186 return (false); \
187 if (atomic_fcmpset_rel_int(count, &old, old - 1)) \
188 return (true); \
189 } \
190}
191_refcount_release_if_cond(old > n, gt)
192_refcount_release_if_cond(old == n, eq)
193
194static __inline __result_use_check bool
195refcount_release_if_gt(volatile u_int *count, u_int n)
196{
197
198 return (_refcount_release_if_gt(count, n));
199}
200
201static __inline __result_use_check bool
202refcount_release_if_last(volatile u_int *count)
203{
204
205 if (_refcount_release_if_eq(count, 1)) {
206 /* See the comment in refcount_releasen(). */
207 atomic_thread_fence_acq();
208 return (true);
209 }
210 return (false);
211}
212
213static __inline __result_use_check bool
214refcount_release_if_not_last(volatile u_int *count)
215{
216
217 return (_refcount_release_if_gt(count, 1));
218}
219
220#endif /* !__SYS_REFCOUNT_H__ */