FreeBSD/Linux Kernel Cross Reference
sys/sys/refcount.h
1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
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 * $FreeBSD$
28 */
29
30 #ifndef __SYS_REFCOUNT_H__
31 #define __SYS_REFCOUNT_H__
32
33 #include <machine/atomic.h>
34
35 #if defined(_KERNEL) || defined(_STANDALONE)
36 #include <sys/systm.h>
37 #else
38 #include <stdbool.h>
39 #define KASSERT(exp, msg) /* */
40 #endif
41
42 #define REFCOUNT_SATURATED(val) (((val) & (1U << 31)) != 0)
43 #define REFCOUNT_SATURATION_VALUE (3U << 30)
44
45 /*
46 * Attempt to handle reference count overflow and underflow. Force the counter
47 * to stay at the saturation value so that a counter overflow cannot trigger
48 * destruction of the containing object and instead leads to a less harmful
49 * memory leak.
50 */
51 static __inline void
52 _refcount_update_saturated(volatile u_int *count)
53 {
54 #ifdef INVARIANTS
55 panic("refcount %p wraparound", count);
56 #else
57 atomic_store_int(count, REFCOUNT_SATURATION_VALUE);
58 #endif
59 }
60
61 static __inline void
62 refcount_init(volatile u_int *count, u_int value)
63 {
64 KASSERT(!REFCOUNT_SATURATED(value),
65 ("invalid initial refcount value %u", value));
66 atomic_store_int(count, value);
67 }
68
69 static __inline u_int
70 refcount_load(volatile u_int *count)
71 {
72 return (atomic_load_int(count));
73 }
74
75 static __inline u_int
76 refcount_acquire(volatile u_int *count)
77 {
78 u_int old;
79
80 old = atomic_fetchadd_int(count, 1);
81 if (__predict_false(REFCOUNT_SATURATED(old)))
82 _refcount_update_saturated(count);
83
84 return (old);
85 }
86
87 static __inline u_int
88 refcount_acquiren(volatile u_int *count, u_int n)
89 {
90 u_int old;
91
92 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
93 ("refcount_acquiren: n=%u too large", n));
94 old = atomic_fetchadd_int(count, n);
95 if (__predict_false(REFCOUNT_SATURATED(old)))
96 _refcount_update_saturated(count);
97
98 return (old);
99 }
100
101 static __inline __result_use_check bool
102 refcount_acquire_checked(volatile u_int *count)
103 {
104 u_int old;
105
106 old = atomic_load_int(count);
107 for (;;) {
108 if (__predict_false(REFCOUNT_SATURATED(old + 1)))
109 return (false);
110 if (__predict_true(atomic_fcmpset_int(count, &old,
111 old + 1) == 1))
112 return (true);
113 }
114 }
115
116 /*
117 * This functions returns non-zero if the refcount was
118 * incremented. Else zero is returned.
119 */
120 static __inline __result_use_check bool
121 refcount_acquire_if_gt(volatile u_int *count, u_int n)
122 {
123 u_int old;
124
125 old = atomic_load_int(count);
126 for (;;) {
127 if (old <= n)
128 return (false);
129 if (__predict_false(REFCOUNT_SATURATED(old)))
130 return (true);
131 if (atomic_fcmpset_int(count, &old, old + 1))
132 return (true);
133 }
134 }
135
136 static __inline __result_use_check bool
137 refcount_acquire_if_not_zero(volatile u_int *count)
138 {
139
140 return (refcount_acquire_if_gt(count, 0));
141 }
142
143 static __inline bool
144 refcount_releasen(volatile u_int *count, u_int n)
145 {
146 u_int old;
147
148 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
149 ("refcount_releasen: n=%u too large", n));
150
151 atomic_thread_fence_rel();
152 old = atomic_fetchadd_int(count, -n);
153 if (__predict_false(old < n || REFCOUNT_SATURATED(old))) {
154 _refcount_update_saturated(count);
155 return (false);
156 }
157 if (old > n)
158 return (false);
159
160 /*
161 * Last reference. Signal the user to call the destructor.
162 *
163 * Ensure that the destructor sees all updates. This synchronizes with
164 * release fences from all routines which drop the count.
165 */
166 atomic_thread_fence_acq();
167 return (true);
168 }
169
170 static __inline bool
171 refcount_release(volatile u_int *count)
172 {
173
174 return (refcount_releasen(count, 1));
175 }
176
177 #define _refcount_release_if_cond(cond, name) \
178 static __inline __result_use_check bool \
179 _refcount_release_if_##name(volatile u_int *count, u_int n) \
180 { \
181 u_int old; \
182 \
183 KASSERT(n > 0, ("%s: zero increment", __func__)); \
184 old = atomic_load_int(count); \
185 for (;;) { \
186 if (!(cond)) \
187 return (false); \
188 if (__predict_false(REFCOUNT_SATURATED(old))) \
189 return (false); \
190 if (atomic_fcmpset_rel_int(count, &old, old - 1)) \
191 return (true); \
192 } \
193 }
194 _refcount_release_if_cond(old > n, gt)
195 _refcount_release_if_cond(old == n, eq)
196
197 static __inline __result_use_check bool
198 refcount_release_if_gt(volatile u_int *count, u_int n)
199 {
200
201 return (_refcount_release_if_gt(count, n));
202 }
203
204 static __inline __result_use_check bool
205 refcount_release_if_last(volatile u_int *count)
206 {
207
208 if (_refcount_release_if_eq(count, 1)) {
209 /* See the comment in refcount_releasen(). */
210 atomic_thread_fence_acq();
211 return (true);
212 }
213 return (false);
214 }
215
216 static __inline __result_use_check bool
217 refcount_release_if_not_last(volatile u_int *count)
218 {
219
220 return (_refcount_release_if_gt(count, 1));
221 }
222
223 #endif /* !__SYS_REFCOUNT_H__ */
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