Make temporary variable in _rcu_dereference non-const
[urcu.git] / include / urcu / static / pointer.h
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1#ifndef _URCU_POINTER_STATIC_H
2#define _URCU_POINTER_STATIC_H
3
4/*
6cd23d47 5 * urcu/static/pointer.h
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6 *
7 * Userspace RCU header. Operations on pointers.
8 *
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9 * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
10 * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
7e30abe3 11 *
6982d6d7 12 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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13 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
14 *
15 * This library is free software; you can redistribute it and/or
16 * modify it under the terms of the GNU Lesser General Public
17 * License as published by the Free Software Foundation; either
18 * version 2.1 of the License, or (at your option) any later version.
19 *
20 * This library is distributed in the hope that it will be useful,
21 * but WITHOUT ANY WARRANTY; without even the implied warranty of
22 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
23 * Lesser General Public License for more details.
24 *
25 * You should have received a copy of the GNU Lesser General Public
26 * License along with this library; if not, write to the Free Software
27 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
28 *
29 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
30 */
31
32#include <urcu/compiler.h>
33#include <urcu/arch.h>
34#include <urcu/system.h>
a2e7bf9c 35#include <urcu/uatomic.h>
7e30abe3 36
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37#ifdef __cplusplus
38extern "C" {
67ecffc0 39#endif
36bc70a8 40
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41/**
42 * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
43 * into a RCU read-side critical section. The pointer can later be safely
44 * dereferenced within the critical section.
45 *
46 * This ensures that the pointer copy is invariant thorough the whole critical
47 * section.
48 *
49 * Inserts memory barriers on architectures that require them (currently only
50 * Alpha) and documents which pointers are protected by RCU.
51 *
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52 * With C standards prior to C11/C++11, the compiler memory barrier in
53 * CMM_LOAD_SHARED() ensures that value-speculative optimizations (e.g.
54 * VSS: Value Speculation Scheduling) does not perform the data read
55 * before the pointer read by speculating the value of the pointer.
56 * Correct ordering is ensured because the pointer is read as a volatile
57 * access. This acts as a global side-effect operation, which forbids
58 * reordering of dependent memory operations.
59 *
60 * With C standards C11/C++11, concerns about dependency-breaking
61 * optimizations are taken care of by the "memory_order_consume" atomic
62 * load.
63 *
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64 * Use the gcc __atomic_load() rather than C11/C++11 atomic load
65 * explicit because the pointer used as input argument is a pointer,
66 * not an _Atomic type as required by C11/C++11.
67 *
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68 * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
69 * volatile access to implement rcu_dereference rather than
70 * memory_order_consume load from the C11/C++11 standards.
71 *
72 * This may improve performance on weakly-ordered architectures where
73 * the compiler implements memory_order_consume as a
74 * memory_order_acquire, which is stricter than required by the
75 * standard.
76 *
77 * Note that using volatile accesses for rcu_dereference may cause
78 * LTO to generate incorrectly ordered code starting from C11/C++11.
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79 *
80 * Should match rcu_assign_pointer() or rcu_xchg_pointer().
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81 *
82 * This macro is less than 10 lines long. The intent is that this macro
83 * meets the 10-line criterion in LGPL, allowing this function to be
84 * expanded directly in non-LGPL code.
7e30abe3 85 */
380f4b19 86
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87#if !defined (URCU_DEREFERENCE_USE_VOLATILE) && \
88 ((defined (__cplusplus) && __cplusplus >= 201103L) || \
89 (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L))
90# define __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
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91#endif
92
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93/*
94 * If p is const (the pointer itself, not what it points to), using
95 * __typeof__(p) would declare a const variable, leading to
96 * -Wincompatible-pointer-types errors. Using `+ 0` makes it an rvalue and
97 * gets rid of the const-ness.
98 */
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99#ifdef __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
100# define _rcu_dereference(p) __extension__ ({ \
1e41ec3b 101 __typeof__(p + 0) _________p1; \
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102 __atomic_load(&(p), &_________p1, __ATOMIC_CONSUME); \
103 (_________p1); \
104 })
105#else
106# define _rcu_dereference(p) __extension__ ({ \
107 __typeof__(p) _________p1 = CMM_LOAD_SHARED(p); \
108 cmm_smp_read_barrier_depends(); \
109 (_________p1); \
110 })
111#endif
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112
113/**
114 * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
115 * is as expected by "old". If succeeds, returns the previous pointer to the
116 * data structure, which can be safely freed after waiting for a quiescent state
117 * using synchronize_rcu(). If fails (unexpected value), returns old (which
118 * should not be freed !).
a5a9f428 119 *
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120 * uatomic_cmpxchg() acts as both release and acquire barriers.
121 *
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122 * This macro is less than 10 lines long. The intent is that this macro
123 * meets the 10-line criterion in LGPL, allowing this function to be
124 * expanded directly in non-LGPL code.
7e30abe3 125 */
2b5554c9 126#define _rcu_cmpxchg_pointer(p, old, _new) \
1b85da85 127 __extension__ \
2b5554c9 128 ({ \
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129 __typeof__(*p) _________pold = (old); \
130 __typeof__(*p) _________pnew = (_new); \
bf9de1b7 131 uatomic_cmpxchg(p, _________pold, _________pnew); \
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132 })
133
134/**
135 * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
136 * pointer to the data structure, which can be safely freed after waiting for a
137 * quiescent state using synchronize_rcu().
a5a9f428 138 *
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139 * uatomic_xchg() acts as both release and acquire barriers.
140 *
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141 * This macro is less than 10 lines long. The intent is that this macro
142 * meets the 10-line criterion in LGPL, allowing this function to be
143 * expanded directly in non-LGPL code.
7e30abe3 144 */
7e30abe3 145#define _rcu_xchg_pointer(p, v) \
1b85da85 146 __extension__ \
7e30abe3 147 ({ \
bdffa73a 148 __typeof__(*p) _________pv = (v); \
2b5554c9 149 uatomic_xchg(p, _________pv); \
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150 })
151
152
153#define _rcu_set_pointer(p, v) \
3daae22a 154 do { \
bdffa73a 155 __typeof__(*p) _________pv = (v); \
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156 if (!__builtin_constant_p(v) || \
157 ((v) != NULL)) \
5481ddb3 158 cmm_wmb(); \
bf9de1b7 159 uatomic_set(p, _________pv); \
3daae22a 160 } while (0)
7e30abe3 161
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162/**
163 * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
164 * meant to be read by RCU read-side critical sections. Returns the assigned
165 * value.
166 *
167 * Documents which pointers will be dereferenced by RCU read-side critical
168 * sections and adds the required memory barriers on architectures requiring
169 * them. It also makes sure the compiler does not reorder code initializing the
170 * data structure before its publication.
171 *
edf8de69 172 * Should match rcu_dereference().
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173 *
174 * This macro is less than 10 lines long. The intent is that this macro
175 * meets the 10-line criterion in LGPL, allowing this function to be
176 * expanded directly in non-LGPL code.
7e30abe3 177 */
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178#define _rcu_assign_pointer(p, v) _rcu_set_pointer(&(p), v)
179
67ecffc0 180#ifdef __cplusplus
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181}
182#endif
183
7e30abe3 184#endif /* _URCU_POINTER_STATIC_H */
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