1 /*
2 * Copyright (C) 2006 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include <endian.h>
18 #include <limits.h>
19 #include <stdatomic.h>
20 #include <stddef.h>
21
22 #include "private/bionic_futex.h"
23
24 // This file contains C++ ABI support functions for one time
25 // constructors as defined in the "Run-time ABI for the ARM Architecture"
26 // section 4.4.2
27 //
28 // ARM C++ ABI and Itanium/x86 C++ ABI has different definition for
29 // one time construction:
30 //
31 // ARM C++ ABI defines the LSB of guard variable should be tested
32 // by compiler-generated code before calling __cxa_guard_acquire et al.
33 //
34 // The Itanium/x86 C++ ABI defines the low-order _byte_ should be
35 // tested instead.
36 //
37 // Meanwhile, guard variable are 32bit aligned for ARM, and 64bit
38 // aligned for x86.
39 //
40 // Reference documentation:
41 //
42 // section 3.2.3 of ARM IHI 0041C (for ARM)
43 // section 3.3.2 of the Itanium C++ ABI specification v1.83 (for x86).
44 //
45 // There is no C++ ABI available for other ARCH. But the gcc source
46 // shows all other ARCH follow the definition of Itanium/x86 C++ ABI.
47
48 #if defined(__arm__)
49 // The ARM C++ ABI mandates that guard variables are 32-bit aligned, 32-bit
50 // values. The LSB is tested by the compiler-generated code before calling
51 // __cxa_guard_acquire.
52 union _guard_t {
53 atomic_int state;
54 int32_t aligner;
55 };
56
57 #else
58 // The Itanium/x86 C++ ABI (used by all other architectures) mandates that
59 // guard variables are 64-bit aligned, 64-bit values. The LSB is tested by
60 // the compiler-generated code before calling __cxa_guard_acquire.
61 union _guard_t {
62 atomic_int state;
63 int64_t aligner;
64 };
65
66 #endif
67
68 // Set construction state values according to reference documentation.
69 // 0 is the initialization value.
70 // Arm requires ((*gv & 1) == 1) after __cxa_guard_release, ((*gv & 3) == 0) after __cxa_guard_abort.
71 // X86 requires first byte not modified by __cxa_guard_acquire, first byte is non-zero after
72 // __cxa_guard_release.
73
74 #define CONSTRUCTION_NOT_YET_STARTED 0
75 #define CONSTRUCTION_COMPLETE 1
76 #define CONSTRUCTION_UNDERWAY_WITHOUT_WAITER 0x100
77 #define CONSTRUCTION_UNDERWAY_WITH_WAITER 0x200
78
__cxa_guard_acquire(_guard_t * gv)79 extern "C" int __cxa_guard_acquire(_guard_t* gv) {
80 int old_value = atomic_load_explicit(&gv->state, memory_order_acquire);
81 // In the common CONSTRUCTION_COMPLETE case we have to ensure that all the stores performed by
82 // the construction function are observable on this CPU after we exit. A similar constraint may
83 // apply in the CONSTRUCTION_NOT_YET_STARTED case with a prior abort.
84
85 while (true) {
86 if (old_value == CONSTRUCTION_COMPLETE) {
87 return 0;
88 } else if (old_value == CONSTRUCTION_NOT_YET_STARTED) {
89 if (!atomic_compare_exchange_weak_explicit(&gv->state, &old_value,
90 CONSTRUCTION_UNDERWAY_WITHOUT_WAITER,
91 memory_order_acquire /* or relaxed in C++17 */,
92 memory_order_acquire)) {
93 continue;
94 }
95 return 1;
96 } else if (old_value == CONSTRUCTION_UNDERWAY_WITHOUT_WAITER) {
97 if (!atomic_compare_exchange_weak_explicit(&gv->state, &old_value,
98 CONSTRUCTION_UNDERWAY_WITH_WAITER,
99 memory_order_acquire /* or relaxed in C++17 */,
100 memory_order_acquire)) {
101 continue;
102 }
103 }
104
105 __futex_wait_ex(&gv->state, false, CONSTRUCTION_UNDERWAY_WITH_WAITER);
106 old_value = atomic_load_explicit(&gv->state, memory_order_acquire);
107 }
108 }
109
__cxa_guard_release(_guard_t * gv)110 extern "C" void __cxa_guard_release(_guard_t* gv) {
111 // Release fence is used to make all stores performed by the construction function
112 // visible in other threads.
113 int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_COMPLETE, memory_order_release);
114 if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) {
115 __futex_wake_ex(&gv->state, false, INT_MAX);
116 }
117 }
118
__cxa_guard_abort(_guard_t * gv)119 extern "C" void __cxa_guard_abort(_guard_t* gv) {
120 // Release fence is used to make all stores performed by the construction function
121 // visible in other threads.
122 int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_NOT_YET_STARTED, memory_order_release);
123 if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) {
124 __futex_wake_ex(&gv->state, false, INT_MAX);
125 }
126 }
127