1 /* 2 * Copyright (C) 2007 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 #ifndef ANDROID_CUTILS_ATOMIC_H 18 #define ANDROID_CUTILS_ATOMIC_H 19 20 #include <stdint.h> 21 #include <sys/types.h> 22 23 #ifdef __cplusplus 24 extern "C" { 25 #endif 26 27 /* 28 * A handful of basic atomic operations. The appropriate pthread 29 * functions should be used instead of these whenever possible. 30 * 31 * The "acquire" and "release" terms can be defined intuitively in terms 32 * of the placement of memory barriers in a simple lock implementation: 33 * - wait until compare-and-swap(lock-is-free --> lock-is-held) succeeds 34 * - barrier 35 * - [do work] 36 * - barrier 37 * - store(lock-is-free) 38 * In very crude terms, the initial (acquire) barrier prevents any of the 39 * "work" from happening before the lock is held, and the later (release) 40 * barrier ensures that all of the work happens before the lock is released. 41 * (Think of cached writes, cache read-ahead, and instruction reordering 42 * around the CAS and store instructions.) 43 * 44 * The barriers must apply to both the compiler and the CPU. Note it is 45 * legal for instructions that occur before an "acquire" barrier to be 46 * moved down below it, and for instructions that occur after a "release" 47 * barrier to be moved up above it. 48 * 49 * The ARM-driven implementation we use here is short on subtlety, 50 * and actually requests a full barrier from the compiler and the CPU. 51 * The only difference between acquire and release is in whether they 52 * are issued before or after the atomic operation with which they 53 * are associated. To ease the transition to C/C++ atomic intrinsics, 54 * you should not rely on this, and instead assume that only the minimal 55 * acquire/release protection is provided. 56 * 57 * NOTE: all int32_t* values are expected to be aligned on 32-bit boundaries. 58 * If they are not, atomicity is not guaranteed. 59 */ 60 61 /* 62 * Basic arithmetic and bitwise operations. These all provide a 63 * barrier with "release" ordering, and return the previous value. 64 * 65 * These have the same characteristics (e.g. what happens on overflow) 66 * as the equivalent non-atomic C operations. 67 */ 68 int32_t android_atomic_inc(volatile int32_t* addr); 69 int32_t android_atomic_dec(volatile int32_t* addr); 70 int32_t android_atomic_add(int32_t value, volatile int32_t* addr); 71 int32_t android_atomic_and(int32_t value, volatile int32_t* addr); 72 int32_t android_atomic_or(int32_t value, volatile int32_t* addr); 73 74 /* 75 * Perform an atomic load with "acquire" or "release" ordering. 76 * 77 * This is only necessary if you need the memory barrier. A 32-bit read 78 * from a 32-bit aligned address is atomic on all supported platforms. 79 */ 80 int32_t android_atomic_acquire_load(volatile const int32_t* addr); 81 int32_t android_atomic_release_load(volatile const int32_t* addr); 82 83 /* 84 * Perform an atomic store with "acquire" or "release" ordering. 85 * 86 * This is only necessary if you need the memory barrier. A 32-bit write 87 * to a 32-bit aligned address is atomic on all supported platforms. 88 */ 89 void android_atomic_acquire_store(int32_t value, volatile int32_t* addr); 90 void android_atomic_release_store(int32_t value, volatile int32_t* addr); 91 92 /* 93 * Compare-and-set operation with "acquire" or "release" ordering. 94 * 95 * This returns zero if the new value was successfully stored, which will 96 * only happen when *addr == oldvalue. 97 * 98 * (The return value is inverted from implementations on other platforms, 99 * but matches the ARM ldrex/strex result.) 100 * 101 * Implementations that use the release CAS in a loop may be less efficient 102 * than possible, because we re-issue the memory barrier on each iteration. 103 */ 104 int android_atomic_acquire_cas(int32_t oldvalue, int32_t newvalue, 105 volatile int32_t* addr); 106 int android_atomic_release_cas(int32_t oldvalue, int32_t newvalue, 107 volatile int32_t* addr); 108 109 /* 110 * Aliases for code using an older version of this header. These are now 111 * deprecated and should not be used. The definitions will be removed 112 * in a future release. 113 */ 114 #define android_atomic_write android_atomic_release_store 115 #define android_atomic_cmpxchg android_atomic_release_cas 116 117 #ifdef __cplusplus 118 } // extern "C" 119 #endif 120 121 #endif // ANDROID_CUTILS_ATOMIC_H 122