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1 /*
2  * Copyright (C) 2022 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 ART_RUNTIME_BASE_ATOMIC_PAIR_H_
18 #define ART_RUNTIME_BASE_ATOMIC_PAIR_H_
19 
20 #include <android-base/logging.h>
21 
22 #include <atomic>
23 #include <type_traits>
24 
25 #include "base/macros.h"
26 
27 namespace art HIDDEN {
28 
29 // Implement 16-byte atomic pair using the seq-lock synchronization algorithm.
30 // This is currently only used for DexCache.
31 //
32 // This uses top 4-bytes of the key as version counter and lock bit,
33 // which means the stored pair key can not use those bytes.
34 //
35 // This allows us to read the cache without exclusive access to the cache line.
36 //
37 // The 8-byte atomic pair uses the normal single-instruction implementation.
38 //
39 static constexpr uint64_t kSeqMask = (0xFFFFFFFFull << 32);
40 static constexpr uint64_t kSeqLock = (0x80000000ull << 32);
41 static constexpr uint64_t kSeqIncr = (0x00000001ull << 32);
42 
43 // std::pair<> is not trivially copyable and as such it is unsuitable for atomic operations.
44 template <typename IntType>
45 struct PACKED(2 * sizeof(IntType)) AtomicPair {
46   static_assert(std::is_integral_v<IntType>);
47 
AtomicPairAtomicPair48   AtomicPair(IntType f, IntType s) : key(f), val(s) {}
49 
50   IntType key;
51   IntType val;
52 };
53 
54 template <typename IntType>
AtomicPairLoadAcquire(AtomicPair<IntType> * pair)55 ALWAYS_INLINE static inline AtomicPair<IntType> AtomicPairLoadAcquire(AtomicPair<IntType>* pair) {
56   static_assert(std::is_trivially_copyable<AtomicPair<IntType>>::value);
57   auto* target = reinterpret_cast<std::atomic<AtomicPair<IntType>>*>(pair);
58   return target->load(std::memory_order_acquire);
59 }
60 
61 template <typename IntType>
AtomicPairStoreRelease(AtomicPair<IntType> * pair,AtomicPair<IntType> value)62 ALWAYS_INLINE static inline void AtomicPairStoreRelease(AtomicPair<IntType>* pair,
63                                                         AtomicPair<IntType> value) {
64   static_assert(std::is_trivially_copyable<AtomicPair<IntType>>::value);
65   auto* target = reinterpret_cast<std::atomic<AtomicPair<IntType>>*>(pair);
66   target->store(value, std::memory_order_release);
67 }
68 
AtomicPairLoadAcquire(AtomicPair<uint64_t> * pair)69 ALWAYS_INLINE static inline AtomicPair<uint64_t> AtomicPairLoadAcquire(AtomicPair<uint64_t>* pair) {
70   auto* key_ptr = reinterpret_cast<std::atomic_uint64_t*>(&pair->key);
71   auto* val_ptr = reinterpret_cast<std::atomic_uint64_t*>(&pair->val);
72   while (true) {
73     uint64_t key0 = key_ptr->load(std::memory_order_acquire);
74     uint64_t val = val_ptr->load(std::memory_order_acquire);
75     uint64_t key1 = key_ptr->load(std::memory_order_relaxed);
76     uint64_t key = key0 & ~kSeqMask;
77     if (LIKELY((key0 & kSeqLock) == 0 && key0 == key1)) {
78       return {key, val};
79     }
80   }
81 }
82 
AtomicPairStoreRelease(AtomicPair<uint64_t> * pair,AtomicPair<uint64_t> value)83 ALWAYS_INLINE static inline void AtomicPairStoreRelease(AtomicPair<uint64_t>* pair,
84                                                         AtomicPair<uint64_t> value) {
85   DCHECK((value.key & kSeqMask) == 0) << "Key=0x" << std::hex << value.key;
86   auto* key_ptr = reinterpret_cast<std::atomic_uint64_t*>(&pair->key);
87   auto* val_ptr = reinterpret_cast<std::atomic_uint64_t*>(&pair->val);
88   uint64_t key = key_ptr->load(std::memory_order_relaxed);
89   do {
90     key &= ~kSeqLock;  // Ensure that the CAS below fails if the lock bit is already set.
91   } while (!key_ptr->compare_exchange_weak(key, key | kSeqLock));
92   key = (((key & kSeqMask) + kSeqIncr) & ~kSeqLock) | (value.key & ~kSeqMask);
93   val_ptr->store(value.val, std::memory_order_release);
94   key_ptr->store(key, std::memory_order_release);
95 }
96 
97 }  // namespace art
98 
99 #endif  // ART_RUNTIME_BASE_ATOMIC_PAIR_H_
100