1 | /* |
2 | * Copyright 2016-present Facebook, Inc. |
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 | #pragma once |
18 | |
19 | #include <atomic> |
20 | #include <mutex> |
21 | #include <utility> |
22 | |
23 | #include <folly/Likely.h> |
24 | #include <folly/Portability.h> |
25 | #include <folly/SharedMutex.h> |
26 | #include <folly/functional/Invoke.h> |
27 | |
28 | namespace folly { |
29 | |
30 | template <typename Mutex, template <typename> class Atom = std::atomic> |
31 | class basic_once_flag; |
32 | |
33 | // call_once |
34 | // |
35 | // Drop-in replacement for std::call_once. |
36 | // |
37 | // The libstdc++ implementation has two flaws: |
38 | // * it lacks a fast path, and |
39 | // * it deadlocks (in explicit violation of the standard) when invoked twice |
40 | // with a given flag, and the callable passed to the first invocation throws. |
41 | // |
42 | // This implementation corrects both flaws. |
43 | // |
44 | // The tradeoff is a slightly larger once_flag struct at 8 bytes, vs 4 bytes |
45 | // with libstdc++ on Linux/x64. |
46 | // |
47 | // Does not work with std::once_flag. |
48 | // |
49 | // mimic: std::call_once |
50 | template < |
51 | typename Mutex, |
52 | template <typename> class Atom, |
53 | typename F, |
54 | typename... Args> |
55 | FOLLY_ALWAYS_INLINE void |
56 | call_once(basic_once_flag<Mutex, Atom>& flag, F&& f, Args&&... args) { |
57 | flag.call_once(std::forward<F>(f), std::forward<Args>(args)...); |
58 | } |
59 | |
60 | // test_once |
61 | // |
62 | // Tests whether any invocation to call_once with the given flag has succeeded. |
63 | // |
64 | // May help with space usage in certain esoteric scenarios compared with caller |
65 | // code tracking a separate and possibly-padded bool. |
66 | // |
67 | // Note: This is has no parallel in the std::once_flag interface. |
68 | template <typename Mutex, template <typename> class Atom> |
69 | FOLLY_ALWAYS_INLINE bool test_once(basic_once_flag<Mutex, Atom> const& flag) { |
70 | return flag.called_.load(std::memory_order_acquire); |
71 | } |
72 | |
73 | // basic_once_flag |
74 | // |
75 | // The flag template to be used with call_once. Parameterizable by the mutex |
76 | // type and atomic template. The mutex type is required to mimic std::mutex and |
77 | // the atomic type is required to mimic std::atomic. |
78 | template <typename Mutex, template <typename> class Atom> |
79 | class basic_once_flag { |
80 | public: |
81 | constexpr basic_once_flag() noexcept = default; |
82 | basic_once_flag(const basic_once_flag&) = delete; |
83 | basic_once_flag& operator=(const basic_once_flag&) = delete; |
84 | |
85 | private: |
86 | template < |
87 | typename Mutex_, |
88 | template <typename> class Atom_, |
89 | typename F, |
90 | typename... Args> |
91 | friend void call_once(basic_once_flag<Mutex_, Atom_>&, F&&, Args&&...); |
92 | |
93 | template <typename Mutex_, template <typename> class Atom_> |
94 | friend bool test_once(basic_once_flag<Mutex_, Atom_> const& flag); |
95 | |
96 | template <typename F, typename... Args> |
97 | FOLLY_ALWAYS_INLINE void call_once(F&& f, Args&&... args) { |
98 | if (LIKELY(called_.load(std::memory_order_acquire))) { |
99 | return; |
100 | } |
101 | call_once_slow(std::forward<F>(f), std::forward<Args>(args)...); |
102 | } |
103 | |
104 | template <typename F, typename... Args> |
105 | FOLLY_NOINLINE void call_once_slow(F&& f, Args&&... args) { |
106 | std::lock_guard<Mutex> lock(mutex_); |
107 | if (called_.load(std::memory_order_relaxed)) { |
108 | return; |
109 | } |
110 | invoke(std::forward<F>(f), std::forward<Args>(args)...); |
111 | called_.store(true, std::memory_order_release); |
112 | } |
113 | |
114 | Atom<bool> called_{false}; |
115 | Mutex mutex_; |
116 | }; |
117 | |
118 | // once_flag |
119 | // |
120 | // The flag type to be used with call_once. An instance of basic_once_flag. |
121 | // |
122 | // Does not work with sd::call_once. |
123 | // |
124 | // mimic: std::once_flag |
125 | using once_flag = basic_once_flag<SharedMutex>; |
126 | |
127 | } // namespace folly |
128 | |