1/*
2 * Copyright 2015-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/*
18 * N.B. You most likely do _not_ want to use MicroSpinLock or any
19 * other kind of spinlock. Consider MicroLock instead.
20 *
21 * In short, spinlocks in preemptive multi-tasking operating systems
22 * have serious problems and fast mutexes like std::mutex are almost
23 * certainly the better choice, because letting the OS scheduler put a
24 * thread to sleep is better for system responsiveness and throughput
25 * than wasting a timeslice repeatedly querying a lock held by a
26 * thread that's blocked, and you can't prevent userspace
27 * programs blocking.
28 *
29 * Spinlocks in an operating system kernel make much more sense than
30 * they do in userspace.
31 */
32
33#pragma once
34
35/*
36 * @author Keith Adams <kma@fb.com>
37 * @author Jordan DeLong <delong.j@fb.com>
38 */
39
40#include <array>
41#include <atomic>
42#include <cassert>
43#include <cstdint>
44#include <mutex>
45#include <type_traits>
46
47#include <folly/Portability.h>
48#include <folly/lang/Align.h>
49#include <folly/synchronization/detail/Sleeper.h>
50
51namespace folly {
52
53/*
54 * A really, *really* small spinlock for fine-grained locking of lots
55 * of teeny-tiny data.
56 *
57 * Zero initializing these is guaranteed to be as good as calling
58 * init(), since the free state is guaranteed to be all-bits zero.
59 *
60 * This class should be kept a POD, so we can used it in other packed
61 * structs (gcc does not allow __attribute__((__packed__)) on structs that
62 * contain non-POD data). This means avoid adding a constructor, or
63 * making some members private, etc.
64 */
65struct MicroSpinLock {
66 enum { FREE = 0, LOCKED = 1 };
67 // lock_ can't be std::atomic<> to preserve POD-ness.
68 uint8_t lock_;
69
70 // Initialize this MSL. It is unnecessary to call this if you
71 // zero-initialize the MicroSpinLock.
72 void init() noexcept {
73 payload()->store(FREE);
74 }
75
76 bool try_lock() noexcept {
77 return cas(FREE, LOCKED);
78 }
79
80 void lock() noexcept {
81 detail::Sleeper sleeper;
82 while (!try_lock()) {
83 do {
84 sleeper.wait();
85 } while (payload()->load(std::memory_order_relaxed) == LOCKED);
86 }
87 assert(payload()->load() == LOCKED);
88 }
89
90 void unlock() noexcept {
91 assert(payload()->load() == LOCKED);
92 payload()->store(FREE, std::memory_order_release);
93 }
94
95 private:
96 std::atomic<uint8_t>* payload() noexcept {
97 return reinterpret_cast<std::atomic<uint8_t>*>(&this->lock_);
98 }
99
100 bool cas(uint8_t compare, uint8_t newVal) noexcept {
101 return std::atomic_compare_exchange_strong_explicit(
102 payload(),
103 &compare,
104 newVal,
105 std::memory_order_acquire,
106 std::memory_order_relaxed);
107 }
108};
109static_assert(
110 std::is_pod<MicroSpinLock>::value,
111 "MicroSpinLock must be kept a POD type.");
112
113//////////////////////////////////////////////////////////////////////
114
115/**
116 * Array of spinlocks where each one is padded to prevent false sharing.
117 * Useful for shard-based locking implementations in environments where
118 * contention is unlikely.
119 */
120
121// TODO: generate it from configure (`getconf LEVEL1_DCACHE_LINESIZE`)
122#define FOLLY_CACHE_LINE_SIZE 64
123
124template <class T, size_t N>
125struct alignas(max_align_v) SpinLockArray {
126 T& operator[](size_t i) noexcept {
127 return data_[i].lock;
128 }
129
130 const T& operator[](size_t i) const noexcept {
131 return data_[i].lock;
132 }
133
134 constexpr size_t size() const noexcept {
135 return N;
136 }
137
138 private:
139 struct PaddedSpinLock {
140 PaddedSpinLock() : lock() {}
141 T lock;
142 char padding[FOLLY_CACHE_LINE_SIZE - sizeof(T)];
143 };
144 static_assert(
145 sizeof(PaddedSpinLock) == FOLLY_CACHE_LINE_SIZE,
146 "Invalid size of PaddedSpinLock");
147
148 // Check if T can theoretically cross a cache line.
149 static_assert(
150 max_align_v > 0 && FOLLY_CACHE_LINE_SIZE % max_align_v == 0 &&
151 sizeof(T) <= max_align_v,
152 "T can cross cache line boundaries");
153
154 char padding_[FOLLY_CACHE_LINE_SIZE];
155 std::array<PaddedSpinLock, N> data_;
156};
157
158//////////////////////////////////////////////////////////////////////
159
160typedef std::lock_guard<MicroSpinLock> MSLGuard;
161
162//////////////////////////////////////////////////////////////////////
163
164} // namespace folly
165