2 * Copyright 2017 Facebook, Inc.
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
8 * http://www.apache.org/licenses/LICENSE-2.0
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.
18 * N.B. You most likely do _not_ want to use MicroSpinLock or any
19 * other kind of spinlock. Consider MicroLock instead.
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
29 * Spinlocks in an operating system kernel make much more sense than
30 * they do in userspace.
36 * @author Keith Adams <kma@fb.com>
37 * @author Jordan DeLong <delong.j@fb.com>
45 #include <type_traits>
47 #include <folly/Portability.h>
48 #include <folly/lang/Align.h>
49 #include <folly/synchronization/detail/Sleeper.h>
54 * A really, *really* small spinlock for fine-grained locking of lots
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.
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.
65 struct MicroSpinLock {
66 enum { FREE = 0, LOCKED = 1 };
67 // lock_ can't be std::atomic<> to preserve POD-ness.
70 // Initialize this MSL. It is unnecessary to call this if you
71 // zero-initialize the MicroSpinLock.
73 payload()->store(FREE);
77 return cas(FREE, LOCKED);
81 detail::Sleeper sleeper;
83 while (payload()->load() != FREE) {
86 } while (!try_lock());
87 assert(payload()->load() == LOCKED);
91 assert(payload()->load() == LOCKED);
92 payload()->store(FREE, std::memory_order_release);
96 std::atomic<uint8_t>* payload() {
97 return reinterpret_cast<std::atomic<uint8_t>*>(&this->lock_);
100 bool cas(uint8_t compare, uint8_t newVal) {
101 return std::atomic_compare_exchange_strong_explicit(payload(), &compare, newVal,
102 std::memory_order_acquire,
103 std::memory_order_relaxed);
107 std::is_pod<MicroSpinLock>::value,
108 "MicroSpinLock must be kept a POD type.");
110 //////////////////////////////////////////////////////////////////////
113 * Array of spinlocks where each one is padded to prevent false sharing.
114 * Useful for shard-based locking implementations in environments where
115 * contention is unlikely.
118 // TODO: generate it from configure (`getconf LEVEL1_DCACHE_LINESIZE`)
119 #define FOLLY_CACHE_LINE_SIZE 64
121 template <class T, size_t N>
122 struct alignas(max_align_v) SpinLockArray {
123 T& operator[](size_t i) {
124 return data_[i].lock;
127 const T& operator[](size_t i) const {
128 return data_[i].lock;
131 constexpr size_t size() const { return N; }
134 struct PaddedSpinLock {
135 PaddedSpinLock() : lock() {}
137 char padding[FOLLY_CACHE_LINE_SIZE - sizeof(T)];
139 static_assert(sizeof(PaddedSpinLock) == FOLLY_CACHE_LINE_SIZE,
140 "Invalid size of PaddedSpinLock");
142 // Check if T can theoretically cross a cache line.
144 max_align_v > 0 && FOLLY_CACHE_LINE_SIZE % max_align_v == 0 &&
145 sizeof(T) <= max_align_v,
146 "T can cross cache line boundaries");
148 char padding_[FOLLY_CACHE_LINE_SIZE];
149 std::array<PaddedSpinLock, N> data_;
152 //////////////////////////////////////////////////////////////////////
154 typedef std::lock_guard<MicroSpinLock> MSLGuard;
156 //////////////////////////////////////////////////////////////////////