2 * Copyright 2016 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>
42 #include <type_traits>
43 #include <boost/noncopyable.hpp>
49 #include <glog/logging.h>
50 #include <folly/detail/Sleeper.h>
51 #include <folly/Portability.h>
56 * A really, *really* small spinlock for fine-grained locking of lots
59 * Zero initializing these is guaranteed to be as good as calling
60 * init(), since the free state is guaranteed to be all-bits zero.
62 * This class should be kept a POD, so we can used it in other packed
63 * structs (gcc does not allow __attribute__((__packed__)) on structs that
64 * contain non-POD data). This means avoid adding a constructor, or
65 * making some members private, etc.
67 struct MicroSpinLock {
68 enum { FREE = 0, LOCKED = 1 };
69 // lock_ can't be std::atomic<> to preserve POD-ness.
72 // Initialize this MSL. It is unnecessary to call this if you
73 // zero-initialize the MicroSpinLock.
75 payload()->store(FREE);
79 return cas(FREE, LOCKED);
83 detail::Sleeper sleeper;
85 while (payload()->load() != FREE) {
88 } while (!try_lock());
89 DCHECK(payload()->load() == LOCKED);
93 CHECK(payload()->load() == LOCKED);
94 payload()->store(FREE, std::memory_order_release);
98 std::atomic<uint8_t>* payload() {
99 return reinterpret_cast<std::atomic<uint8_t>*>(&this->lock_);
102 bool cas(uint8_t compare, uint8_t newVal) {
103 return std::atomic_compare_exchange_strong_explicit(payload(), &compare, newVal,
104 std::memory_order_acquire,
105 std::memory_order_relaxed);
109 std::is_pod<MicroSpinLock>::value,
110 "MicroSpinLock must be kept a POD type.");
112 //////////////////////////////////////////////////////////////////////
115 * Array of spinlocks where each one is padded to prevent false sharing.
116 * Useful for shard-based locking implementations in environments where
117 * contention is unlikely.
120 // TODO: generate it from configure (`getconf LEVEL1_DCACHE_LINESIZE`)
121 #define FOLLY_CACHE_LINE_SIZE 64
123 template <class T, size_t N>
124 struct FOLLY_ALIGNED_MAX SpinLockArray {
125 T& operator[](size_t i) {
126 return data_[i].lock;
129 const T& operator[](size_t i) const {
130 return data_[i].lock;
133 constexpr size_t size() const { return N; }
136 struct PaddedSpinLock {
137 PaddedSpinLock() : lock() {}
139 char padding[FOLLY_CACHE_LINE_SIZE - sizeof(T)];
141 static_assert(sizeof(PaddedSpinLock) == FOLLY_CACHE_LINE_SIZE,
142 "Invalid size of PaddedSpinLock");
144 // Check if T can theoretically cross a cache line.
145 static_assert(alignof(std::max_align_t) > 0 &&
146 FOLLY_CACHE_LINE_SIZE % alignof(std::max_align_t) == 0 &&
147 sizeof(T) <= alignof(std::max_align_t),
148 "T can cross cache line boundaries");
150 char padding_[FOLLY_CACHE_LINE_SIZE];
151 std::array<PaddedSpinLock, N> data_;
154 //////////////////////////////////////////////////////////////////////
156 typedef std::lock_guard<MicroSpinLock> MSLGuard;
158 //////////////////////////////////////////////////////////////////////