folly/folly/synchronization/MicroSpinLock.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * N.B. You most likely do _not_ want to use MicroSpinLock or any
 * other kind of spinlock.  Consider MicroLock instead.
 *
 * In short, spinlocks in preemptive multi-tasking operating systems
 * have serious problems and fast mutexes like std::mutex are almost
 * certainly the better choice, because letting the OS scheduler put a
 * thread to sleep is better for system responsiveness and throughput
 * than wasting a timeslice repeatedly querying a lock held by a
 * thread that's blocked, and you can't prevent userspace
 * programs blocking.
 *
 * Spinlocks in an operating system kernel make much more sense than
 * they do in userspace.
 */

#pragma once

#include <array>
#include <atomic>
#include <cassert>
#include <cstdint>
#include <mutex>
#include <type_traits>

#include <folly/Portability.h>
#include <folly/lang/Align.h>
#include <folly/synchronization/SanitizeThread.h>
#include <folly/synchronization/detail/Sleeper.h>

namespace folly {

/*
 * A really, *really* small spinlock for fine-grained locking of lots
 * of teeny-tiny data.
 *
 * Zero initializing these is guaranteed to be as good as calling
 * init(), since the free state is guaranteed to be all-bits zero.
 *
 * This class should be kept a POD, so we can used it in other packed
 * structs (gcc does not allow __attribute__((__packed__)) on structs that
 * contain non-POD data).  This means avoid adding a constructor, or
 * making some members private, etc.
 */
struct MicroSpinLock { … };
static_assert …;

//////////////////////////////////////////////////////////////////////

/**
 * Array of spinlocks where each one is padded to prevent false sharing.
 * Useful for shard-based locking implementations in environments where
 * contention is unlikely.
 */

template <class T, size_t N>
struct alignas(max_align_v) SpinLockArray { … };

//////////////////////////////////////////////////////////////////////

MSLGuard;

//////////////////////////////////////////////////////////////////////

} // namespace folly