SpinLock.h

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/*
 * Copyright 2014-present Facebook, Inc.
 *
 * 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 SpinLock or any other
 * kind of spinlock.  Use std::mutex 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 <type_traits>

#include <boost/noncopyable.hpp>

#include <folly/Portability.h>
#include <folly/synchronization/SmallLocks.h>

namespace folly {

class SpinLock {
 public:
  FOLLY_ALWAYS_INLINE SpinLock() noexcept {
    lock_.init();
  }
  FOLLY_ALWAYS_INLINE void lock() const noexcept {
    lock_.lock();
  }
  FOLLY_ALWAYS_INLINE void unlock() const noexcept {
    lock_.unlock();
  }
  FOLLY_ALWAYS_INLINE bool try_lock() const noexcept {
    return lock_.try_lock();
  }

 private:
  mutable folly::MicroSpinLock lock_;
};

template <typename LOCK>
class SpinLockGuardImpl : private boost::noncopyable {
 public:
  FOLLY_ALWAYS_INLINE explicit SpinLockGuardImpl(LOCK& lock) noexcept(
      noexcept(lock.lock()))
      : lock_(lock) {
    lock_.lock();
  }
  FOLLY_ALWAYS_INLINE ~SpinLockGuardImpl() {
    lock_.unlock();
  }

 private:
  LOCK& lock_;
};

typedef SpinLockGuardImpl<SpinLock> SpinLockGuard;

} // namespace folly