rocksdb/util/mutexlock.h

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#pragma once
#include <assert.h>
#include <atomic>
#include <mutex>
#include <thread>
#include "port/port.h"

namespace rocksdb {

// Helper class that locks a mutex on construction and unlocks the mutex when
// the destructor of the MutexLock object is invoked.
//
// Typical usage:
//
//   void MyClass::MyMethod() {
//     MutexLock l(&mu_);       // mu_ is an instance variable
//     ... some complex code, possibly with multiple return paths ...
//   }

class MutexLock {
 public:
  explicit MutexLock(port::Mutex *mu) : mu_(mu) {
    this->mu_->Lock();
  }
  ~MutexLock() { this->mu_->Unlock(); }

 private:
  port::Mutex *const mu_;
  // No copying allowed
  MutexLock(const MutexLock&);
  void operator=(const MutexLock&);
};

//
// Acquire a ReadLock on the specified RWMutex.
// The Lock will be automatically released then the
// object goes out of scope.
//
class ReadLock {
 public:
  explicit ReadLock(port::RWMutex *mu) : mu_(mu) {
    this->mu_->ReadLock();
  }
  ~ReadLock() { this->mu_->ReadUnlock(); }

 private:
  port::RWMutex *const mu_;
  // No copying allowed
  ReadLock(const ReadLock&);
  void operator=(const ReadLock&);
};

//
// Automatically unlock a locked mutex when the object is destroyed
//
class ReadUnlock {
 public:
  explicit ReadUnlock(port::RWMutex *mu) : mu_(mu) { mu->AssertHeld(); }
  ~ReadUnlock() { mu_->ReadUnlock(); }

 private:
  port::RWMutex *const mu_;
  // No copying allowed
  ReadUnlock(const ReadUnlock &) = delete;
  ReadUnlock &operator=(const ReadUnlock &) = delete;
};

//
// Acquire a WriteLock on the specified RWMutex.
// The Lock will be automatically released then the
// object goes out of scope.
//
class WriteLock {
 public:
  explicit WriteLock(port::RWMutex *mu) : mu_(mu) {
    this->mu_->WriteLock();
  }
  ~WriteLock() { this->mu_->WriteUnlock(); }

 private:
  port::RWMutex *const mu_;
  // No copying allowed
  WriteLock(const WriteLock&);
  void operator=(const WriteLock&);
};

//
// SpinMutex has very low overhead for low-contention cases.  Method names
// are chosen so you can use std::unique_lock or std::lock_guard with it.
//
class SpinMutex {
 public:
  SpinMutex() : locked_(false) {}

  bool try_lock() {
    auto currently_locked = locked_.load(std::memory_order_relaxed);
    return !currently_locked &&
           locked_.compare_exchange_weak(currently_locked, true,
                                         std::memory_order_acquire,
                                         std::memory_order_relaxed);
  }

  void lock() {
    for (size_t tries = 0;; ++tries) {
      if (try_lock()) {
        // success
        break;
      }
      port::AsmVolatilePause();
      if (tries > 100) {
        std::this_thread::yield();
      }
    }
  }

  void unlock() { locked_.store(false, std::memory_order_release); }

 private:
  std::atomic<bool> locked_;
};

}  // namespace rocksdb