arangodb/lib/Basics/DeadlockDetector.h

////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2016 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// 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.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Jan Steemann
////////////////////////////////////////////////////////////////////////////////

#ifndef LIB_BASICS_DEADLOCK_DETECTOR_H
#define LIB_BASICS_DEADLOCK_DETECTOR_H 1

#include "Basics/Common.h"
#include "Basics/Mutex.h"
#include "Basics/MutexLocker.h"
#include "Basics/threads.h"

namespace triagens {
namespace basics {


template <typename T>
class DeadlockDetector {
  
 public:
  DeadlockDetector() = default;
  ~DeadlockDetector() = default;

  DeadlockDetector(DeadlockDetector const&) = delete;
  DeadlockDetector& operator=(DeadlockDetector const&) = delete;

  
 public:
  bool isDeadlocked(T const* value) {
    auto tid = TRI_CurrentThreadId();
    std::unordered_set<TRI_tid_t> watchFor({tid});

    std::vector<TRI_tid_t> stack;

    TRI_tid_t writerTid = value->getCurrentWriterThread();

    if (writerTid == 0) {
      return false;
    }

    stack.push_back(writerTid);

    MUTEX_LOCKER(_readersLock);

    while (!stack.empty()) {
      TRI_tid_t current = stack.back();
      stack.pop_back();

      watchFor.emplace(current);
      auto it2 = _readersBlocked.find(current);

      if (it2 == _readersBlocked.end()) {
        return false;
      }

      if (watchFor.find((*it2).second) != watchFor.end()) {
        // deadlock!
        return true;
      }

      stack.push_back((*it2).second);
    }

    // no deadlock found
    return false;
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief insert a reader into the list of blocked readers
  /// returns true if a deadlock was detected and false otherwise
  //////////////////////////////////////////////////////////////////////////////

  bool setReaderBlocked(T const* value) {
    auto tid = TRI_CurrentThreadId();
    std::unordered_set<TRI_tid_t> watchFor({tid});

    std::vector<TRI_tid_t> stack;

    TRI_tid_t writerTid = value->getCurrentWriterThread();

    if (writerTid == 0) {
      return false;
    }

    stack.push_back(writerTid);

    MUTEX_LOCKER(_readersLock);
    _readersBlocked.emplace(tid, writerTid);

    try {
      while (!stack.empty()) {
        TRI_tid_t current = stack.back();
        stack.pop_back();

        watchFor.emplace(current);
        auto it2 = _readersBlocked.find(current);

        if (it2 == _readersBlocked.end()) {
          return false;
        }

        if (watchFor.find((*it2).second) != watchFor.end()) {
          // deadlock!
          _readersBlocked.erase(tid);
          return true;
        }

        stack.push_back((*it2).second);
      }

      // no deadlock found
      return false;
    } catch (...) {
      // clean up and re-throw
      _readersBlocked.erase(tid);
      throw;
    }
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief remove a reader from the list of blocked readers
  //////////////////////////////////////////////////////////////////////////////

  void setReaderUnblocked(T const* value) noexcept {
    auto tid = TRI_CurrentThreadId();

    try {
      MUTEX_LOCKER(_readersLock);
      _readersBlocked.erase(tid);
    } catch (...) {
    }
  }

  
 private:
  //////////////////////////////////////////////////////////////////////////////
  /// @brief lock for managing the readers
  //////////////////////////////////////////////////////////////////////////////

  triagens::basics::Mutex _readersLock;

  //////////////////////////////////////////////////////////////////////////////
  /// @brief readers that are blocked on writers
  //////////////////////////////////////////////////////////////////////////////

  std::unordered_map<TRI_tid_t, TRI_tid_t> _readersBlocked;
};

}  // namespace triagens::basics
}  // namespace triagens

#endif