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/* Copyright (C) 2009, 2011 Free Software Foundation, Inc.

   Contributed by Richard Henderson <rth@redhat.com>.

   This file is part of the GNU Transactional Memory Library (libitm).

   Libitm is free software; you can redistribute it and/or modify it

   under the terms of the GNU General Public License as published by

   the Free Software Foundation; either version 3 of the License, or

   (at your option) any later version.

   Libitm is distributed in the hope that it will be useful, but WITHOUT ANY

   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS

   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for

   more details.

   Under Section 7 of GPL version 3, you are granted additional

   permissions described in the GCC Runtime Library Exception, version

   3.1, as published by the Free Software Foundation.

   You should have received a copy of the GNU General Public License and

   a copy of the GCC Runtime Library Exception along with this program;

   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

   <http://www.gnu.org/licenses/>.  */

#ifndef GTM_RWLOCK_H

#define GTM_RWLOCK_H

#include <pthread.h>

#include "local_atomic"

namespace GTM HIDDEN {

struct gtm_thread;

// This datastructure is the blocking, mutex-based side of the Dekker-style

// reader-writer lock used to provide mutual exclusion between active and

// serial transactions. It has similarities to POSIX pthread_rwlock_t except

// that we also provide for upgrading a reader->writer lock, with a

// positive indication of failure (another writer acquired the lock

// before we were able to acquire). While the writer flag (a_writer below) is

// global and protected by the mutex, there are per-transaction reader flags,

// which are stored in a transaction's shared state.

// See libitm's documentation for further details.

//

// In this implementation, writers are given highest priority access but

// read-to-write upgrades do not have a higher priority than writers.

class gtm_rwlock

{

  pthread_mutex_t mutex;                // Held if manipulating any field.

  pthread_cond_t c_readers;             // Readers wait here

  pthread_cond_t c_writers;             // Writers wait here for writers

  pthread_cond_t c_confirmed_writers;   // Writers wait here for readers

  static const unsigned a_writer  = 1;  // An active writer.

  static const unsigned w_writer  = 2;  // The w_writers field != 0

  static const unsigned w_reader  = 4;  // The w_readers field != 0

  std::atomic<unsigned int> summary;    // Bitmask of the above.

  unsigned int a_readers;       // Nr active readers as observed by a writer

  unsigned int w_readers;       // Nr waiting readers

  unsigned int w_writers;       // Nr waiting writers

 public:

  gtm_rwlock();

  ~gtm_rwlock();

  void read_lock (gtm_thread *tx);

  void read_unlock (gtm_thread *tx);

  void write_lock ();

  void write_unlock ();

  bool write_upgrade (gtm_thread *tx);

  void write_upgrade_finish (gtm_thread *tx);

 protected:

  bool write_lock_generic (gtm_thread *tx);

};

} // namespace GTM

#endif // GTM_RWLOCK_H