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diff --git a/libitm/config/posix/rwlock.h b/libitm/config/posix/rwlock.h
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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