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diff --git a/libitm/config/posix/rwlock.h b/libitm/config/posix/rwlock.h
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+++ b/libitm/config/posix/rwlock.h
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+/* Copyright (C) 2009 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>
+
+namespace GTM HIDDEN {
+
+// This datastructure is similar to the 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).
+//
+// In this implementation, rw upgrade is given highest priority access,
+// and writers are given priority over readers.
+
+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
+  pthread_cond_t c_upgrade;	// An upgrader waits here
+
+  static const unsigned w_upgrade  = 1;		// A reader waiting for upgrade
+  static const unsigned a_writer   = 2;		// An active writer.
+  static const unsigned w_writer   = 4;		// The w_writers field != 0
+  static const unsigned a_reader   = 8;		// The a_readers field != 0
+  static const unsigned w_reader   = 16;	// The w_readers field != 0
+
+  unsigned int summary;		// Bitmask of the above.
+  unsigned int a_readers;	// Nr active readers
+  unsigned int w_readers;	// Nr waiting readers
+  unsigned int w_writers;	// Nr waiting writers
+
+ public:
+  gtm_rwlock();
+  ~gtm_rwlock();
+
+  void read_lock ();
+  void read_unlock ();
+
+  void write_lock ();
+  void write_unlock ();
+
+  bool write_upgrade ();
+};
+
+} // namespace GTM
+
+#endif // GTM_RWLOCK_H