mutex.h (gomp_mutex_unlock): Add comment about needed memory barrier semantics.

2009-08-04  David Daney  <ddaney@caviumnetworks.com>

	* config/linux/mutex.h (gomp_mutex_unlock): Add comment about
	needed memory barrier semantics.
	* config/linux/mips/mutex.h: New file.

From-SVN: r150463

libgomp/ChangeLog

+2009-08-04  David Daney  <ddaney@caviumnetworks.com>
+
+	* config/linux/mutex.h (gomp_mutex_unlock): Add comment about
+	needed memory barrier semantics.
+	* config/linux/mips/mutex.h: New file.
+
 2009-07-30  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>
 
 	* configure.ac (_AC_ARG_VAR_PRECIOUS): Use m4_rename_force.

libgomp/config/linux/mips/mutex.h

+/* Copyright (C) 2009 Free Software Foundation, Inc.
+
+   This file is part of the GNU OpenMP Library (libgomp).
+
+   Libgomp 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, or (at your option)
+   any later version.
+
+   Libgomp 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/>.  */
+
+/* MIPS needs the same correct usage of __sync_synchronize and
+   __sync_lock_test_and_set as ia64.  So we just use its mutex.h.  */
+
+#include "config/linux/ia64/mutex.h"

libgomp/config/linux/mutex.h

@@ -48,6 +48,16 @@ static inline void gomp_mutex_lock (gomp_mutex_t *mutex)
 extern void gomp_mutex_unlock_slow (gomp_mutex_t *mutex);
 static inline void gomp_mutex_unlock (gomp_mutex_t *mutex)
 {
+  /* Warning: By definition __sync_lock_test_and_set() does not have
+     proper memory barrier semantics for a mutex unlock operation.
+     However, this default implementation is written assuming that it
+     does, which is true for some targets.
+
+     Targets that require additional memory barriers before
+     __sync_lock_test_and_set to achieve the release semantics of
+     mutex unlock, are encouraged to include
+     "config/linux/ia64/mutex.h" in a target specific mutex.h instead
+     of using this file.  */
   int val = __sync_lock_test_and_set (mutex, 0);
   if (__builtin_expect (val > 1, 0))
     gomp_mutex_unlock_slow (mutex);