uatomic/x86: Remove redundant memory barriers

[urcu.git] / include / urcu / uatomic / arm.h

diff --git a/include/urcu/uatomic/arm.h b/include/urcu/uatomic/arm.h
index fd8fe6abfc6c22348a354e49db5e0690a609d514..5124a71aa9d4dfda80c41e04500c6706a60c0638 100644 (file)
--- a/include/urcu/uatomic/arm.h
+++ b/include/urcu/uatomic/arm.h
@@ -1,38 +1,44 @@
+// SPDX-FileCopyrightText: 1991-1994 by Xerox Corporation.  All rights reserved.
+// SPDX-FileCopyrightText: 1996-1999 by Silicon Graphics.  All rights reserved.
+// SPDX-FileCopyrightText: 1999-2004 Hewlett-Packard Development Company, L.P.
+// SPDX-FileCopyrightText: 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+// SPDX-FileCopyrightText: 2010 Paul E. McKenney, IBM Corporation
+//
+// SPDX-License-Identifier: LicenseRef-Boehm-GC
+
 #ifndef _URCU_ARCH_UATOMIC_ARM_H
 #define _URCU_ARCH_UATOMIC_ARM_H
 
 /*
  * Atomics for ARM.  This approach is usable on kernels back to 2.6.15.
  *
- * Copyright (c) 1991-1994 by Xerox Corporation.  All rights reserved.
- * Copyright (c) 1996-1999 by Silicon Graphics.  All rights reserved.
- * Copyright (c) 1999-2004 Hewlett-Packard Development Company, L.P.
- * Copyright (c) 2009      Mathieu Desnoyers
- * Copyright (c) 2010      Paul E. McKenney, IBM Corporation
- *                        (Adapted from uatomic_arch_ppc.h)
- *
- * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
- * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
- *
- * Permission is hereby granted to use or copy this program
- * for any purpose,  provided the above notices are retained on all copies.
- * Permission to modify the code and to distribute modified code is granted,
- * provided the above notices are retained, and a notice that the code was
- * modified is included with the above copyright notice.
- *
  * Code inspired from libuatomic_ops-1.2, inherited in part from the
  * Boehm-Demers-Weiser conservative garbage collector.
  */
 
 #include <urcu/compiler.h>
 #include <urcu/system.h>
+#include <urcu/arch.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 /* xchg */
-#define uatomic_xchg(addr, v) __sync_lock_test_and_set(addr, v)
+
+/*
+ * Based on [1], __sync_lock_test_and_set() is not a full barrier, but
+ * instead only an acquire barrier. Given that uatomic_xchg() acts as
+ * both release and acquire barriers, we therefore need to have our own
+ * release barrier before this operation.
+ *
+ * [1] https://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html
+ */
+#define uatomic_xchg(addr, v)                          \
+       ({                                              \
+               cmm_smp_mb();                           \
+               __sync_lock_test_and_set(addr, v);      \
+       })
 
 #ifdef __cplusplus
 }