urcu/arch/generic: Use atomic builtins if configured

[userspace-rcu.git] / include / urcu / arch / generic.h

diff --git a/include/urcu/arch/generic.h b/include/urcu/arch/generic.h
index be6e41e24d88a40f16d07ae08d7d391aeb650821..f9e115f14e190c6da199b6d005d9515641b4be56 100644 (file)
--- a/include/urcu/arch/generic.h
+++ b/include/urcu/arch/generic.h
@@ -1,24 +1,12 @@
+// SPDX-FileCopyrightText: 2010 Paolo Bonzini <pbonzini@redhat.com>
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
 #ifndef _URCU_ARCH_GENERIC_H
 #define _URCU_ARCH_GENERIC_H
 
 /*
  * arch_generic.h: common definitions for multiple architectures.
- *
- * Copyright (c) 2010 Paolo Bonzini <pbonzini@redhat.com>
- *
- * This library is free software; you can redistribute it and/or
- * modify it under the terms of the GNU Lesser General Public
- * License as published by the Free Software Foundation; either
- * version 2.1 of the License, or (at your option) any later version.
- *
- * This library 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
- * Lesser General Public License for more details.
- *
- * You should have received a copy of the GNU Lesser General Public
- * License along with this library; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
 #include <urcu/compiler.h>
@@ -43,6 +31,28 @@ extern "C" {
  * GCC builtins) as well as cmm_rmb and cmm_wmb (defaulting to cmm_mb).
  */
 
+#ifdef CONFIG_RCU_USE_ATOMIC_BUILTINS
+
+# ifndef cmm_smp_mb
+#  define cmm_smp_mb() __atomic_thread_fence(__ATOMIC_SEQ_CST)
+# endif
+
+#endif /* CONFIG_RCU_USE_ATOMIC_BUILTINS */
+
+
+/*
+ * cmm_mb() expands to __sync_synchronize() instead of __atomic_thread_fence
+ * with SEQ_CST because the former "issues a full memory barrier" while the
+ * latter "acts as a synchronization fence between threads" which is too weak
+ * for what we want, for example with I/O devices.
+ *
+ * Even though sync_synchronize seems to be an alias for a sequential consistent
+ * atomic thread fence on every architecture on GCC and Clang, this assumption
+ * might be untrue in future.  Therefore, the definitions above are used to
+ * ensure correct behavior in the future.
+ *
+ * The above defintions are quoted from the GCC manual.
+ */
 #ifndef cmm_mb
 #define cmm_mb()    __sync_synchronize()
 #endif