X-Git-Url: http://git.liburcu.org/?a=blobdiff_plain;f=urcu%2Fuatomic%2Farm.h;h=95f32f3d47b497c79373c7fcbf71bb65c8aa5dda;hb=a4850cc6c0eb0a7a61086eca3076c2ea378ef65c;hp=e0016b8e3153572caac53956df86a5b612481c0c;hpb=bc2433a9853a945a825c244e9ccfe341b73b2ceb;p=userspace-rcu.git

diff --git a/urcu/uatomic/arm.h b/urcu/uatomic/arm.h
index e0016b8..95f32f3 100644
--- a/urcu/uatomic/arm.h
+++ b/urcu/uatomic/arm.h
@@ -1,7 +1,7 @@
 #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.
@@ -26,15 +26,29 @@
 
 #include <urcu/compiler.h>
 #include <urcu/system.h>
+#include <urcu/arch.h>
 
 #ifdef __cplusplus
 extern "C" {
-#endif 
+#endif
 
 /* xchg */
-#define uatomic_xchg(addr, v) __sync_lock_test_and_set(addr, v)
 
-#ifdef __cplusplus 
+/*
+ * 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
 }
 #endif