[ltt-dev] [PATCH 3/3] Add a test for the dmb instruction on ARM

[Jason Wessel]

The dmb instruction is used to implement a memory barrier on modern
ARM hardware.  The older generations of processors which are included
in single processor systems can get by on the generic implementation
of the cmm_mb().  This will allow the user space rcu to work on many
other ARM systems.

Signed-off-by: Jason Wessel <jason.wessel at windriver.com>
---
 configure.ac    |   20 ++++++++++++++++++++
 urcu/arch_arm.h |    2 ++
 2 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/configure.ac b/configure.ac
index 69ce396..3e1b501 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,6 +20,7 @@ AH_TEMPLATE([CONFIG_RCU_HAVE_FENCE], [Defined when on a system that has memory f
 AH_TEMPLATE([CONFIG_RCU_HAVE_FUTEX], [Defined when on a system with futex support.])
 AH_TEMPLATE([CONFIG_RCU_COMPAT_ARCH], [Compatibility mode for i386 which lacks
 cmpxchg instruction.])
+AH_TEMPLATE([CONFIG_ARM_HAVE_DMB], [Use the dmb instruction is available for use on ARM.])
 
 # Checks for programs.
 AC_PROG_CC
@@ -56,6 +57,25 @@ case $host_cpu in
 	*) ARCHTYPE="unknown";;
 esac
 
+if test "x$ARCHTYPE" = "xarm" ; then
+AC_MSG_CHECKING([checking for dmb instruction])
+AC_TRY_COMPILE(
+[
+],
+[
+asm volatile("dmb":::"memory");
+],
+[
+	AC_MSG_RESULT([yes])
+	AC_DEFINE([CONFIG_ARM_HAVE_DMB], [1])
+]
+,
+[
+	AC_MSG_RESULT([no])
+]
+)
+fi
+
 UATOMICSRC=urcu/uatomic_arch_$ARCHTYPE.h
 ARCHSRC=urcu/arch_$ARCHTYPE.h
 if test "x$ARCHTYPE" != xx86 -a "x$ARCHTYPE" != xppc; then
diff --git a/urcu/arch_arm.h b/urcu/arch_arm.h
index e25457d..83cbb60 100644
--- a/urcu/arch_arm.h
+++ b/urcu/arch_arm.h
@@ -29,7 +29,9 @@
 extern "C" {
 #endif 
 
+#ifdef CONFIG_ARM_HAVE_DMB
 #define cmm_mb()    asm volatile("dmb":::"memory")
+#endif /* CONFIG_ARM_HAVE_DMB */
 
 #include <stdlib.h>
 #include <sys/time.h>
-- 
1.7.0