x86: Add support for cmpxchg_double

A simple implementation that only supports the word size and does not
have a fallback mode (would require a spinlock).

Add 32 and 64 bit support for cmpxchg_double. cmpxchg double uses
the cmpxchg8b or cmpxchg16b instruction on x86 processors to compare
and swap 2 machine words. This allows lockless algorithms to move more
context information through critical sections.

Set a flag CONFIG_CMPXCHG_DOUBLE to signal that support for double word
cmpxchg detection has been build into the kernel. Note that each subsystem
using cmpxchg_double has to implement a fall back mechanism as long as
we offer support for processors that do not implement cmpxchg_double.

Reviewed-by: H. Peter Anvin <hpa@zytor.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Christoph Lameter <cl@linux.com>
Link: http://lkml.kernel.org/r/20110601172614.173427964@linux.com
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index 6a7cfdf8ff69..e3ca7e0d858c 100644
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -312,6 +312,9 @@ config X86_CMPXCHG
 config CMPXCHG_LOCAL
         def_bool X86_64 || (X86_32 && !M386)
 
+config CMPXCHG_DOUBLE
+        def_bool y
+
 config X86_L1_CACHE_SHIFT
         int
         default "7" if MPENTIUM4 || MPSC