x86: asm -- Add cmpxchg helpers

Will need it for atomic data conversion.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

arch/x86/include/asm/cmpxchg.h

+#ifndef __CR_CMPXCHG_H__
+#define __CR_CMPXCHG_H__
+
+#define __X86_CASE_B	1
+#define __X86_CASE_W	2
+#define __X86_CASE_L	4
+#define __X86_CASE_Q	8
+
+/*
+ * An exchange-type operation, which takes a value and a pointer, and
+ * returns the old value. Make sure you never reach non-case statement
+ * here, otherwise behaviour is undefined.
+ */
+#define __xchg_op(ptr, arg, op, lock)					\
+	({								\
+	        __typeof__ (*(ptr)) __ret = (arg);			\
+		switch (sizeof(*(ptr))) {				\
+		case __X86_CASE_B:					\
+			asm volatile (lock #op "b %b0, %1\n"		\
+				      : "+q" (__ret), "+m" (*(ptr))	\
+				      : : "memory", "cc");		\
+			break;						\
+		case __X86_CASE_W:					\
+			asm volatile (lock #op "w %w0, %1\n"		\
+				      : "+r" (__ret), "+m" (*(ptr))	\
+				      : : "memory", "cc");		\
+			break;						\
+		case __X86_CASE_L:					\
+			asm volatile (lock #op "l %0, %1\n"		\
+				      : "+r" (__ret), "+m" (*(ptr))	\
+				      : : "memory", "cc");		\
+			break;						\
+		case __X86_CASE_Q:					\
+			asm volatile (lock #op "q %q0, %1\n"		\
+				      : "+r" (__ret), "+m" (*(ptr))	\
+				      : : "memory", "cc");		\
+			break;						\
+		}							\
+		__ret;							\
+	})
+
+#define __xadd(ptr, inc, lock)	__xchg_op((ptr), (inc), xadd, lock)
+#define xadd(ptr, inc)		__xadd((ptr), (inc), "lock ;")
+
+#endif /* __CR_CMPXCHG_H__ */