This is just a hack to compile 2.6.5 kernel + 2.6.4-um1 patch. As Jeff Dike 
put it "you might post that to uml-devel for the impatient" ;), but it's not 
official fix...

Here it goes:
diff -ruN linux/include/asm-i386/system.h uml/linux/include/asm-i386/system.h
--- linux/include/asm-i386/system.h	2004-04-07 23:29:03.000000000 +0200
+++ uml/linux/include/asm-i386/system.h	2004-04-07 23:24:39.000000000 +0200
@@ -243,6 +243,7 @@
 #define __HAVE_ARCH_CMPXCHG 1
 #endif
 
+#ifndef __UM_SYSTEM_I386_H 
 static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 				      unsigned long new, int size)
 {
@@ -270,6 +271,8 @@
 	return old;
 }
 
+#endif
+
 #define cmpxchg(ptr,o,n)\
 	((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\
 					(unsigned long)(n),sizeof(*(ptr))))
--- linux/arch/um/Makefile-i386	2004-04-07 23:29:20.521333000 +0200
+++ uml/linux/arch/um/Makefile-i386	2004-04-07 23:22:31.000000000 +0200
@@ -30,7 +30,7 @@
 $(SYS_DIR)/thread.h: $(SYS_UTIL_DIR)/mk_thread 
 	$(call filechk,$@)
 
-$(SYS_UTIL_DIR)/mk_sc: scripts/fixdep include/config/MARKER FORCE ; 
+$(SYS_UTIL_DIR)/mk_sc: scripts/basic/fixdep include/config/MARKER FORCE ; 
 	$(Q)$(MAKE) $(build)=$(SYS_UTIL_DIR) $@
 
 $(SYS_UTIL_DIR)/mk_thread: $(ARCH_SYMLINKS) $(GEN_HEADERS) sys_prepare 
FORCE ;