arm64: fix unreachable code issue with cmpxchg

On arm64 build with clang, sometimes the __cmpxchg_mb is not inlined
when CONFIG_OPTIMIZE_INLINING is set.
Clang then fails a compile-time assertion, because it cannot tell at
compile time what the size of the argument is:

mm/memcontrol.o: In function `__cmpxchg_mb':
memcontrol.c:(.text+0x1a4c): undefined reference to `__compiletime_assert_175'
memcontrol.c:(.text+0x1a4c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `__compiletime_assert_175'

Mark all of the cmpxchg() style functions as __always_inline to
ensure that the compiler can see the result.
Acked-by: NNick Desaulniers <ndesaulniers@google.com>
Reported-by: NNathan Chancellor <natechancellor@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/648Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Tested-by: NNathan Chancellor <natechancellor@gmail.com>
Reviewed-by: NAndrew Murray <andrew.murray@arm.com>
Tested-by: NAndrew Murray <andrew.murray@arm.com>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NWill Deacon <will@kernel.org>

arch/arm64/include/asm/cmpxchg.h

@@ -62,7 +62,7 @@ __XCHG_CASE( ,  ,  mb_, 64, dmb ish, nop,  , a, l, "memory")
 #undef __XCHG_CASE
 
 #define __XCHG_GEN(sfx)							\
-static inline unsigned long __xchg##sfx(unsigned long x,		\
+static __always_inline  unsigned long __xchg##sfx(unsigned long x,	\
 					volatile void *ptr,		\
 					int size)			\
 {									\
@@ -148,7 +148,7 @@ __CMPXCHG_DBL(_mb)
 #undef __CMPXCHG_DBL
 
 #define __CMPXCHG_GEN(sfx)						\
-static inline unsigned long __cmpxchg##sfx(volatile void *ptr,		\
+static __always_inline unsigned long __cmpxchg##sfx(volatile void *ptr,	\
 					   unsigned long old,		\
 					   unsigned long new,		\
 					   int size)			\
@@ -255,7 +255,7 @@ __CMPWAIT_CASE( ,  , 64);
 #undef __CMPWAIT_CASE
 
 #define __CMPWAIT_GEN(sfx)						\
-static inline void __cmpwait##sfx(volatile void *ptr,			\
+static __always_inline void __cmpwait##sfx(volatile void *ptr,		\
 				  unsigned long val,			\
 				  int size)				\
 {									\