[PATCH 5/5] xtensa: Emulate two-byte cmpxchg

[Bradley Morgan <brads@mainlining.org>]

Xtensa has no two-byte atomic compare and swap, so the __cmpxchg()
switch lets case 2 fall through to __cmpxchg_called_with_bad_pointer(),
which is declared but never defined, so a two-byte cmpxchg() fails at
link time. Route case 2 through the new cmpxchg_emu_u16(), which takes
the old and new values as unsigned long, so the (unsigned long) casts
on _o_ and _n_ move off the call and into _old_ and _new_ declarations
that type check the old and new arguments against *ptr through
(unsigned long)(0 ? *ptr : (o)), the idiom David Laight suggested,
so cmpxchg(&p, 4, 5) no longer compiles silently.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/xtensa/include/asm/cmpxchg.h | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/xtensa/include/asm/cmpxchg.h b/arch/xtensa/include/asm/cmpxchg.h
index b6db4838b175..f6bed4221b43 100644
--- a/arch/xtensa/include/asm/cmpxchg.h
+++ b/arch/xtensa/include/asm/cmpxchg.h
@@ -76,6 +76,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
 {
 	switch (size) {
 	case 1:  return cmpxchg_emu_u8(ptr, old, new);
+	case 2:  return cmpxchg_emu_u16(ptr, old, new);
 	case 4:  return __cmpxchg_u32(ptr, old, new);
 	default: __cmpxchg_called_with_bad_pointer();
 		 return old;
@@ -83,10 +84,10 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
 }
 
 #define arch_cmpxchg(ptr,o,n)						      \
-	({ __typeof__(*(ptr)) _o_ = (o);				      \
-	   __typeof__(*(ptr)) _n_ = (n);				      \
-	   (__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_,	      \
-	   			        (unsigned long)_n_, sizeof (*(ptr))); \
+	({ unsigned long _old_ = (unsigned long)(0 ? *(ptr) : (o));	      \
+	unsigned long _new_ = (unsigned long)(0 ? *(ptr) : (n));	      \
+	(__typeof__(*(ptr))) __cmpxchg((ptr), _old_,		      \
+				_new_, sizeof (*(ptr))); \
 	})
 
 #include <asm-generic/cmpxchg-local.h>
-- 
2.47.3