[PATCH v3 4/5] sh: Emulate two-byte cmpxchg

[Bradley Morgan <brads@mainlining.org>]

SH has no byte or halfword atomic memory operations, so the __cmpxchg()
switch routes case 1 through cmpxchg_emu_u8() and 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 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.

Acked-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/sh/include/asm/cmpxchg.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/sh/include/asm/cmpxchg.h b/arch/sh/include/asm/cmpxchg.h
index 1e5dc5ccf7bf..b87f59107b4e 100644
--- a/arch/sh/include/asm/cmpxchg.h
+++ b/arch/sh/include/asm/cmpxchg.h
@@ -59,6 +59,8 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
 	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);
 	}
@@ -70,8 +72,10 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
   ({									 \
      __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