[PATCH] atomic_cmpxchg return type error

[PATCH] atomic_cmpxchg return type error

[Mathieu Desnoyers]

Hi,

I just noticed that a atomic_cmpxchg, that would be given an atomic64_t
parameter, would cast the return value as a (int). In the typical use of this
primitive, the result would be that the 32 MSB would be lost when comparing
against the original value. It also affects atomic_add_unless. Note that there
is no atomic64_cmpxchg nor atomic64_add_unless, which might make things a
little clearer.

Here is a quick fix for this against 2.6.18.

Regards,

Mathieu

-- BEGIN --
--- a/include/asm-x86_64/atomic.h
+++ b/include/asm-x86_64/atomic.h
@@ -388,7 +388,8 @@ static __inline__ long atomic64_sub_retu
 #define atomic64_inc_return(v)  (atomic64_add_return(1,v))
 #define atomic64_dec_return(v)  (atomic64_sub_return(1,v))
 
-#define atomic_cmpxchg(v, old, new) ((int)cmpxchg(&((v)->counter), old, new))
+#define atomic_cmpxchg(v, old, new) \
+	((__typeof__((v)->counter))cmpxchg(&((v)->counter), old, new))
 #define atomic_xchg(v, new) (xchg(&((v)->counter), new))
 
 /**
@@ -402,7 +403,7 @@ #define atomic_xchg(v, new) (xchg(&((v)-
  */
 #define atomic_add_unless(v, a, u)				\
 ({								\
-	int c, old;						\
+	__typeof__((v)->counter) c, old;			\
 	c = atomic_read(v);					\
 	for (;;) {						\
 		if (unlikely(c == (u)))				\
-- END --


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Re: [PATCH] atomic_cmpxchg return type error

[Andi Kleen]

On Thu, Nov 30, 2006 at 04:17:06PM -0500, Mathieu Desnoyers wrote:
> Hi,
> 
> I just noticed that a atomic_cmpxchg, that would be given an atomic64_t
> parameter, would cast the return value as a (int). In the typical use of this
> primitive, the result would be that the 32 MSB would be lost when comparing
> against the original value. It also affects atomic_add_unless. Note that there
> is no atomic64_cmpxchg nor atomic64_add_unless, which might make things a
> little clearer.

Normally you're supposed to only use atomic64_* with atomic64_t

While it works for most by accident on x86 to just use atomic_* 
with atomic64_t it's not portable to other architectures.

If you want to use those with atomic64_t suitable macros would need
to be added on all architectures.

-Andi