[PATCH 2/3][RFC] atomic_cmpxchg, atomic_inc_not_zero

[Nick Piggin <nickpiggin@yahoo.com.au>]

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2/3 is a sample atomic_inc_not_zero for ppc64 implemented with
atomic_cmpxchg. I couldn't be bothered to go through the motions
without a first round of comments...

Ideally, ppc64 will implement this with ll/sc instructions and
hopefully shave a cycle or so. But anyone who can implement
atomic_cmpxchg can implement atomic_inc_not_zero as demonstrated.

-- 
SUSE Labs, Novell Inc.


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Index: linux-2.6/include/asm-ppc64/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-ppc64/atomic.h
+++ linux-2.6/include/asm-ppc64/atomic.h
@@ -164,6 +164,23 @@ static __inline__ int atomic_dec_return(
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+/**
+ * atomic_inc_not_zero - increment if not zero
+ * @v: pointer of type atomic_t
+ *
+ * Atomically increments @v by 1, so long as it was not 0.
+ * Returns non-zero on successful increment and zero otherwise.
+ */
+#define atomic_inc_not_zero(v)					\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c && (old = atomic_cmpxchg((v), c, c + 1)) != c)	\
+		c = old;					\
+	c;							\
+})
+
+
 #define atomic_sub_and_test(a, v)	(atomic_sub_return((a), (v)) == 0)
 #define atomic_dec_and_test(v)		(atomic_dec_return((v)) == 0)