atomic: implement generic atomic_dec_if_positive()

The x86 implementation of atomic_dec_if_positive is quite generic, so make
it available to all architectures.

This is needed for "swap: add a simple detector for inappropriate swapin
readahead".

[[e-mail]: do the "#define foo foo" trick in the conventional manner]
Signed-off-by: Shaohua Li <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Michal Simek <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/microblaze/include/asm/atomic.h

···
 
 	return res;
 }
+#define atomic_dec_if_positive atomic_dec_if_positive
 
 #endif /* _ASM_MICROBLAZE_ATOMIC_H */

arch/powerpc/include/asm/atomic.h

···
 
 	return t;
 }
+#define atomic_dec_if_positive atomic_dec_if_positive
 
 #define smp_mb__before_atomic_dec()     smp_mb()
 #define smp_mb__after_atomic_dec()      smp_mb()

arch/x86/include/asm/atomic.h

···
 	return c;
 }
 
-
-/*
- * atomic_dec_if_positive - decrement by 1 if old value positive
- * @v: pointer of type atomic_t
- *
- * The function returns the old value of *v minus 1, even if
- * the atomic variable, v, was not decremented.
- */
-static inline int atomic_dec_if_positive(atomic_t *v)
-{
-	int c, old, dec;
-	c = atomic_read(v);
-	for (;;) {
-		dec = c - 1;
-		if (unlikely(dec < 0))
-			break;
-		old = atomic_cmpxchg((v), c, dec);
-		if (likely(old == c))
-			break;
-		c = old;
-	}
-	return dec;
-}
-
 /**
  * atomic_inc_short - increment of a short integer
  * @v: pointer to type int

include/linux/atomic.h

···
 }
 #endif
 
+/*
+ * atomic_dec_if_positive - decrement by 1 if old value positive
+ * @v: pointer of type atomic_t
+ *
+ * The function returns the old value of *v minus 1, even if
+ * the atomic variable, v, was not decremented.
+ */
+#ifndef atomic_dec_if_positive
+static inline int atomic_dec_if_positive(atomic_t *v)
+{
+	int c, old, dec;
+	c = atomic_read(v);
+	for (;;) {
+		dec = c - 1;
+		if (unlikely(dec < 0))
+			break;
+		old = atomic_cmpxchg((v), c, dec);
+		if (likely(old == c))
+			break;
+		c = old;
+	}
+	return dec;
+}
+#endif
+
 #ifndef CONFIG_ARCH_HAS_ATOMIC_OR
 static inline void atomic_or(int i, atomic_t *v)
 {