percpu_counter: change return value and add comments

The percpu_counter_*_positive() API in UP case doesn't check if return
value is positive.  Add comments to explain why we don't.  Also if count <
0, returns 0 instead of 1 for *read_positive().

[akpm@linux-foundation.org: tweak comment]
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Shaohua Li

include/linux/percpu_counter.h

@@ -75,7 +75,7 @@ static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
 	barrier();		/* Prevent reloads of fbc->count */
 	if (ret >= 0)
 		return ret;
-	return 1;
+	return 0;
 }
 
 static inline int percpu_counter_initialized(struct percpu_counter *fbc)
@@ -133,6 +133,10 @@ static inline s64 percpu_counter_read(struct percpu_counter *fbc)
 	return fbc->count;
 }
 
+/*
+ * percpu_counter is intended to track positive numbers. In the UP case the
+ * number should never be negative.
+ */
 static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
 {
 	return fbc->count;