kref: minor cleanup

 - make warning smp-safe
 - result of atomic _unless_zero functions should be checked by caller
   to avoid use-after-free error
 - trivial whitespace fix.

Link: https://lkml.org/lkml/2013/4/12/391

Tested: compile x86, boot machine and run xfstests
Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
[ Removed line-break, changed to use WARN_ON_ONCE()  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kref.h

@@ -39,8 +39,11 @@ static inline void kref_init(struct kref *kref)
  */
 static inline void kref_get(struct kref *kref)
 {
-	WARN_ON(!atomic_read(&kref->refcount));
-	atomic_inc(&kref->refcount);
+	/* If refcount was 0 before incrementing then we have a race
+	 * condition when this kref is freeing by some other thread right now.
+	 * In this case one should use kref_get_unless_zero()
+	 */
+	WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
 }
 
 /**

lib/kobject.c

@@ -529,7 +529,7 @@ struct kobject *kobject_get(struct kobject *kobj)
 	return kobj;
 }
 
-static struct kobject *kobject_get_unless_zero(struct kobject *kobj)
+static struct kobject * __must_check kobject_get_unless_zero(struct kobject *kobj)
 {
 	if (!kref_get_unless_zero(&kobj->kref))
 		kobj = NULL;