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>

2 files changed, 7 insertions, 4 deletions

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 4972e6e9ca93..e15828fd71f1 100644
--- a/include/linux/kref.h
+++ b/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);
 }
 
 /**
@@ -100,7 +103,7 @@ static inline int kref_put_mutex(struct kref *kref,
                                  struct mutex *lock)
 {
         WARN_ON(release == NULL);
-        if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
+        if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
                 mutex_lock(lock);
                 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
                         mutex_unlock(lock);
diff --git a/lib/kobject.c b/lib/kobject.c
index a65486613d79..b7e29a6056d3 100644
--- a/lib/kobject.c
+++ b/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;