@@ -93,4 +93,25 @@ static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref)
return kref_sub(kref, 1, release);
+ * kref_get_unless_zero - Increment refcount for object unless it is zero.
+ * @kref: object.
+ * Return non-zero if the increment succeeded. Otherwise return 0.
+ * This function is intended to simplify locking around refcounting for
+ * objects that can be looked up from a lookup structure, and which are
+ * removed from that lookup structure in the object destructor.
+ * Operations on such objects require at least a read lock around
+ * lookup + kref_get, and a write lock around kref_put + remove from lookup
+ * structure. Furthermore, RCU implementations become extremely tricky.
+ * With a lookup followed by a kref_get_unless_zero *with return value check*
+ * locking in the kref_put path can be deferred to the actual removal from
+ * the lookup structure and RCU lookups become trivial.
+static inline int __must_check kref_get_unless_zero(struct kref *kref)
+ return atomic_add_unless(&kref->refcount, 1, 0);
#endif /* _KREF_H_ */