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yaml --- r: 345343 b: refs/heads/master c: 4b20db3 h: refs/heads/master i: 345341: 9e5f65d 345339: 1448aa2 345335: de73c17 345327: ed083f8 345311: f651214 345279: 461a9b4 345215: 3baeb37 345087: b8c0a95 v: v3
git-mirror · Nov 20, 2012 · b8d0a94 · b8d0a94
1 parent c532b24
commit b8d0a94
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Showing 2 changed files with 22 additions and 1 deletion.
diff --git a/[refs] b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: d7144556195ab04a148e722a93103da5438bdf78
+refs/heads/master: 4b20db3de8dab005b07c74161cb041db8c5ff3a7
diff --git a/trunk/include/linux/kref.h b/trunk/include/linux/kref.h
@@ -111,4 +111,25 @@ static inline int kref_put_mutex(struct kref *kref,
 	}
 	return 0;
 }
+
+/**
+ * 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_ */