From b8d0a94533e8a45f425899b66391367c86b375e7 Mon Sep 17 00:00:00 2001
From: Thomas Hellstrom <thellstrom@vmware.com>
Date: Tue, 6 Nov 2012 11:31:49 +0000
Subject: [PATCH] --- yaml --- r: 345343 b: refs/heads/master c:
 4b20db3de8dab005b07c74161cb041db8c5ff3a7 h: refs/heads/master i:   345341:
 9e5f65d077e9d8359a0c18430a2829c17751fa03   345339:
 1448aa2c8df4b061323c160d57977595e3b5b99e   345335:
 de73c17cc1f3128989273c3693a24205120aaa37   345327:
 ed083f872aee8d59e40221acec116162eda8a966   345311:
 f651214afe5ce26de4936c66e5061977040148f5   345279:
 461a9b44a72a6768103f8bd00c56e18d11bd8d43   345215:
 3baeb372d98ea7aaa1dbb7c18e76bd685f31c4ca   345087:
 b8c0a9506e5aeb843fb076c5da60f80ba12cf217 v: v3

---
 [refs]                     |  2 +-
 trunk/include/linux/kref.h | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/[refs] b/[refs]
index d60ae80d53f9..7fb9da71825a 100644
--- 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
index 65af6887872f..4972e6e9ca93 100644
--- 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_ */