[ 3.5.y.z extended stable ] Patch "kref: Implement kref_get_unless_zero v3" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    kref: Implement kref_get_unless_zero v3

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From a19781fe5f90f97da1361ae8d832fb983a536d52 Mon Sep 17 00:00:00 2001
From: Thomas Hellstrom <thellstrom at vmware.com>
Date: Tue, 6 Nov 2012 11:31:49 +0000
Subject: [PATCH] kref: Implement kref_get_unless_zero v3

commit 4b20db3de8dab005b07c74161cb041db8c5ff3a7 upstream.

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.

v2: Formatting fixes.
v3: Invert the return value.

Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
Signed-off-by: Dave Airlie <airlied at redhat.com>
[ luis: adjust context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 include/linux/kref.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 9c07dce..fd2a616 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -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_ */
--
1.8.1.2