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authorThomas Hellstrom <thellstrom@vmware.com>2012-11-06 11:31:49 +0000
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-04-16 21:27:26 -0700
commite3a55052f4773105dbd23f72dec4aeac82dea871 (patch)
tree287ba056309faab0f819fa03b74f6770430d5b49 /include/linux
parent7077c66b3ab3a1d336648cf88f54df43709ebac3 (diff)
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@vmware.com> Signed-off-by: Dave Airlie <airlied@redhat.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/kref.h21
1 files changed, 21 insertions, 0 deletions
diff --git a/include/linux/kref.h b/include/linux/kref.h
index 9c07dcebded7..fd2a6162ad3d 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_ */