From 3fa8ee5fafec620e0dadb3ce226124a75d599288 Mon Sep 17 00:00:00 2001 From: Thomas Hellstrom Date: Tue, 6 Nov 2012 11:31:49 +0000 Subject: 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 Signed-off-by: Dave Airlie [bwh: Backported to 3.2: - Adjust context - Add #include ] Signed-off-by: Ben Hutchings --- include/linux/kref.h | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) (limited to 'include') diff --git a/include/linux/kref.h b/include/linux/kref.h index d4a62ab2ee5..d0645026c89 100644 --- a/include/linux/kref.h +++ b/include/linux/kref.h @@ -16,6 +16,7 @@ #define _KREF_H_ #include +#include struct kref { atomic_t refcount; @@ -27,4 +28,24 @@ int kref_put(struct kref *kref, void (*release) (struct kref *kref)); int kref_sub(struct kref *kref, unsigned int count, void (*release) (struct kref *kref)); +/** + * 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_ */ -- cgit v1.2.3-18-g5258