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.
Signed-off-by: Thomas Hellstrom thellstrom@vmware.com --- include/linux/kref.h | 21 +++++++++++++++++++++ 1 files changed, 21 insertions(+), 0 deletions(-)
diff --git a/include/linux/kref.h b/include/linux/kref.h index 65af688..bae91d0 100644 --- a/include/linux/kref.h +++ b/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 0 if the increment succeeded. Otherwise return non-zero. + * + * 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_ */