From c9cb68da74db0f3b3524f1462d485241dcf324ed Mon Sep 17 00:00:00 2001 From: Thomas Hellstrom Date: Tue, 6 Aug 2013 16:42:15 -0600 Subject: [PATCH] kref: Implement kref_get_unless_zero v3 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. Change-Id: Ibcb524294c2cebe1d832e77714895822adc1c89a Signed-off-by: Thomas Hellstrom Signed-off-by: Dave Airlie Git-commit: 4b20db3de8dab005b07c74161cb041db8c5ff3a7 Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git [shrenujb@codeaurora.org: resolve trivial merge conflicts] Signed-off-by: Shrenuj Bansal --- include/linux/kref.h | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/include/linux/kref.h b/include/linux/kref.h index 9c07dcebded..aa5acc26019 100644 --- a/include/linux/kref.h +++ b/include/linux/kref.h @@ -93,4 +93,26 @@ 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_ */