From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Hellstrom Subject: [PATCH 2/4] kref: Implement kref_get_unless_zero v2 Date: Mon, 5 Nov 2012 14:55:42 +0100 Message-ID: <1352123744-13269-3-git-send-email-thellstrom@vmware.com> References: <1352123744-13269-1-git-send-email-thellstrom@vmware.com> Return-path: In-Reply-To: <1352123744-13269-1-git-send-email-thellstrom@vmware.com> Sender: linux-kernel-owner@vger.kernel.org To: airlied@gmail.com, airlied@redhat.com Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Thomas Hellstrom List-Id: dri-devel@lists.freedesktop.org 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 --- 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_ */ -- 1.7.4.4