Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755172Ab3DOCZq (ORCPT ); Sun, 14 Apr 2013 22:25:46 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:36078 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755029Ab3DOCZn (ORCPT ); Sun, 14 Apr 2013 22:25:43 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Thomas Hellstrom , Dave Airlie Subject: [ 10/17] kref: Implement kref_get_unless_zero v3 Date: Sun, 14 Apr 2013 19:25:20 -0700 Message-Id: <20130415022353.392438843@linuxfoundation.org> X-Mailer: git-send-email 1.8.2.1.368.g99c2266 In-Reply-To: <20130415022352.616689034@linuxfoundation.org> References: <20130415022352.616689034@linuxfoundation.org> User-Agent: quilt/0.60-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2498 Lines: 64 3.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Thomas Hellstrom 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 Signed-off-by: Greg Kroah-Hartman --- include/linux/kref.h | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) --- a/include/linux/kref.h +++ b/include/linux/kref.h @@ -93,4 +93,25 @@ static inline int kref_put(struct 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_ */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/