From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966804Ab3DRJVB (ORCPT ); Thu, 18 Apr 2013 05:21:01 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:40332 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759400Ab3DRJSe (ORCPT ); Thu, 18 Apr 2013 05:18:34 -0400 From: Luis Henriques To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Thomas Hellstrom , Dave Airlie , Luis Henriques Subject: [PATCH 65/72] kref: Implement kref_get_unless_zero v3 Date: Thu, 18 Apr 2013 10:16:50 +0100 Message-Id: <1366276617-3553-66-git-send-email-luis.henriques@canonical.com> X-Mailer: git-send-email 1.8.1.2 In-Reply-To: <1366276617-3553-1-git-send-email-luis.henriques@canonical.com> References: <1366276617-3553-1-git-send-email-luis.henriques@canonical.com> X-Extended-Stable: 3.5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.5.7.11 -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 [ luis: adjust context ] Signed-off-by: Luis Henriques --- include/linux/kref.h | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/include/linux/kref.h b/include/linux/kref.h index 9c07dce..fd2a616 100644 --- a/include/linux/kref.h +++ b/include/linux/kref.h @@ -93,4 +93,25 @@ 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_ */ -- 1.8.1.2