Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932486AbcLHWDU (ORCPT ); Thu, 8 Dec 2016 17:03:20 -0500 Received: from mail-pg0-f51.google.com ([74.125.83.51]:33393 "EHLO mail-pg0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932094AbcLHWDT (ORCPT ); Thu, 8 Dec 2016 17:03:19 -0500 From: Joel Fernandes To: linux-kernel@vger.kernel.org Cc: Joel Fernandes , Huang Ying , Ingo Molnar , Will Deacon , Paul McKenney Subject: [RFC] llist: Fix code comments about llist_del_first locking Date: Thu, 8 Dec 2016 13:54:40 -0800 Message-Id: <1481234081-61472-1-git-send-email-joelaf@google.com> X-Mailer: git-send-email 2.8.0.rc3.226.g39d4020 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2321 Lines: 52 Usage llist_del_first needs lock protection, however the table in the comments of llist.h show a '-'. Correct this, and also add better comments on top. Cc: Huang Ying Cc: Ingo Molnar Cc: Will Deacon Cc: Paul McKenney Signed-off-by: Joel Fernandes --- include/linux/llist.h | 19 ++++++++++--------- 1 file changed, 10 insertions(+), 9 deletions(-) diff --git a/include/linux/llist.h b/include/linux/llist.h index fd4ca0b..15e4949 100644 --- a/include/linux/llist.h +++ b/include/linux/llist.h @@ -3,14 +3,15 @@ /* * Lock-less NULL terminated single linked list * - * If there are multiple producers and multiple consumers, llist_add - * can be used in producers and llist_del_all can be used in - * consumers. They can work simultaneously without lock. But - * llist_del_first can not be used here. Because llist_del_first - * depends on list->first->next does not changed if list->first is not - * changed during its operation, but llist_del_first, llist_add, - * llist_add (or llist_del_all, llist_add, llist_add) sequence in - * another consumer may violate that. + * If there are multiple producers and multiple consumers, llist_add can be + * used in producers and llist_del_all can be used in consumers. They can work + * simultaneously without lock. But llist_del_first will need to use a lock + * with any other operation (ABA problem). This is because llist_del_first + * depends on list->first->next not changing but there's no way to be sure + * about that and the cmpxchg in llist_del_first may succeed if list->first is + * the same after concurrent operations. For example, a llist_del_first, + * llist_add, llist_add (or llist_del_all, llist_add, llist_add) sequence in + * another consumer may cause violations. * * If there are multiple producers and one consumer, llist_add can be * used in producers and llist_del_all or llist_del_first can be used @@ -19,7 +20,7 @@ * This can be summarized as follow: * * | add | del_first | del_all - * add | - | - | - + * add | - | L | - * del_first | | L | L * del_all | | | - * -- 2.8.0.rc3.226.g39d4020