Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754050AbcLIAoZ (ORCPT ); Thu, 8 Dec 2016 19:44:25 -0500 Received: from mail-vk0-f53.google.com ([209.85.213.53]:36309 "EHLO mail-vk0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753072AbcLIAoY (ORCPT ); Thu, 8 Dec 2016 19:44:24 -0500 MIME-Version: 1.0 In-Reply-To: References: <1481234081-61472-1-git-send-email-joelaf@google.com> <878trq3pnd.fsf@yhuang-dev.intel.com> From: Joel Fernandes Date: Thu, 8 Dec 2016 16:43:24 -0800 Message-ID: Subject: Re: [RFC] llist: Fix code comments about llist_del_first locking To: "Huang, Ying" Cc: LKML , Ingo Molnar , Will Deacon , Paul McKenney Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3206 Lines: 67 On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes wrote: > On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying wrote: >> Joel Fernandes writes: >> >>> 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 | - >> >> If there are only one consumer which only calls llist_del_first(), lock >> is unnecessary. So '-' is shown here originally. But if there are >> multiple consumers which call llist_del_first() or llist_del_all(), lock >> is needed. > > I think this needs to be made more clear in the table. The table > doesn't clear say whether it describes the preceding paragraph > (multiple producers and one consumer), or if it describes the multiple > producers and one consumer case. So either we should have 2 tables, or Sorry, I meant "or if it describes the multiple producer and multiple consumer case". Regards, Joel