Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933111AbcLICMn (ORCPT ); Thu, 8 Dec 2016 21:12:43 -0500 Received: from mga06.intel.com ([134.134.136.31]:14501 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932822AbcLICMm (ORCPT ); Thu, 8 Dec 2016 21:12:42 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.33,322,1477983600"; d="scan'208";a="38417665" From: "Huang\, Ying" To: Joel Fernandes Cc: "Huang\, Ying" , LKML , Ingo Molnar , "Will Deacon" , Paul McKenney Subject: Re: [RFC] llist: Fix code comments about llist_del_first locking References: <1481234081-61472-1-git-send-email-joelaf@google.com> <878trq3pnd.fsf@yhuang-dev.intel.com> Date: Fri, 09 Dec 2016 10:12:39 +0800 In-Reply-To: (Joel Fernandes's message of "Thu, 8 Dec 2016 16:43:24 -0800") Message-ID: <87lgvp3l60.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3782 Lines: 81 Joel Fernandes writes: > 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". I tried to describe both cases in the original table. * | add | del_first | del_all * add | - | - | - * del_first | | L | L * del_all | | | - The 'L' for "del_first * del_first" means multiple consumers uses llist_del_first() need lock. And the 'L' for 'del_first * del_all' means multiple consumers uses llist_del_first() and llist_del_all() need lock. Best Regards, Huang, Ying