Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933268AbcLIC0o (ORCPT ); Thu, 8 Dec 2016 21:26:44 -0500 Received: from mga05.intel.com ([192.55.52.43]:57048 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932502AbcLIC0n (ORCPT ); Thu, 8 Dec 2016 21:26:43 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.33,322,1477983600"; d="scan'208";a="1096714016" 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> <87lgvp3l60.fsf@yhuang-dev.intel.com> Date: Fri, 09 Dec 2016 10:26:41 +0800 In-Reply-To: (Joel Fernandes's message of "Thu, 8 Dec 2016 18:22:18 -0800") Message-ID: <87h96d3kim.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: 4406 Lines: 92 Joel Fernandes writes: > On Thu, Dec 8, 2016 at 6:12 PM, Huang, Ying wrote: >> 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. > > Ok, now I get it - so basically the table describes one > producer/consumer vs another producer/consumer, in other words you are > just describing contention between any 2 operations. Thanks for > clarifying. I will respin the comments to explain this a bit better if > that's Ok with you. It is good for me to improve the comments. Best Regards, Huang, Ying