Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933216AbcLICXA (ORCPT ); Thu, 8 Dec 2016 21:23:00 -0500 Received: from mail-ua0-f180.google.com ([209.85.217.180]:36210 "EHLO mail-ua0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932672AbcLICW6 (ORCPT ); Thu, 8 Dec 2016 21:22:58 -0500 MIME-Version: 1.0 In-Reply-To: <87lgvp3l60.fsf@yhuang-dev.intel.com> References: <1481234081-61472-1-git-send-email-joelaf@google.com> <878trq3pnd.fsf@yhuang-dev.intel.com> <87lgvp3l60.fsf@yhuang-dev.intel.com> From: Joel Fernandes Date: Thu, 8 Dec 2016 18:22:18 -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: 4215 Lines: 88 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. Regards, Joel