Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752464AbcLJSUp (ORCPT ); Sat, 10 Dec 2016 13:20:45 -0500 Received: from mail-ua0-f176.google.com ([209.85.217.176]:34103 "EHLO mail-ua0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751965AbcLJSUo (ORCPT ); Sat, 10 Dec 2016 13:20:44 -0500 MIME-Version: 1.0 In-Reply-To: <376665889.35293.1481393758914.JavaMail.zimbra@efficios.com> References: <1481393024-22310-1-git-send-email-joelaf@google.com> <376665889.35293.1481393758914.JavaMail.zimbra@efficios.com> From: Joel Fernandes Date: Sat, 10 Dec 2016 10:20:42 -0800 Message-ID: Subject: Re: [PATCH v2] llist: Clarify comments about when locking is needed To: Mathieu Desnoyers Cc: linux-kernel , Huang Ying , Ingo Molnar , Will Deacon , "Paul E. 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: 4071 Lines: 99 On Sat, Dec 10, 2016 at 10:15 AM, Mathieu Desnoyers wrote: > ----- On Dec 10, 2016, at 7:03 PM, Joel Fernandes joelaf@google.com wrote: > >> llist.h comments are a bit confusing about when locking is needed versus when >> it isn't. Clarify these comments a bit more by being a bit more descriptive >> about why locking is needed for llist_del_first. > > As I stated in my earlier review, please remove a couple of "a bit" > from the changelog. > I'm sorry I missed that. I will update it correctly in the next rev. Regards, Joel > Thanks, > > Mathieu > >> >> Cc: Huang Ying >> Cc: Ingo Molnar >> Cc: Will Deacon >> Cc: Paul McKenney >> Acked-by: Mathieu Desnoyers >> Signed-off-by: Joel Fernandes >> --- >> v2 changes: >> Minor changes to comment and commit message based on Mathieu's suggestions >> (https://lkml.org/lkml/2016/12/10/39) >> >> include/linux/llist.h | 37 +++++++++++++++++++++---------------- >> 1 file changed, 21 insertions(+), 16 deletions(-) >> >> diff --git a/include/linux/llist.h b/include/linux/llist.h >> index fd4ca0b..31822bb 100644 >> --- a/include/linux/llist.h >> +++ b/include/linux/llist.h >> @@ -3,28 +3,33 @@ >> /* >> * 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 one consumer, llist_add can be >> - * used in producers and llist_del_all or llist_del_first can be used >> - * in the consumer. >> - * >> - * This can be summarized as follow: >> + * Cases where locking is not needed: >> + * If there are multiple producers and multiple consumers, llist_add can be >> + * used in producers and llist_del_all can be used in consumers simultaneously >> + * without locking. Also a single consumer can use llist_del_first while >> + * multiple producers simultaneously use llist_add, without any locking. >> + * >> + * Cases where locking is needed: >> + * If we have multiple consumers with llist_del_first used in one consumer, and >> + * llist_del_first or llist_del_all used in other consumers, then a lock is >> + * needed. This is because llist_del_first depends on list->first->next not >> + * changing, but without lock protection, there's no way to be sure about that >> + * if a preemption happens in the middle of the delete operation and on being >> + * preempted back, the list->first is the same as before causing the cmpxchg in >> + * llist_del_first to succeed. For example, while a llist_del_first operation >> + * is in progress in one consumer, then a llist_del_first, llist_add, >> + * llist_add (or llist_del_all, llist_add, llist_add) sequence in another >> + * consumer may cause violations. >> + * >> + * This can be summarized as follows: >> * >> * | add | del_first | del_all >> * add | - | - | - >> * del_first | | L | L >> * del_all | | | - >> * >> - * Where "-" stands for no lock is needed, while "L" stands for lock >> - * is needed. >> + * Where, a particular row's operation can happen concurrently with a column's >> + * operation, with "-" being no lock needed, while "L" being lock is needed. >> * >> * The list entries deleted via llist_del_all can be traversed with >> * traversing function such as llist_for_each etc. But the list >> -- >> 2.8.0.rc3.226.g39d4020 > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com