Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932645AbcLMCCY (ORCPT ); Mon, 12 Dec 2016 21:02:24 -0500 Received: from mail-pg0-f45.google.com ([74.125.83.45]:33665 "EHLO mail-pg0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752965AbcLMCCV (ORCPT ); Mon, 12 Dec 2016 21:02:21 -0500 From: Joel Fernandes To: linux-kernel@vger.kernel.org Cc: Huang Ying , Mathieu Desnoyers , Joel Fernandes , Ingo Molnar , Will Deacon , Paul McKenney Subject: [PATCH v3] llist: Clarify comments about when locking is needed Date: Mon, 12 Dec 2016 18:01:45 -0800 Message-Id: <1481594505-2779-1-git-send-email-joelaf@google.com> X-Mailer: git-send-email 2.8.0.rc3.226.g39d4020 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3412 Lines: 74 llist.h comments are confusing about when locking is needed versus when it isn't. Clarify these comments by being more descriptive about why locking is needed for llist_del_first. Cc: Ingo Molnar Cc: Will Deacon Cc: Paul McKenney Acked-by: Huang Ying Acked-by: Mathieu Desnoyers Signed-off-by: Joel Fernandes --- Changes since before: 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