From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753819AbeDROk3 (ORCPT ); Wed, 18 Apr 2018 10:40:29 -0400 Received: from mail-it0-f46.google.com ([209.85.214.46]:55584 "EHLO mail-it0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753658AbeDROkX (ORCPT ); Wed, 18 Apr 2018 10:40:23 -0400 X-Google-Smtp-Source: AIpwx48o5Ru1o703HmiQyPa8xJsC4ZwU5RUIzhRwIpttEZh2yCyEKSjH8HUIa8NqyHqp3kDwUoiB9w== Subject: Re: [PATCH] blkcg: not hold blkcg lock when deactivating policy. To: jiang.biao2@zte.com.cn, paolo.valente@linaro.org Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org, zhong.weidong@zte.com.cn, wen.yang99@zte.com.cn, ulf.hansson@linaro.org, linus.walleij@linaro.org, broonie@kernel.org References: <201804181718167747438@zte.com.cn> From: Jens Axboe Message-ID: <3a699198-edf4-7337-f5e7-faaa6b058713@kernel.dk> Date: Wed, 18 Apr 2018 08:40:20 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <201804181718167747438@zte.com.cn> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/18/18 3:18 AM, jiang.biao2@zte.com.cn wrote: > Hi, >>> Il giorno 17 apr 2018, alle ore 09:10, Jiang Biao ha scritto: >>> >>> As described in the comment of blkcg_activate_policy(), >>> *Update of each blkg is protected by both queue and blkcg locks so >>> that holding either lock and testing blkcg_policy_enabled() is >>> always enough for dereferencing policy data.* >>> with queue lock held, there is no need to hold blkcg lock in >>> blkcg_deactivate_policy(). Similar case is in >>> blkcg_activate_policy(), which has removed holding of blkcg lock in >>> commit 4c55f4f9ad3001ac1fefdd8d8ca7641d18558e23. >>> >> >> Hi, >> by chance, did you check whether this may cause problems with bfq, >> being the latter not protected by the queue lock as cfq? > Checked the bfq code, bfq seems never used blkcg lock derectly, and > update of blkg in the common code is protected by both queue and > blkcg locks, so IMHO this patch would not introduce any new problem > with bfq, even though bfq is not protected by queue lock. > On the other hand, the locks (queue lock/blkcg lock) used to protected > the update of blkg seems a bit too heavyweight, especially the queue lock > which is used too widely may cause races with other contexts. I wonder > if there is any way to ease the case? e.g. add a new lock for blkg's own.:) It might make sense to lock it separately, but I would not worry about it unless it shows up as hot in your testing. I've applied your patch, thanks. -- Jens Axboe