From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33102C76188 for ; Mon, 3 Apr 2023 17:42:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231563AbjDCRm1 (ORCPT ); Mon, 3 Apr 2023 13:42:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47436 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231320AbjDCRmZ (ORCPT ); Mon, 3 Apr 2023 13:42:25 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 099F9B6 for ; Mon, 3 Apr 2023 10:41:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1680543698; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=du5ube66GFFv6eiC7GgKY4F8MRZGEnQkABRtMIiA1tM=; b=hXorWAGsTukYq6UNdHWyZtINHi3zGmsi59FNNItcGo4RCqbDGiGG2ktcu7ywtWS+OWcWNR LJPA7HbiTGYRtfjLBzTymXvvYZEx0Wc+cVJPOf0r5YsoCyt8vlaYGQNP0tvVsHYFs2UxzP fsVvcgqAVwCdwqyCG16IhFc6wQ+7fxs= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-584-vsOxvpMkMAm9e-diM8GuYA-1; Mon, 03 Apr 2023 13:41:37 -0400 X-MC-Unique: vsOxvpMkMAm9e-diM8GuYA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id A37E4101A54F; Mon, 3 Apr 2023 17:41:34 +0000 (UTC) Received: from [10.18.17.153] (dhcp-17-153.bos.redhat.com [10.18.17.153]) by smtp.corp.redhat.com (Postfix) with ESMTP id ED04C40C6EC4; Mon, 3 Apr 2023 17:41:33 +0000 (UTC) Message-ID: <24b67530-62ce-4f9c-7b74-d41d2ccc710e@redhat.com> Date: Mon, 3 Apr 2023 13:41:33 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH 3/3] cgroup/cpuset: Allow only one active attach operation per cpuset Content-Language: en-US To: =?UTF-8?Q?Michal_Koutn=c3=bd?= Cc: Tejun Heo , Zefan Li , Johannes Weiner , Christian Brauner , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Juri Lelli , Dietmar Eggemann , gscrivan@redhat.com References: <20230331145045.2251683-1-longman@redhat.com> <20230331145045.2251683-4-longman@redhat.com> <20230403164736.lpjdpzxxnjlpxrqv@blackpad> From: Waiman Long In-Reply-To: <20230403164736.lpjdpzxxnjlpxrqv@blackpad> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/3/23 12:47, Michal Koutný wrote: > On Fri, Mar 31, 2023 at 10:50:45AM -0400, Waiman Long wrote: >> The current cpuset code uses the global cpuset_attach_old_cs variable >> to store the old cpuset value between consecutive cpuset_can_attach() >> and cpuset_attach() calls. Since a caller of cpuset_can_attach() may >> not need to hold the global cgroup_threadgroup_rwsem, parallel cpuset >> attach operations are possible. > Do I understand correctly this consequence of the cpuset_attach_task() > on the clone path? > In that particular case (with CLONE_INTO_CGROUP) cgroup_mutex is taken, > so the access the the old_cs variable should still be synchronized with > regular migrations that are also under cgroup_mutex. This patch is actually not related to the CLONE_INTO_GROUP problem in patch 1. It is a generic problem when multiple users are moving threads into cgroup.threads of the same or different cpusets simultaneously. >> When there are concurrent cpuset attach operations in progress, >> cpuset_attach() may fetch the wrong value from cpuset_attach_old_cs >> causing incorrect result. To avoid this problem while still allowing >> certain level of parallelism, drop cpuset_attach_old_cs and use a >> per-cpuset attach_old_cs value. Also restrict to at most one active >> attach operation per cpuset to avoid corrupting the value of the >> per-cpuset attach_old_cs value. > Secondly, semantically wouldn't a `void *ss_priv[CGROUP_SUBSYS_COUNT]` > in struct cgroup_taskset make it simpler wrt the exclusivity guarantees? I guess we can put the old_cs value into cgroup_taskset. Since the related attach_in_progress value is in cpuset, so I put the old_cs there too. > Thirdly, if my initial assumptino is right -- I'd suggest ordering this > before the patch `cgroup/cpuset: Make cpuset_fork() handle > CLONE_INTO_CGROUP properly` to spare backporters possible troubles if > this is would be a fixup to that. I don't believe this patch has a dependency on patch 1. I had thought about adding a fixes tag so it will be backported to stable. However, this problem should be rare. Let's see what others think about it. Cheers, Longman