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 X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 18554C2BB40 for ; Mon, 14 Dec 2020 18:40:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CC12B22525 for ; Mon, 14 Dec 2020 18:40:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2408828AbgLNSkK (ORCPT ); Mon, 14 Dec 2020 13:40:10 -0500 Received: from mga12.intel.com ([192.55.52.136]:11436 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2408733AbgLNSjp (ORCPT ); Mon, 14 Dec 2020 13:39:45 -0500 IronPort-SDR: MyT16XC7SQF91cjIKRNwBVdhhx9dj9SAwb5vVZ1AirJ3zamkW08CvT3rJNvNyGn8sda/kv1QuE fadza7zn0kzw== X-IronPort-AV: E=McAfee;i="6000,8403,9834"; a="153987574" X-IronPort-AV: E=Sophos;i="5.78,420,1599548400"; d="scan'208";a="153987574" Received: from orsmga001.jf.intel.com ([10.7.209.18]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Dec 2020 10:39:00 -0800 IronPort-SDR: hZy/5jPNvSjNnkEXZkIC+n0IJ0iuhaeVeLbHyDwJxv0Ii96UhSgyBDYiv2D34ZytIdtw72xqbi MFYJKrNbKaYQ== X-IronPort-AV: E=Sophos;i="5.78,420,1599548400"; d="scan'208";a="411361295" Received: from rchatre-mobl3.amr.corp.intel.com (HELO [10.212.45.112]) ([10.212.45.112]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Dec 2020 10:38:59 -0800 Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group To: Valentin Schneider Cc: tglx@linutronix.de, fenghua.yu@intel.com, bp@alien8.de, tony.luck@intel.com, kuo-lang.tseng@intel.com, shakeelb@google.com, mingo@redhat.com, babu.moger@amd.com, james.morse@arm.com, hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org References: From: Reinette Chatre Message-ID: <07ccc96d-a875-af0e-5169-24b1f84c46da@intel.com> Date: Mon, 14 Dec 2020 10:38:58 -0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Valentin, On 12/11/2020 12:46 PM, Valentin Schneider wrote: > On 03/12/20 23:25, Reinette Chatre wrote: >> Valentin's series in [2] ends by adding memory barriers to support the >> updating of the task_struct from one CPU and the usage of the task_struct data >> from another CPU. This work is still needed and as discussed with Valentin in >> that thread the work would be re-evaluated by him after seeing how this series >> turns out. >> Thank you very much for taking a look. > > So the "problematic" pattern is still there: a context switch can happen > concurrently with a write to the switching-to-tasks's {closid, rmid}. > Accesses to these fields would thus need to be wrapped by READ_ONCE() & > WRITE_ONCE(). ok. > > Thinking a bit more (too much?) about it, we could limit ourselves to > wrapping only reads not protected by the rdtgroup_mutex: the only two > task_struct {closid, rmid} writers are > - rdtgroup_move_task() > - rdt_move_group_tasks() > and they are both invoked while holding said mutex. Thus, a reader holding > the mutex cannot race with a write, so load tearing ought to be safe. The reads that are not protected by the rdtgroup_mutex can be found in __resctrl_sched_in(). It thus sounds to me like your proposed changes to this function found in your patch [1] is what is needed? It is not clear to me how the pairing would work in this case though. If I understand correctly the goal is for the write to the closid/rmid in the functions you mention above to be paired with the reads in resctrl_sched_in() and it is not clear how adding a single READ_ONCE would accomplish this pairing by itself. It is also not entirely clear to me what the problematic scenario could be. If I understand correctly, the risk is (as you explained in your commit message), that a CPU could have its {closid, rmid} fields read locally (resctrl_sched_in()) while they are concurrently being written to from another CPU (in rdtgroup_move_task() and rdt_move_group_tasks() as you state above). If this happens then a task being moved may be scheduled in with its old closid/rmid. The update of closid/rmid in rdtgroup_move_task()/rdt_move_group_tasks() is followed by smp_call_function_xx() where the registers are updated with preemption disabled and thus protected against __switch_to. If a task was thus incorrectly scheduled in with old closid/rmid, would it not be corrected at this point? Thank you Reinette [1] https://lore.kernel.org/lkml/20201123022433.17905-4-valentin.schneider@arm.com/