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 15B14C4332F for ; Tue, 18 Oct 2022 06:55:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229956AbiJRGzY (ORCPT ); Tue, 18 Oct 2022 02:55:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56084 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229959AbiJRGzU (ORCPT ); Tue, 18 Oct 2022 02:55:20 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C133F13E37 for ; Mon, 17 Oct 2022 23:55:17 -0700 (PDT) Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.56]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4Ms4Hs0SlxzmV85; Tue, 18 Oct 2022 14:50:33 +0800 (CST) Received: from kwepemm600005.china.huawei.com (7.193.23.191) by dggemv711-chm.china.huawei.com (10.1.198.66) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 18 Oct 2022 14:55:11 +0800 Received: from [10.67.103.158] (10.67.103.158) by kwepemm600005.china.huawei.com (7.193.23.191) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 18 Oct 2022 14:55:10 +0800 Subject: Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to migration driver To: Shameerali Kolothum Thodi , "John Garry" , "alex.williamson@redhat.com" , "jgg@nvidia.com" CC: "cohuck@redhat.com" , "linux-kernel@vger.kernel.org" , "linuxarm@openeuler.org" References: <20221014025757.39415-1-liulongfang@huawei.com> <20221014025757.39415-2-liulongfang@huawei.com> <42d12308-4777-47a7-a1ae-50dfada050cf@huawei.com> <7c041671-71bf-d558-d47a-70296fe5c30e@huawei.com> From: liulongfang Message-ID: <43a109b8-7cde-7bc6-f05d-73d322cea3c2@huawei.com> Date: Tue, 18 Oct 2022 14:55:09 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.103.158] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To kwepemm600005.china.huawei.com (7.193.23.191) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/10/17 21:57, Shameerali Kolothum Thodi Wrote: > > >> -----Original Message----- >> From: liulongfang >> Sent: 17 October 2022 10:21 >> To: John Garry ; alex.williamson@redhat.com; >> jgg@nvidia.com; Shameerali Kolothum Thodi >> >> Cc: cohuck@redhat.com; linux-kernel@vger.kernel.org; >> linuxarm@openeuler.org >> Subject: Re: [Linuxarm] [PATCH 1/2] hisi_acc_vfio_pci: Add debugfs to >> migration driver >> >> On 2022/10/14 17:20, John Garry wrote: >>> On 14/10/2022 03:57, Longfang Liu wrote: >>>> +static void hisi_acc_vf_debugfs_init(struct hisi_acc_vf_core_device >> *hisi_acc_vdev) >>>> +{ >>>> +    struct pci_dev *vf_pdev = hisi_acc_vdev->vf_dev; >>>> +    struct device *dev = &vf_pdev->dev; >>>> +    int ret; >>>> + >>>> +    if (!atomic_read(&hisi_acc_root_ref)) >>>> +        hisi_acc_debugfs_root = debugfs_create_dir("hisi_vfio_acc", >> NULL); >>>> +    atomic_inc(&hisi_acc_root_ref); >>>> + >>> >>> This looks totally racy, such that I wonder why even bother using an atomic >> for hisi_acc_root_ref. >> >> >> When enabling VF, it is possible for multiple VMs to enable VF at the same >> time. The atomic variable >> is used to ensure that only one "hisi_vfio_acc" is created. When other VFs >> are enabled, >> it will not be created again, but will be used directly. >> >> Indeed, why is hisi_acc_debugfs_root not created in the driver module init? >>> >> Because the normal function of VF is to perform encryption and decryption >> services, the live migration >> function is an auxiliary function, which no need to be used in scenarios >> where only encryption and >> decryption services are performed. >> >> During module init, it can register ops(hisi_acc_vfio_pci_ops) that only >> perform encryption and >> decryption services, and can also register with live migration function >> ops(hisi_acc_vfio_pci_migrn_ops), >> and this debugfs only needs to register it when the the ops is >> hisi_acc_vfio_pci_migrn_ops. > > Isn't the ops registration happens at probe()? In any case, I think you can move > the hisi_acc_debugfs_root creation to module_init() as suggested above > and avoid the race. > mig_root_ref uses atomic to prevent race, and another function is for reference counting. Even if this hisi_acc_debugfs_root() is moved to module_init(), this mig_root_ref is still needed, because the driver needs to ensure that the debugfs file is created when the first VF is enabled, destroy it when the last VF have unloaded. > Thanks, > Shameer > Thanks, Longfang.