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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 583E1C33C8C for ; Mon, 6 Jan 2020 16:00:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 24B7B20707 for ; Mon, 6 Jan 2020 16:00:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726448AbgAFQAR (ORCPT ); Mon, 6 Jan 2020 11:00:17 -0500 Received: from mx2.suse.de ([195.135.220.15]:40794 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726296AbgAFQAR (ORCPT ); Mon, 6 Jan 2020 11:00:17 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 4BF3CAE8A; Mon, 6 Jan 2020 16:00:15 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 5180EDA78B; Mon, 6 Jan 2020 17:00:05 +0100 (CET) Date: Mon, 6 Jan 2020 17:00:04 +0100 From: David Sterba To: Anand Jain Cc: dsterba@suse.cz, linux-btrfs@vger.kernel.org Subject: Re: [PATCH 4/4] btrfs: sysfs, add devid/dev_state kobject and attribute Message-ID: <20200106160004.GL3929@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Anand Jain , linux-btrfs@vger.kernel.org References: <20191205112706.8125-1-anand.jain@oracle.com> <20191205112706.8125-5-anand.jain@oracle.com> <20191205142148.GQ2734@twin.jikos.cz> <78560abd-7d85-c95d-ed76-7810b1d03789@oracle.com> <20191205151428.GS2734@twin.jikos.cz> <673babd8-90ec-2f7e-532a-df8c98a844cf@oracle.com> <8bd3d9b9-11b1-4c9a-8b59-ccfe0c6d92c4@oracle.com> <20191213164332.GA3929@twin.jikos.cz> <20191213170215.GB3929@twin.jikos.cz> <1faa5860-130a-9f3c-3e44-724ce9a26adb@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1faa5860-130a-9f3c-3e44-724ce9a26adb@oracle.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Sat, Dec 14, 2019 at 08:26:24AM +0800, Anand Jain wrote: > > > On 14/12/19 1:02 AM, David Sterba wrote: > > On Fri, Dec 13, 2019 at 05:43:32PM +0100, David Sterba wrote: > >>> Looked into this further, actually we don't need any lock here > >>> the device delete thread which calls kobject_put() makes sure > >>> sysfs read is closed. So an existing sysfs read thread will have > >>> to complete before device free. > >>> > >>> > >>> CPU1 CPU2 > >>> > >>> btrfs_rm_device > >>> open file > >>> btrfs_sysfs_rm_device_link > >>> call read, access freed device > >>> sysfs waits for the open file > >>> to close. > >> > >> How exactly does sysfs wait for the device? Is it eg wait_event checking > >> number of references? If the file stays open by an evil process is it > >> going to block the device removal indefinitelly? > > > > Yeah, sysfs waits until the file is closed. Eg. umount can be stalled > > that way too. > > And similar to umount, I don't think we should return EBUSY > for btrfs_rm_device if the device sysfs attribute is opened, > as sysfs show attributes are non blocking and would be completed > in the timely manner. While I don't think the blocking behaviour is totally OK, returning EBUSY could be confusing without any other explanation. Also leaving sysfs files open but not read is kind of strange on itself.