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,URIBL_BLOCKED,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 CB558C433FF for ; Tue, 30 Jul 2019 10:16:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9E2E420693 for ; Tue, 30 Jul 2019 10:16:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732459AbfG3KQs (ORCPT ); Tue, 30 Jul 2019 06:16:48 -0400 Received: from mx2.suse.de ([195.135.220.15]:46056 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726078AbfG3KQs (ORCPT ); Tue, 30 Jul 2019 06:16:48 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id EC570AE2D; Tue, 30 Jul 2019 10:16:46 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id A5DA91E440D; Tue, 30 Jul 2019 12:16:46 +0200 (CEST) Date: Tue, 30 Jul 2019 12:16:46 +0200 From: Jan Kara To: John Lenton Cc: Jan Kara , Kai-Heng Feng , Jens Axboe , Tetsuo Handa , linux-block@vger.kernel.org, jean-baptiste.lallement@canonical.com Subject: Re: [PATCH] loop: Don't change loop device under exclusive opener Message-ID: <20190730101646.GC28829@quack2.suse.cz> References: <20190516140127.23272-1-jack@suse.cz> <50edd0fa-9cfa-38e1-8870-0fbc5c618522@kernel.dk> <20190527122915.GB9998@quack2.suse.cz> <894DDAA8-2ADD-467C-8E4F-4DE6B9A50625@me.com> <20190730092939.GB28829@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue 30-07-19 10:36:59, John Lenton wrote: > On Tue, 30 Jul 2019 at 10:29, Jan Kara wrote: > > > > Thanks for the notice and the references. What's your version of > > util-linux? What your test script does is indeed racy. You have there: > > > > echo Running: > > for i in {a..z}{a..z}; do > > mount $i.squash /mnt/$i & > > done > > > > So all mount(8) commands will run in parallel and race to setup loop > > devices with LOOP_SET_FD and mount them. However util-linux (at least in > > the current version) seems to handle EBUSY from LOOP_SET_FD just fine and > > retries with the new loop device. So at this point I don't see why the patch > > makes difference... I guess I'll need to reproduce and see what's going on > > in detail. > > We've observed this in arch with util-linux 2.34, and ubuntu 19.10 > (eoan ermine) with util-linux 2.33. > > just to be clear, the initial reports didn't involve a zany loop of > mounts, but were triggered by effectively the same thing as systemd > booted a system with a lot of snaps. The reroducer tries to makes > things simpler to reproduce :-). FWIW, systemd versions were 244 and > 242 for those systems, respectively. Thanks for info! So I think I see what's going on. The two mounts race like: MOUNT1 MOUNT2 num = ioctl(LOOP_CTL_GET_FREE) num = ioctl(LOOP_CTL_GET_FREE) ioctl("/dev/loop$num", LOOP_SET_FD, ..) - returns OK ioctl("/dev/loop$num", LOOP_SET_FD, ..) - acquires exclusine loop$num reference mount("/dev/loop$num", ...) - sees exclusive reference from MOUNT2 and fails - sees loop device is already bound and fails It is a bug in the scheme I've chosen that racing LOOP_SET_FD can block perfectly valid mount. I'll think how to fix this... Honza -- Jan Kara SUSE Labs, CR