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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 D1286C433FF for ; Wed, 7 Aug 2019 09:52:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AD1F921BE6 for ; Wed, 7 Aug 2019 09:52:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729107AbfHGJwV (ORCPT ); Wed, 7 Aug 2019 05:52:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:46136 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726902AbfHGJwV (ORCPT ); Wed, 7 Aug 2019 05:52:21 -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 68BA0B0B6; Wed, 7 Aug 2019 09:52:19 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id EBD7B1E43FE; Wed, 7 Aug 2019 11:45:20 +0200 (CEST) Date: Wed, 7 Aug 2019 11:45:20 +0200 From: Jan Kara To: Bart Van Assche Cc: Jan Kara , John Lenton , 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: <20190807094520.GB14658@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> <20190730101646.GC28829@quack2.suse.cz> <20190730133607.GD28829@quack2.suse.cz> <43344436-99b5-f0a7-b71e-2bbb025dfd09@acm.org> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="nFreZHaLTZJo0R7j" Content-Disposition: inline In-Reply-To: <43344436-99b5-f0a7-b71e-2bbb025dfd09@acm.org> 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 --nFreZHaLTZJo0R7j Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Mon 05-08-19 09:41:39, Bart Van Assche wrote: > On 7/30/19 6:36 AM, Jan Kara wrote: > > On Tue 30-07-19 12:16:46, Jan Kara wrote: > > > 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... > > > > So how about attached patch? It fixes the regression for me. Hi Bart, > A new kernel warning is triggered by blktests block/001 that did not happen > without this patch. Reverting commit 89e524c04fa9 ("loop: Fix mount(2) > failure due to race with LOOP_SET_FD") makes that kernel warning disappear. > Is this reproducible on your setup? Thanks for report! Hum, no, it seems the warning doesn't trigger in my test VM. But reviewing the mentioned commit with fresh head, I can see where I did a mistake during my conversion of blkdev_get(). Does attached patch fix the warning for you? Honza -- Jan Kara SUSE Labs, CR --nFreZHaLTZJo0R7j Content-Type: text/x-patch; charset=us-ascii Content-Disposition: attachment; filename="0001-bdev-Fixup-error-handling-in-blkdev_get.patch" >From c4cd39244088d8de548264bc33dc9fb8f0f1db2d Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Wed, 7 Aug 2019 11:36:47 +0200 Subject: [PATCH] bdev: Fixup error handling in blkdev_get() Commit 89e524c04fa9 ("loop: Fix mount(2) failure due to race with LOOP_SET_FD") converted blkdev_get() to use the new helpers for finishing claiming of a block device. However the conversion botched the error handling in blkdev_get() and thus the bdev has been marked as held even in case __blkdev_get() returned error. This led to occasional warnings with block/001 test from blktests like: kernel: WARNING: CPU: 5 PID: 907 at fs/block_dev.c:1899 __blkdev_put+0x396/0x3a0 Correct the error handling. CC: stable@vger.kernel.org Fixes: 89e524c04fa9 ("loop: Fix mount(2) failure due to race with LOOP_SET_FD") Signed-off-by: Jan Kara --- fs/block_dev.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/fs/block_dev.c b/fs/block_dev.c index a6f7c892cb4a..7db181581606 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -1754,7 +1754,10 @@ int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder) /* finish claiming */ mutex_lock(&bdev->bd_mutex); - bd_finish_claiming(bdev, whole, holder); + if (!res) + bd_finish_claiming(bdev, whole, holder); + else + bd_abort_claiming(bdev, whole, holder); /* * Block event polling for write claims if requested. Any * write holder makes the write_holder state stick until -- 2.16.4 --nFreZHaLTZJo0R7j--