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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 0B1A3C67863 for ; Sat, 20 Oct 2018 05:25:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B5B4521470 for ; Sat, 20 Oct 2018 05:25:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B5B4521470 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ZenIV.linux.org.uk Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727017AbeJTNeW (ORCPT ); Sat, 20 Oct 2018 09:34:22 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:46278 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726312AbeJTNeW (ORCPT ); Sat, 20 Oct 2018 09:34:22 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gDjl3-0006jD-Nu; Sat, 20 Oct 2018 05:25:05 +0000 Date: Sat, 20 Oct 2018 06:25:05 +0100 From: Al Viro To: David Howells Cc: Alan Jenkins , torvalds@linux-foundation.org, ebiederm@xmission.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, mszeredi@redhat.com Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12] Message-ID: <20181020052505.GK32577@ZenIV.linux.org.uk> References: <97872123-70be-2833-ea7a-a463ce204b53@gmail.com> <862e36a2-2a6f-4e26-3228-8cab4b4cf230@gmail.com> <153754740781.17872.7869536526927736855.stgit@warthog.procyon.org.uk> <153754743491.17872.12115848333103740766.stgit@warthog.procyon.org.uk> <6518.1539956277@warthog.procyon.org.uk> <29902.1539988579@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <29902.1539988579@warthog.procyon.org.uk> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 19, 2018 at 11:36:19PM +0100, David Howells wrote: > Alan Jenkins wrote: > > > # open_tree_clone 3 > # cd /proc/self/fd/3 > > # mount --move . /mnt > > [ 41.747831] mnt_flags=1020 umount=0 > > # cd / > > # umount /mnt > > umount: /mnt: target is busy > > > > ^ a newly introduced bug? I do not remember having this problem before. > > The reason EBUSY is returned is because propagate_mount_busy() is called by > do_umount() with refcnt == 2, but mnt_count == 3: > > umount-3577 M=f8898a34 u=3 0x555 sp=__x64_sys_umount+0x12/0x15 > > the trace line being added here: > > if (!propagate_mount_busy(mnt, 2)) { > if (!list_empty(&mnt->mnt_list)) > umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC); > retval = 0; > } else { > trace_mnt_count(mnt, mnt->mnt_id, > atomic_read(&mnt->mnt_count), > 0x555, __builtin_return_address(0)); > } > > The busy evaluation is a result of this check: > > if (!list_empty(&mnt->mnt_mounts) || do_refcount_check(mnt, refcnt)) > > in propagate_mount_busy(). > > > The problem apparently being that mnt_count counts both refs from mountings > and refs from other sources, such as file descriptors or pathwalk. As it bloody well should. Once the tree has been attached, that open_ctree() descriptor is refering to vfsmount of /mnt (what else could it be?) IOW, it *is* genuinely busy. The livelock on umount -l you've mentioned is a different story - that's definitely a bug, but this -EBUSY is correct.