From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753076Ab1HDJqZ (ORCPT ); Thu, 4 Aug 2011 05:46:25 -0400 Received: from cantor2.suse.de ([195.135.220.15]:59846 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752760Ab1HDJqS (ORCPT ); Thu, 4 Aug 2011 05:46:18 -0400 Date: Thu, 4 Aug 2011 11:46:16 +0200 From: Jan Kara To: Miklos Szeredi Cc: Al Viro , Miklos Szeredi , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, jack@suse.cz, hch@infradead.org, akpm@linux-foundation.org, toshi.okajima@jp.fujitsu.com Subject: Re: [PATCH 2/4] vfs: keep list of mounts for each superblock Message-ID: <20110804094616.GC17196@quack.suse.cz> References: <1312368521-7911-1-git-send-email-miklos@szeredi.hu> <1312368521-7911-3-git-send-email-miklos@szeredi.hu> <20110803221707.GM2203@ZenIV.linux.org.uk> <1312449054.6032.22.camel@tucsk.pomaz.szeredi.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1312449054.6032.22.camel@tucsk.pomaz.szeredi.hu> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 04-08-11 11:10:54, Miklos Szeredi wrote: > On Wed, 2011-08-03 at 23:17 +0100, Al Viro wrote: > > On Wed, Aug 03, 2011 at 12:48:39PM +0200, Miklos Szeredi wrote: > > > @@ -696,6 +696,11 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void > > > mnt->mnt_sb = root->d_sb; > > > mnt->mnt_mountpoint = mnt->mnt_root; > > > mnt->mnt_parent = mnt; > > > + > > > + br_write_lock(vfsmount_lock); > > > + list_add_tail(&mnt->mnt_instance, &mnt->mnt_sb->s_mounts); > > > + br_write_unlock(vfsmount_lock); > > > > Racy. > > > > > @@ -745,6 +750,10 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root, > > > if (!list_empty(&old->mnt_expire)) > > > list_add(&mnt->mnt_expire, &old->mnt_expire); > > > } > > > + > > > + br_write_lock(vfsmount_lock); > > > + list_add_tail(&mnt->mnt_instance, &mnt->mnt_sb->s_mounts); > > > + br_write_unlock(vfsmount_lock); > > > > Ditto. If you expect to be able to find *all* vfsmounts over given sb, > > this locking is simply wrong. > > I don't understand. All accesses to mnt_instance/s_mounts are protected > by vfsmount_lock. What else is needed? I guess Al meant that sb_prepare_remount_readonly() from the next patch could race with new mountpoint being added to the list and the check is thus still unreliable? Quickly checking the locking seems to confirm that since e.g. clone_mnt() does not hold s_umount semaphore (only namespace_sem?) and do_remount_sb() has it the other way around... Honza -- Jan Kara SUSE Labs, CR