Re: [BUGFIX][PATCH 3/3] configfs: Fix failing symlink() making rmdir() fail

Louis Rilling
2008-06-18T11:40:54Z

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On Tue, Jun 17, 2008 at 03:15:28PM -0700, Joel Becker wrote:
> On Tue, Jun 17, 2008 at 07:37:23PM +0200, Louis Rilling wrote:
> > For the parent's rmdir() case, we can use the same solution as with mkdir() vs
> > rmdir(). For the target's rmdir() case, we cannot, since we do not and cannot
> > lock the target's inode while in symlink(). Fortunately, once create_link()
> > terminates, no further operation can fail in symlink(). So we first reorder the
> > operations in create_link() to attach the new symlink to its target in last
> > place, and second handle symlink creation failure the same way as a new item
> > creation failure.
>
> Oh, no, ugh. We don't want to create vfs objects first and ask
> questions later. Otherwise we wouldn't need ATTACHING - we'd just
> create the symlink, then check dropping.
> If you have ATTACHING set, the rmdir cannot continue - you can
> check dropping at that time. That is, you keep the DROPPING check where
> it is - if it is already set, you know that rmdir() is going to complete
> successfully. You can bail before even calling configfs_create_link().
> If, however, it isn't set, your ATTACHING protects you from rmdir
> throughout.

The problem is rmdir() of the target item (see below). ATTACHING only protects
us from rmdir() of the parent. This is the exact reason why I attach the link to
the target in last place, where we know that we won't have to rollback.
And AFAICS, creating a VFS object can not hurt as long as we hold the
parent i_mutex, right? Otherwise there already is a problem in
configfs_attach_item() where a failure in populate_attrs() leads to rollback the
creation of the VFS object already created for the item.

>
> sl = kmalloc(sizeof(struct configfs_symlink), GFP_KERNEL);
> if (sl) {
> sl->sl_target = config_item_get(item);
> spin_lock(&configfs_dirent_lock);
> if (target_sd->s_type & CONFIGFS_USET_DROPPING) {
> spin_unlock(&configfs_dirent_lock);
> config_item_put(item);
> kfree(sl);
> return -ENOENT;
> /*
> * Force rmdir() of parent_item to wait until we know
> * if we succeed.
> */
> parent_sd->s_type |= CONFIGFS_USET_ATTACHING;
> }
> list_add(&sl->sl_list, &target_sd->s_links);
> spin_unlock(&configfs_dirent_lock);
> ret = configfs_create_link(sl, parent_item->ci_dentry,
> dentry);
> spin_lock(&configfs_dirent_lock);
> parent_sd->s_type &= ~CONFIGFS_USET_ATTACHING;
> if (ret) {

Here, if detach_prep() of the target failed because of the link attached above,
it had no means to retry. rmdir() of the target fails because of this
temporary link, which results in a failing symlink() making rmdir() of the
target fail.

> list_del_init(&sl->sl_list);
> spin_unlock(&configfs_dirent_lock);
> config_item_put(item);
> kfree(sl);
> } else
> spin_unlock(&configfs_dirent_lock);
> }
>
> return ret;
>

--
Dr Louis Rilling Kerlabs
Skype: louis.rilling Batiment Germanium
Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
http://www.kerlabs.com/ 35700 Rennes

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