[Ocfs2-devel] [RFC] configfs: Pin configfs subsystems separately from new config_items.

[Louis Rilling]

On Mon, Jun 23, 2008 at 12:10:28PM -0700, Joel Becker wrote:
> On Mon, Jun 23, 2008 at 05:44:57PM +0200, Louis Rilling wrote:
> > make_item()
> > 	new_item = kmalloc();
> > 	config_item_init_type_long_name();
> > 	return new_item;
> > 
> > drop_item(item)
> > 	config_item_put(item);
> > 	kfree(item);
> 
> 	This is never, ever safe.  Consider that someone has an
> attribute file open - it has a reference to the item.  You can still
> rmdir() the item - doing stuff to the attribute after drop_item() will
> just get ignored.  But you can't free it in drop_item().

Yup, I realized it this night (prevented me from sleeping by the way).
The previous arguments remain valid though. I hope that you won't discard
them because of this buggy one :)

While looking at file.c, I found this in configfs_release():

struct config_item * item = to_item(filp->f_path.dentry->d_parent);
[...]
if (item)
	config_item_put(item);

It looks strange:
1/ either item may be NULL, and there is a probable memory leak because of the
reference grabbed in configfs_open_file();
2/ or item may never be NULL and this check is just useless and (at least for
me) misleading.

If I understand correctly, option 2 is correct because
a/ even if .dentry gets unhashed and .dentry->d_parent gets unhashed as well,
VFS ensures that filp->f_path.dentry->d_parent is unchanged unless .dentry was
renamed, which is not permitted by configfs, and, I guess, will never be
permitted for a configfs attribute;
b/ once dentry->d_fsdata points to a configfs_dirent, it never changes
and remains valid for the rest of dentry's life (until dentry_iput());
c/ configfs_dirent->s_element never changes once it is set.

Am I wrong somewhere?

Thanks

Louis

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Dr Louis Rilling			Kerlabs - IRISA
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