Re: [PATCH v1] kobject: make sure parent is not released before children

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Apr 15, 2020 at 10:47 AM Heikki Krogerus
<heikki.krogerus@linux.intel.com> wrote:
>
> Hi Greg,
>
> On Wed, Apr 15, 2020 at 08:11:54AM +0200, Greg KH wrote:
> > > diff --git a/lib/kobject.c b/lib/kobject.c
> > > index 83198cb37d8d..5921e2470b46 100644
> > > --- a/lib/kobject.c
> > > +++ b/lib/kobject.c
> > > @@ -663,6 +663,7 @@ EXPORT_SYMBOL(kobject_get_unless_zero);
> > >   */
> > >  static void kobject_cleanup(struct kobject *kobj)
> > >  {
> > > +   struct kobject *parent = kobj->parent;
> > >     struct kobj_type *t = get_ktype(kobj);
> > >     const char *name = kobj->name;
> > >
> > > @@ -680,6 +681,9 @@ static void kobject_cleanup(struct kobject *kobj)
> > >             kobject_uevent(kobj, KOBJ_REMOVE);
> > >     }
> > >
> > > +   /* make sure the parent is not released before the (last) child */
> > > +   kobject_get(parent);
> > > +
> > >     /* remove from sysfs if the caller did not do it */
> > >     if (kobj->state_in_sysfs) {
> > >             pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
> > > @@ -693,6 +697,8 @@ static void kobject_cleanup(struct kobject *kobj)
> > >             t->release(kobj);
> > >     }
> > >
> > > +   kobject_put(parent);
> > > +
> >
> > No, please don't do this.
> >
> > A child device should have always incremented the parent already if it
> > was correctly registered.  We have had this patch been proposed multiple
> > times over the years, and every time it was, we said no and went and
> > fixed the real issue which was with the user of the interface.
>
> The parent ref count is incremented by the child, that is not the
> problem. The problem is that when that child is released, if it's the
> last child of the parent, and there are no other users for the parent,
> then the parent is actually released _before_ the child. And that
> happens in the above function kobject_cleanup().

In fact, it happens in kobject_del() invoked by kobject_cleanup() AFAICS.

So it appears incorrect to use kobject_del() as is in the latter.

> We can work around the problem by taking a reference to the parent
> separately, but we have to do that everywhere separately (which I
> guess is exactly what has been done so far). That workaroud still does
> not really fix the core problem. The core problem is still that
> lib/kboject.c is allowing the parent kobject to be released before the
> child kobject, and that quite simply should not be allowed to happen.
>
> I don't have a problem if you want to have a better solution for this,
> but the solution really can't anymore be that we are always expected
> to separately increment the parent's ref count with every type of
> kobject.

An alternative might be to define something like __kobject_del() doing
everything that kobject_del() does *without* the
kobject_put(kobj->parent).

Then, kobject_del() could be defined as something like (pseudocode):

kobject_del(kobj)
{
    kobject *perent = kobj->parent;

    __kobject_del(kobj);
    kobject_put(parent);
}

and kobject_cleanup() could call __kobject_del() instead of
kobject_del() and then do the last kobject_put(parent) when it is done
with the child.

Would that work?