On Wed, Apr 15, 2020 at 10:47 AM Heikki Krogerus
<heikki.krogerus@xxxxxxxxxxxxxxx> 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?
