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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Mon, 3 Aug 2020 10:25:11 +0300
Message-ID: <CAHp75VcbSR1NSSPemg5dMyfp00uC4wkktVjKSFx4sjSgFC-_vQ@mail.gmail.com>
Subject: Re: [PATCH] kobject: Avoid premature parent object freeing in kobject_cleanup()
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Guenter Roeck <guenter@roeck-us.net>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
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On Mon, Aug 3, 2020 at 9:47 AM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
> On 2020/6/5 =E4=B8=8A=E5=8D=881:46, Rafael J. Wysocki wrote:

...

> > +/**
> > + * kobject_del() - Unlink kobject from hierarchy.
> > + * @kobj: object.
> > + *
> > + * This is the function that should be called to delete an object
> > + * successfully added via kobject_add().
> > + */
> > +void kobject_del(struct kobject *kobj)
> > +{
> > +     struct kobject *parent =3D kobj->parent;
> > +
> > +     __kobject_del(kobj);
> > +     kobject_put(parent);
>
> Could you please add an extra check on kobj before accessing kobj->parent=
?

I do not understand. Where do we access it?
kobject_put() is NULL-aware.

> This patch in fact removes the ability to call kobject_del() on NULL
> pointer while not cause anything wrong.
>
> I know this is not a big deal, but such behavior change has already
> caused some problem for the incoming btrfs code.
> (Now I feels guilty just by looking into the old
> kobject_del()/kobject_put() and utilize that feature in btrfs)
>
> Since the old kobject_del() accepts NULL pointer intentionally, it would
> be much better to keep such behavior.

Can you elaborate, please?

--=20
With Best Regards,
Andy Shevchenko