linux-kernel - Re: [PATCH 2/2] kobject: send KOBJ_REMOVE uevent when the object is removed from sysfs

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Date:   Tue, 26 May 2020 10:26:23 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH 2/2] kobject: send KOBJ_REMOVE uevent when the object is
 removed from sysfs

On Tue, May 26, 2020 at 7:58 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Mon, May 25, 2020 at 03:49:01PM -0700, Dmitry Torokhov wrote:
> > On Sun, May 24, 2020 at 8:34 AM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > It is possible for a KOBJ_REMOVE uevent to be sent to userspace way
> > > after the files are actually gone from sysfs, due to how reference
> > > counting for kobjects work.  This should not be a problem, but it would
> > > be good to properly send the information when things are going away, not
> > > at some later point in time in the future.
> > >
> > > Before this move, if a kobject's parent was torn down before the child,
> >
> > ^^^^ And this is the root of the problem and what has to be fixed.
>
> I fixed that in patch one of this series.  Turns out the user of the
> kobject was not even expecting that to happen.
>
> > > when the call to kobject_uevent() happened, the parent walk to try to
> > > reconstruct the full path of the kobject could be a total mess and cause
> > > crashes.  It's not good to try to tear down a kobject tree from top
> > > down, but let's at least try to not to crash if a user does so.
> >
> > One can try, but if we keep proper reference counting then kobject
> > core should take care of actually releasing objects in the right
> > order. I do not think you should keep this patch, and instead see if
> > we can push call to kobject_put(kobj->parent) into kobject_cleanup().
>
> I tried that, but there was a _lot_ of underflow errors reported, so
> there's something else happening.  Or my attempt was incorrect :)

So it looks like there is something in there that's been overlooked so far.

I'll try to look at the Guenter's traces and figure out what went
wrong after the Heikki's patch.