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

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Message-ID: <20200527075048.GD3284396@kuha.fi.intel.com>
Date:   Wed, 27 May 2020 10:50:48 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 2/2] kobject: send KOBJ_REMOVE uevent when the object is
 removed from sysfs

On Tue, May 26, 2020 at 10:26:23AM +0200, Rafael J. Wysocki wrote:
> 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.

At least one problem with that patch was that I was releasing the
parent reference unconditionally.

thanks,

-- 
heikki