Message-ID: <20080331135653.ZZRA012@mailhub.coreip.homeip.net>
Date:	Mon, 31 Mar 2008 14:01:20 -0400
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Greg KH <greg@...ah.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bj?rn Steinbrink <B.Steinbrink@....de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Johannes Berg <johannes@...solutions.net>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH] evdev: Release eventual input device grabs when getting
	disconnected

On Mon, Mar 31, 2008 at 10:28:13AM -0700, Greg KH wrote:
> On Mon, Mar 31, 2008 at 02:15:39AM -0400, Dmitry Torokhov wrote:
> > Hi Linus,
> > 
> > On Sunday 30 March 2008, Linus Torvalds wrote:
> > > 
> > > On Sun, 30 Mar 2008, Bj?rn Steinbrink wrote:
> > > >
> > > > I can't reproduce the bug on my UP box and currently can't afford
> > > > crashing my SMP box (all the oopses seem to come from SMP kernels, so I
> > > > guess it needs SMP to crash), so while this doesn't show any new
> > > > problems, I can't tell whether it actually fixes anything. Testers
> > > > welcome!
> > > 
> > > Ok, I applied this because I will do an -rc8 today or tomorrow, but I 
> > > really really hope somebody can figure out what made this all start to 
> > > trigger. It does smell like some core device layer change, because we do 
> > > not seem to have a lot of changes since 2.6.24 in evdev.c and input.c that 
> > > seem relevant.
> > > 
> > > Greg, are there any refcounting changes that would cause the input devices 
> > > to be free'd earlier or something?
> > > 
> > 
> > The following commit changed lifetime runes on kobjects breaking input:
> > 
> > commit 0f4dafc0563c6c49e17fe14b3f5f356e4c4b8806
> > Author: Kay Sievers <kay.sievers@...y.org>
> > Date:   Wed Dec 19 01:40:42 2007 +0100
> > 
> >     Kobject: auto-cleanup on final unref
> > 
> >     We save the current state in the object itself, so we can do proper
> >     cleanup when the last reference is dropped.
> > 
> >     If the initial reference is dropped, the object will be removed from
> >     sysfs if needed, if an "add" event was sent, "remove" will be send, and
> >     the allocated resources are released.
> > 
> >     This allows us to clean up some driver core usage as well as allowing us
> >     to do other such changes to the rest of the kernel.
> > 
> >     Signed-off-by: Kay Sievers <kay.sievers@...y.org>
> >     Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
> > 
> > Before we dropped reference to kobject's parent only when child kobject
> > was released (in kobject_cleanup). The changeset above moves the release
> > to kobject_del() which is way too early in my opinion. The kobject is only
> > marked for deletion at that time, not really deleted.
> 
> It was "deleted" from sysfs, and should have never been used again by
> any callers.  If the reference count was dropped to zero with this call,
> it would be cleaned up as well, it seems that you were assuming that it
> would not be?  Perhaps you just need to grab another reference as this
> would have caused you problems without this change anyway, but without
> slab debugging, you never saw it.
> 

Greg, please look at the change again. Before kobject_put(kobj->parent)
was done in kobject_cleanup() and so the parent would only be freed when
all its children are gone. Now parent is deleted early, even if its
children are still referenced by other users. This is lifetime rule
change and should really be announced as such.

If this change it intentional and is here to stay then I will just grab
the references myself, although I wonder what else might be broken by
it.

-- 
Dmitry
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