Date: Tue, 21 Jul 2009 17:28:41 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Daniel Mack <daniel@caiaq.de>
Cc: Alan Stern <stern@rowland.harvard.edu>,
       Kernel development list <linux-kernel@vger.kernel.org>,
       USB list <linux-usb@vger.kernel.org>
Subject: Re: [PATCH] [usb-serial] fix Ooops on uplug
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In-Reply-To: <20090721161928.GD13236@buzzloop.caiaq.de>
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> It was, at least in my case. Did some printk debugging and it showed
> that the hangup callback was entered what freed the serial device.

Ok 
> 
> > > This is only a lockdep warning, and I don't understand its
> > > significance.  Even worse, when I plugged in a USB flash drive
> > > afterward this appeared:
> > 
> > Looks like something freed the resources but didn't hang up when the
> > disconnect occurred
> 
> Probably the real fix would be to let the tty layer know the device died
> from the hangup handler.

The hangup event means the physical hardware must no longer be touched so
that's already the case and we use it for a lot of things (from ISAPnP to
PCMCIA to USB).

The USB code does slightly misimplement it - it calls tty_hangup() in
usb_serial_disconnect when it should call tty_vhangup() as it needs the
event to be synchronous. That is the hangup of the tty should be done
before the USB layer can sneak off and free stuff.

I don't at this point see that as explaining the bug however
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