linux-kernel - Re: [RFC] cdc-acm: Fix potential deadlock (lockdep warning)

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Date:	Wed, 23 Nov 2011 11:22:32 -0800
From:	Havard Skinnemoen <hskinnemoen@...gle.com>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: [RFC] cdc-acm: Fix potential deadlock (lockdep warning)

On Wed, Nov 23, 2011 at 11:22 AM, Alan Cox <alan@...ux.intel.com> wrote:
>        take lock
>        mark device disconnected
>        drop lock
>        for each tty on the device {
>                tty_port_tty_get
>                if (tty) {
>                        tty_vhangup(tty);
>                        tty_kref_put(tty);
>        }
>        then do the USB disconnect processing
>        and drop our USB side kref

Ok, by USB side kref do you mean the tty_port_get/put calls I
introduced in this patch, or a kref associated with the USB device
itself?

> and on the open side they provide their own install() method which
> nicely avoids all the nasty locking problems and lets them use the
> standard tty_port_open/close/hangup etc rather than the partial ones.

Right, that makes sense.

> It might be worth turning the ACM driver to work the same way as it'll
> save doing that work later to achieve the same use of
> tty_port_open/close/..
>
> The main thing is the use of install - which lets you plug the USB and
> tty bits together during the lookup of the device.

Ok, so the basic idea is:
  * USB side: tty_port_get() in probe, tty_port_put() in disconnect
  * TTY side: tty_port_get() in install, tty_port_put() in remove

And I need to check if the device is properly marked as disconnected,
switch to vhangup, and possibly reorder things a bit as well. Right?

Havard
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