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

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Message-ID: <4ECD6375.5050404@suse.cz>
Date:	Wed, 23 Nov 2011 22:19:49 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Havard Skinnemoen <hskinnemoen@...gle.com>
CC:	Jiri Slaby <jirislaby@...il.com>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Alan Cox <alan@...ux.intel.com>
Subject: Re: [RFC] cdc-acm: Fix potential deadlock (lockdep warning)

On 11/23/2011 10:11 PM, Jiri Slaby wrote:
> On 11/23/2011 10:08 PM, Havard Skinnemoen wrote:
>> tty_port_close_start() takes port->lock, so if we try to do that,
>> we'll turn a potential deadlock into a real one. Or did you mean
>> something else?
> 
> I mean the other uses of port->count in your driver.
> 
>> I think I'll try to follow Alan's advice and use tty_port_close()
>> instead of the split functions.
> 
> Yes, that's indeed the best approach. However you have to use them all:
> tty_port_open, tty_port_hangup, tty_port_close. OR you would have to do
> the locking of count properly.

And I see now that the driver uses tty_port_hangup (count under
port->lock) and tty_port_close_start (count under open_mutex and
port->lock). But doesn't use tty_port_open (count under open_mutex and
acm->mutex). Hence there was always a race hangup vs. open wrt. port
counting.

thanks,
-- 
js
suse labs
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