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

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Message-ID: <CAFQmdRbP3qXc2n_Qto0o3Lg1280gg7mPHVSY28S8nBT39YPPQQ@mail.gmail.com>
Date:	Wed, 23 Nov 2011 13:08:34 -0800
From:	Havard Skinnemoen <hskinnemoen@...gle.com>
To:	Jiri Slaby <jslaby@...e.cz>
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 Wed, Nov 23, 2011 at 11:55 AM, Jiri Slaby <jslaby@...e.cz> wrote:
> On 11/23/2011 07:53 PM, Havard Skinnemoen wrote:
>> --- a/drivers/usb/class/cdc-acm.c
>> +++ b/drivers/usb/class/cdc-acm.c
> ...
>> @@ -567,19 +574,10 @@ static void acm_tty_close(struct tty_struct *tty, struct file *filp)
>>
>>       /* Perform the closing process and see if we need to do the hardware
>>          shutdown */
>> -     if (!acm)
>> -             return;
>> -
>> -     mutex_lock(&open_mutex);
>>       if (tty_port_close_start(&acm->port, tty, filp) == 0) {
>
> Note that port->count is protected by port->lock usually. Till now it
> used to be protected by open_mutex in your driver. As of now it is not
> protected by anything. (Well, BTM is still there to save you, but...)

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 think I'll try to follow Alan's advice and use tty_port_close()
instead of the split functions. it shouldn't be too hard once we get
the lifecycle issues out of the way.

Havard
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