linux-kernel - Re: Race condition in TTY ldisc layer in 2.6.33

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Date:	Thu, 11 Mar 2010 17:06:53 +0100
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Greg KH <greg@...ah.com>
Cc:	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: Race condition in TTY ldisc layer in 2.6.33

On Thu, Mar 11, 2010 at 04:50:15PM +0100, Michał Mirosław wrote:
> On Thu, Mar 11, 2010 at 05:50:09AM -0800, Greg KH wrote:
> > On Thu, Mar 11, 2010 at 02:32:22PM +0100, Micha? Miros?aw wrote:
> > > Looks like there's some race contition in switching ldiscs on USB serial
> > > ports. The following warnings trigger sometimes after killing and restarting
> > > process that changes ldisc and waits forever. In case you want to look at
> > > the code, there it is: http://rere.qmqm.pl/~mirq/sermmc/
> > If you apply git commit 638b9648ab51c9c549ff5735d3de519ef6199df3 to the
> > 2.6.33 kernel, does it solve the issue for you?
> I'm running with the patch now. After couple cycles of starting and killing
> the ldisc-setting process I get no warnings. I'll get back to you when/if
> I encounter them again.

Hah. Just seconds after I sent that mail I hit this again. The stack traces
are exactly the same (except for different starting address of
flush_to_ldisc()).

The warnings are triggered by:

	struct usb_serial_port *port = tty->driver_data;
	...
	/* count is managed under the mutex lock for the tty so cannot
	   drop to zero until after the last close completes */
	WARN_ON(!port->port.count);

BTW, Couple of minutes earlier I got this message:

[  201.629616] cp210x ttyUSB0: usb_serial_generic_resubmit_read_urb - failed resubmitting read urb, error -1

But after that I disconnected and reconnected the device, so this is probably
not relevant here.

Best Regards,
Michał Mirosław
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