linux-kernel - Re: [PATCH] tty: Fix close races in USB serial

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Date:	Tue, 4 Nov 2008 09:55:42 -0800
From:	Greg KH <greg@...ah.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Fix close races in USB serial

On Tue, Nov 04, 2008 at 05:37:26PM +0000, Alan Cox wrote:
> > > -	if (port->port.count == (port->console? 1 : 0)) {
> > > +	if (port->port.count == (port->console ? 2 : 1)) {
> > 
> > Why are you testing for 2 here?
> 
> At the point we enter this routine port->port.count is the number of
> users of the port. If this is the final close then this is 1 (us), except
> if it is the USB console (in which case there will be two references).
> 
> The old code dropped the reference count first, the new code drops the
> kref and port->tty first, then shuts down the port and then drops the
> refcount. That avoids the race where we get the usb serial drivers trying
> to stuff bytes into a closed port and the ldisc layer in turn trying to
> echo them
> - which causes the warnings.

Ah, ok, that makes sense, thanks.

You might want to document that in there so that others understand it as
well.

thanks,

greg k-h
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