linux-kernel - Re: [Fwd: [PATCH v2 0/4] TTY: port hangup and close fixes]

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Message-ID: <1362522724.18799.149.camel@thor.lan>
Date:	Tue, 05 Mar 2013 17:32:04 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	jhovold@...il.com, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Stern <stern@...land.harvard.edu>,
	USB list <linux-usb@...r.kernel.org>,
	linux-serial@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Fwd: [PATCH v2 0/4] TTY: port hangup and close fixes]

On Tue, 2013-03-05 at 23:10 +0100, Jiri Slaby wrote:
> On 03/05/2013 11:02 PM, Peter Hurley wrote:
> > On Tue, 2013-03-05 at 22:56 +0100, Jiri Slaby wrote:
> >> On 03/05/2013 06:06 PM, Peter Hurley wrote:
> >>>>> @@ -225,15 +232,13 @@ void tty_port_hangup(struct tty_port *port)
> >>>>         spin_lock_irqsave(&port->lock, flags);
> >>>>         port->count = 0;
> >>>>         port->flags &= ~ASYNC_NORMAL_ACTIVE;
> >>>> -       if (port->tty) {
> >>>> +       if (port->tty)
> >>>>                 set_bit(TTY_IO_ERROR, &port->tty->flags);
> >>>> -               tty_kref_put(port->tty);
> >>>> -       }
> >>>> -       port->tty = NULL;
> >>>>         spin_unlock_irqrestore(&port->lock, flags);
> >>>>> +       tty_port_shutdown(port, port->tty);
> >>>>
> >>>> What prevents port->tty to be NULL here already?
> >>>
> >>> Nothing. That's why it's tested in tty_port_shutdown() above.
> >>
> >> I know :).
> > 
> > Sorry :)
> > 
> >> But the question is rather don't we want to pass the real
> >> refcounted port->tty (take a snapshot inside the lock) instead?
> > 
> > I think that's why he moved the kref release to after the shutdown (via
> > tty_port_set_tty()) -- but I'm tired and maybe I'm missing something
> > here?
> 
> port->tty can be changed right after the unlock.

Right. My bad. Thanks for catching this.

>  So I'm thinking about
> something like this:
> 
> if (port->tty)
>    set_bit(TTY_IO_ERROR, &port->tty->flags);
> tty = port->tty; <=== take a snapshot
> spin_unlock_irqrestore(&port->lock, flags);
> tty_port_shutdown(port, tty); <=== use the snapshot
> set_tty_port(port, NULL); <=== put kref on that tty

Yeah, that's better.

Regards,
Peter Hurley

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