On Wed, Jul 22, 2009 at 10:39:51AM +0100, Alan Cox wrote:
> See if this one looks sensible. It does leave a tiny race window but that
> semes wiser than hacking up the tty kref_put path in the middle of an -rc
> series.
>
> Thanks to Daniel and Alan Stern for chasing this down and getting traces. Also
> to Daniel for being persistent when I took it as a random odd "only seen by one
> user" error which it wasn't.
Thanks Alan for your patience. You know the tty layer well which I have
no clue of, so I missed the bits in the close callback.
I tested your patch and can confirm it fixes the problem for me.
Daniel
> The port lock is used to protect the port state. However the port structure
> is freed on a hangup, then the lock taken on a close. The right fix is to
> drop the port on tty->shutdown() but we can't yet do that due to sleep v
> non-sleeping rules. Instead do the next best thing and fix it up when we are
> not in -rc season.
>
> Reported-by: Daniel Mack <daniel@xxxxxxxx>
> Signed-off-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
Tested-by: Daniel Mack <daniel@xxxxxxxx>
> ---
>
> drivers/usb/serial/usb-serial.c | 19 ++++++++++++++++++-
> 1 files changed, 18 insertions(+), 1 deletions(-)
>
>
> diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
> index bd7581b..228d77c 100644
> --- a/drivers/usb/serial/usb-serial.c
> +++ b/drivers/usb/serial/usb-serial.c
> @@ -340,6 +340,22 @@ static void serial_close(struct tty_struct *tty, struct file *filp)
>
> dbg("%s - port %d", __func__, port->number);
>
> + /* FIXME:
> + This leaves a very narrow race. Really we should do the
> + serial_do_free() on tty->shutdown(), but tty->shutdown can
> + be called from IRQ context and serial_do_free can sleep.
> +
> + The right fix is probably to make the tty free (which is rare)
> + and thus tty->shutdown() occur via a work queue and simplify all
> + the drivers that use it.
> + */
> + if (tty_hung_up_p(filp)) {
> + /* serial_hangup already called serial_down at this point.
> + Another user may have already reopened the port but
> + serial_do_free is refcounted */
> + serial_do_free(port);
> + return;
> + }
>
> if (tty_port_close_start(&port->port, tty, filp) == 0)
> return;
> @@ -355,7 +371,8 @@ static void serial_hangup(struct tty_struct *tty)
> struct usb_serial_port *port = tty->driver_data;
> serial_do_down(port);
> tty_port_hangup(&port->port);
> - serial_do_free(port);
> + /* We must not free port yet - the USB serial layer depends on it's
> + continued existence */
> }
>
> static int serial_write(struct tty_struct *tty, const unsigned char *buf,
>
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