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SubjectRe: [PATCH v3 00/23] ldisc fixes
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[now this discussion has turned to usb gadget
+cc Felipe Balbi, linux-usb, -cc Dave Jones, Ilya Zykov]

On Tue, 2013-03-05 at 23:39 +0100, Jiri Slaby wrote:
> On 03/05/2013 11:20 PM, Peter Hurley wrote:
> > [--cc Alan Cox]
> >
> > On Tue, 2013-03-05 at 21:50 +0100, Sebastian Andrzej Siewior wrote:
> >> * Peter Hurley | 2013-02-05 15:20:15 [-0500]:
> >>
> >>> Please re-test with your dummy_hcd/g_nokia testcase, although
> >>> I'm not convinced that usb gadget is using tty_hangup() appropriately.
> >>> tty drivers use this for async carrier loss coming from an IRQ
> >>> which will be disabled if the tty has been shutdown. Does gserial
> >>> prevent async hangup to a dead tty in a similar fashion?
> >>
> >> Not sure I understood. tty_hangup() is only called from within
> >> gserial_disconnect() which calls right after usb_ep_disable(). After
> >> usb_ep_disable() no further serial packets can be received until the
> >> endpoints are re-enabled. This happens in gserial_connect().
> >
> > That's why I asked. There are two potential issues:
> >
> > First, tty_hangup() is asynchronous -- ie., it returns immediately. It
> > does not wait for the tty device to actually perform the hangup. So if
> > the gadget layers start cleanup immediately after, expecting that they
> > won't get a flurry of tty calls, that would be bad.
>
> Sorry, I missed what driver is this?

g_serial.

drivers/usb/gadget/u_serial.c

> > tty_vhangup() is synchronous -- ie., you wait while it cleans up. This
> > is what the usb serial core does on it's disconnect() method. But I
> > didn't research further if the circumstances were the same.
>
> Even when tty_vhangup returns, it does not guarantee a closed tty. And
> it also does not guarantee that any of tty->ops won't be called. The
> latter is true only for devices that can be consoles. (For those,
> file->ops are not redirected.) In that case one needs to wait for
> port->count to become 0.

Perhaps I was oversimplifying.

But my point was I doubt usb gadget is conducting its teardown safely
wrt tty.

Regards,
Peter Hurley





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