linux-kernel - Re: [PATCH v3 00/23] ldisc fixes

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Message-ID: <1362522029.18799.145.camel@thor.lan>
Date:	Tue, 05 Mar 2013 17:20:29 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Sasha Levin <levinsasha928@...il.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ilya Zykov <ilya@...x.ru>, Dave Jones <davej@...hat.com>
Subject: Re: [PATCH v3 00/23] ldisc fixes

[--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.

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.

Second, when the hangup actually does run -- in __tty_hangup() -- it
expects the tty to exist. I didn't go looking through the gadget layers
to see if the tty was disposed some other way, which might race the
asynchronous tty hangup.

Thanks,
Peter Hurley

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