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

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Date:	Tue, 05 Mar 2013 23:39:47 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Peter Hurley <peter@...leysoftware.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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

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?

> 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.

-- 
js
suse labs
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