linux-kernel - Re: [PATCH] tty: Add driver unthrottle in ioctl(...,TCFLSH,..).

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Date:	Thu, 22 Nov 2012 10:16:39 +0400
From:	Ilya Zykov <ilya@...x.ru>
To:	andrew mcgregor <andrew.mcgregor@...iedtelesis.co.nz>
CC:	Alan Cox <alan@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Add driver unthrottle in ioctl(...,TCFLSH,..).

On 22.11.2012 10:03, Ilya Zykov wrote:
> On 22.11.2012 8:29, Ilya Zykov wrote:
>> On 22.11.2012 4:47, andrew mcgregor wrote:
>>>
>>>
>>>>>> On 11/22/2012 at 10:39 AM, in message <50AD4A01.7060500@...x.ru>,
>>>>>> Ilya Zykov
>>> <ilya@...x.ru> wrote:
>>>> On 22.11.2012 1:30, Alan Cox wrote:
>>>>>> Function reset_buffer_flags() also invoked during the
>>>>>> ioctl(...,TCFLSH,..). At the time of request we can have full buffers
>>>>>> and throttled driver too. If we don't unthrottle driver, we can get
>>>>>> forever throttled driver, because after request, we will have
>>>>>> empty buffers and throttled driver and there is no place to
>>>>>> unthrottle
>>>> driver.
>>>>>> It simple reproduce with "pty" pair then one side sleep on
>>>>>> tty->write_wait,
>>>>>> and other side do ioctl(...,TCFLSH,..). Then there is no place to do
>>>> writers wake up.
>>>>>
>>>>>
>>>>> So instead of revertng it why not just fix it ? Just add an
>>>>> argument to
>>>>> the reset_buffer_flags function to indicate if unthrottling is
>>>>> permitted.
>>>>>
>>>>> Alan
>>>>>
>>>> Because in my opinion, unthrottling permitted always, except release
>>>> last filp (tty->count == 0)
>>>
>>> Maybe so, but the patch was there in the first place to resolve an
>>> actual observed bug, where a driver would lock up.  So the behaviour
>>> needs preserved.
>>>
>>> Andrew
>>>
>>
>> Maybe it was wrong driver, unfortunately, I didn't find full information
>> about this bug. As an example, if driver indirectly call
>> reset_buffer_flags() in driver's close() function it will be before
>> decrement last (tty->count).
>>
>>
>
> Particularly, many drivers and 'serial_core.c' use tty_ldisc_flush() in
> own close() function. tty_ldisc_flush() call reset_buffer_flags()
> indirectly.
> I think is wrong way use tty_ldisc_flush() in driver's close() function,
> because tty layer 'tty_release()' call  tty_ldisc_release() after
> decremented (tty->count), and clear all buffers.
> We don't care about this in driver. And call ldisc's function.
>
>
Sorry, last phrase not correct.
We don't NEED care about this in driver. And call ldisc's function.

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