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

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Date:	Thu, 22 Nov 2012 10:03:01 +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 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.


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