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Message-ID: <50ADAA26.7080103@ilyx.ru>
Date: Thu, 22 Nov 2012 08:29:26 +0400
From: Ilya Zykov <ilya@ilyx.ru>
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To: andrew mcgregor <andrew.mcgregor@alliedtelesis.co.nz>
CC: Alan Cox <alan@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tty: Add driver unthrottle in ioctl(...,TCFLSH,..).
References: <50AD43DB.4030800@ilyx.ru> <20121121213037.154306da@pyramind.ukuu.org.uk> <50AD4A01.7060500@ilyx.ru> <50AE2CE30200003600026B9A@gwia.alliedtelesyn.co.nz>
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On 22.11.2012 4:47, andrew mcgregor wrote:
>
>
>>>> On 11/22/2012 at 10:39 AM, in message <50AD4A01.7060500@ilyx.ru>, Ilya Zykov
> <ilya@ilyx.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).