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

[Ilya Zykov]

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@xxxxxxx>,
> > > > > Ilya Zykov
> > <ilya@xxxxxxx> 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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