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

[Ilya Zykov]

On 22.11.2012 9:25, andrew mcgregor wrote:
> > > > On 11/22/2012 at 05:29 PM, in message <50ADAA26.7080103@xxxxxxx>, Ilya Zykov
> <ilya@xxxxxxx> 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).
>
> Well, the driver in question was just 8250.c, so you should be able to see that the original condition can exist.
>
> Here's the commit message again:
>
> tty: fix "IRQ45: nobody cared"
>
> Unthrottling the TTY during close ends up enabling interrupts
> on a device not on the active list, which will never have the
> interrupts cleared.  Doctor, it hurts when I do this.
>
> > > > On 6/2/2011 at 01:56 AM, in message <20110601145608.3e586e16@xxxxxxxxxxxxxxxx>, Alan Cox <alan@xxxxxxxxxxxxxxx> wrote:
> > On Wed, 01 Jun 2011 10:34:07 +1200
> > "andrew mcgregor" <andrew.mcgregor@xxxxxxxxxxxxxxxxxxx> wrote:
> > > The LKML message
> > > http://kerneltrap.org/mailarchive/linux-kernel/2010/2/25/4541847from
> > > February doesn't seem to have been resolved since.  We struck the
> > > issue, and the patch below (against 2.6.32) fixes it.  Should I
> > > supply a patch against 3.0.0rc?
> >
> > I think that would be sensible. I don't actually see how you hit it as
> > the IRQ ought to be masked by then but it's certainly wrong for n_tty
> > to be calling into check_unthrottle at that point.
> >
> > So yes please send a patch with a suitable Signed-off-by: line to
> > linux-serial and cc GregKH <greg@xxxxxxxxx> as well.
> >
> > Alan
>
> Signed-off-by: Andrew McGregor <andrew.mcgregor@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
>
> What part of this no longer applies?  I'm happy enough if you can prove that this can't happen any more, but otherwise the fix should remain.
>
> Andrew

This patch must help for 8250.c

diff --git a/drivers/tty/serial/serial_core.c 
b/drivers/tty/serial/serial_core.c
index a21dc8e..2e197c3 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1288,8 +1288,6 @@ static void uart_close(struct tty_struct *tty, 
struct file *filp)
        uart_shutdown(tty, state);
        uart_flush_buffer(tty);

-       tty_ldisc_flush(tty);
-
        tty_port_tty_set(port, NULL);
        spin_lock_irqsave(&port->lock, flags);
        tty->closing = 0;

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