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

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Message-Id: <50AE2CE30200003600026B9A@gwia.alliedtelesyn.co.nz>
Date:	Thu, 22 Nov 2012 13:47:15 +1300
From:	"andrew mcgregor" <andrew.mcgregor@...iedtelesis.co.nz>
To:	"Ilya Zykov" <ilya@...x.ru>, "Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: Add driver unthrottle in
 ioctl(...,TCFLSH,..).



>>> 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

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