linux-kernel - Re: stty blocks forever when the line is already opened (and the tx buffer can't be flushed)

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Message-ID: <56A953D0.4070600@hurleysoftware.com>
Date:	Wed, 27 Jan 2016 15:33:36 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Richard Genoud <richard.genoud@...il.com>
Cc:	Jiri Slaby <jslaby@...e.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: stty blocks forever when the line is already opened (and the tx
 buffer can't be flushed)

On 01/27/2016 02:26 AM, Richard Genoud wrote:
> 2016-01-27 10:14 GMT+01:00 Richard Genoud <richard.genoud@...il.com>:
>> 2016-01-26 18:13 GMT+01:00 Peter Hurley <peter@...leysoftware.com>:
>>> Hi Richard,
>>>
>>> On 01/26/2016 08:19 AM, Richard Genoud wrote:
>>>> [ sorry for the noise, I forgot to Cc the lkml ]
>>>>
>>>> Hi,
>>>> I've found a case were calling
>>>> stty -F /dev/ttyS1 clocal
>>>> blocks forever.
>>>> And I don't know if it's a very old bug or if it's meant to be like that.
>>>>
>>>> Here is how to reproduce the lock :
>>>> NB: there's NO modem on ttyS1
>>>> stty -F /dev/ttyS1 clocal cread crtscts
>>>> cat < /dev/ttyS1
>>>>
>>>> #on another terminal :
>>>> echo "dummy" > /dev/ttyS1  # This call doesn't block
>>>>
>>>> stty -F /dev/ttyS1 -crtscts # this blocks forever on ioctl(TCSETSW )
>>>>
>>>>
>>>> looking at tty_port_close_start(), it's pretty clear that nothing is
>>>> flushed until the last user, so it explains why the "echo dummy"
>>>> returns directly, despite the crtscts flags.
>>>> And in tty_mode_ioctl(), there are the lines:
>>>>         case TCSETSW:
>>>>                 return set_termios(real_tty, p, TERMIOS_WAIT | TERMIOS_OLD);
>>>> That explain why the stty blocks.
>>>>
>>>> But this behavior seems really strange.
>>>> ... Or it's meant to be like that ?
>>>
>>> Yeah, meant to be like that.
>>>
>>> When mgetty writes the login prompt but h/w flow control is enabled
>>> and nothing's connected, the output is buffered.
>>>
>>> Since stty uses tcsetattr(TCSADRAIN), the attempt to turn off h/w flow
>>> control blocks, waiting for output to empty.
>> Damn ! I forgot about the TCSADRAIN/FLUSH/NOW flags
>>
>>> In this situation, stopping mgetty will allow the other process
>>> to unblock and advance.
>>>
>>> Hmmm, I could add a -f,--force flag to stty so it uses tcsetattr(TCSANOW)...
>> Yes, I think such a flag will make sense.
> In fact, a similar flag exist in the very last version of coreutils (8.25)
> http://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=cb7d8b90a213c0186a8c8ba66da959e1f5930e78
> :)
> 

Thanks for saving me the time doing it myself.

Regards,
Peter Hurley