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: <CACQ1gAhUJGdgfB4oape9W4-0bgyLhW1AhySSvQtM8A+kv8cASQ@mail.gmail.com>
Date:	Wed, 27 Jan 2016 11:26:57 +0100
From:	Richard Genoud <richard.genoud@...il.com>
To:	Peter Hurley <peter@...leysoftware.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)

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
:)