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

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Date:	Wed, 27 Jan 2016 10:14:31 +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)

2016-01-26 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.

Thank for your enlightenment !

Richard.