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: <56A7A92E.3020407@hurleysoftware.com>
Date:	Tue, 26 Jan 2016 09:13:18 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Richard Genoud <richard.genoud@...il.com>,
	Jiri Slaby <jslaby@...e.com>
Cc:	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)

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.

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)...

Regards,
Peter Hurley


> Regards,
> Richard
> 
> NB: This is actually a real life use case with mgetty, a modem losing
> its power and another process trying to speak to the modem.
>