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:	Tue, 26 Jan 2016 17:19:20 +0100
From:	Richard Genoud <richard.genoud@...il.com>
To:	Jiri Slaby <jslaby@...e.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Peter Hurley <peter@...leysoftware.com>,
	"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)

[ 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 ?


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.