Benoit PAPILLAULT, 24/04/2001

=== PATCHING YOUR KERNEL ===

cd /usr/src/linux
patch -p1 < n_hdlc.c.diff

recompile your kernel. If HDLC was compiled as a module, you only need to do:
make dep modules modules_install
rmmod n_hdlc
modprobe n_hdlc

to get the new HDLC code working.

=== EXPLANATION ===

We consider the case where two process are communicating over a pseudo tty
 allocated with :

master_tty = open(/dev/ptmx,O_RDWR)
grantpt(master_tty);
unlockpt(master_tty);

pts = ptsname (master_tty);
slave_tty = open(pts,O_RDWR|O_NOCTTY);

If either master_tty or slave_tty are closed, the other side read() or selec() returns -1 with errno = EIO.

Now, if I add:

   int disc = N_HDLC;
    if (ioctl(0,TIOCSETD,&disc) < 0)
      perror("ioctl");

When the slave tty is closed, the master tty receives NO notification at all.
This cause pppoa2 not to exit, preventing other pppoa2 to be executed properly.
Why?

When slave tty is closed, <tty_io.c>
tty_release() calls release_dev()
 o_tty = tty->link

it calls :

 wake_up(&tty->read_wait);
 wake_up(&tty->write_wait);
 wake_up(&o_tty->read_wait);
 wake_up(&o_tty->write_wait);

tty_poll <tty_io.c>:
	if (tty->ldisc.poll)
		return (tty->ldisc.poll)(tty, filp, wait);

normal_poll <n_tty.c>:

	poll_wait(file, &tty->read_wait, wait);
	poll_wait(file, &tty->write_wait, wait);

n_hdlc_tty_poll <n_hdlc.c>
	poll_wait(filp,&n_hdlc->read_wait,wait);
	poll_wait(filp,&n_hdlc->write_wait,wait);

[root@hansolo]# ls -la /dev/ptmx
crw-rw-rw-    1 root     root       5,   2 avr 24 00:49 /dev/ptmx

=> major = 5, minor = 2.

cat /proc/devices:
  2 pty

pty.c : call tty_register_driver() define in tty_io.c
avec PTY_MASTER_MAJOR = 2.

Thus : open("/dev/ptmx",O_RDWR) calls tty_open <tty_io.c>

	which call init_dev() for the ptm_driver major/minor.
	which contains : 

		/* Establish the links in both directions */
		tty->link   = o_tty;
		o_tty->link = tty;

	and filp->private_data = tty.
	tty is allocated by alloc_tty_struct() 	
	 & initialized by initialize_tty_struct() which setup
	 a default line disciplineof N_TTY, init tty->write_wait &
	 tty->read_wait	queues.

Thus : select() calls tty_poll(), which calls ldisc->poll.

In the normal case (N_TTY default line discipline), when one side is closed,
if the other side was blocked in read(), the process is awaken and EIO
is returned:

read_chan <n_tty.c>:

		if (!input_available_p(tty, 0)) {
			if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
				retval = -EIO;
				break;
			}

In the N_HDLC case, the code was waiting on different waitqueues and 
is awaken, only return an error code is a signal was received:

n_hdlc_tty_read <n_hdlc.c>:
		interruptible_sleep_on (&tty->read_wait);
		if (signal_pending(current))
			return -EINTR;