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    Subjectldisc writes during tty release_dev() causing problems.
    On Wed, 2004-09-15 at 15:41, Theodore Ts'o wrote:

    > The current (I can't speak to what Alan Cox is going to change) rules
    > with tty drivers is that tty drivers are supposed to close the line
    > discpline in their close routines. That's the much safer and cleaner
    > way of fixing this problem, and it is in line with what most of the
    > other tty drivers are doing.
    > - Ted


    I greped through the drivers directory and it appears that the only tty
    drivers that actually invoke ldisc.close() are those drivers that don't
    use the N_TTY line discipline (serial drivers).

    Many of the drivers actually call ldisc.flush_buffers() which
    effectively does the same as a ldisc.close(), cleaning up the
    tty->read_buf and tasks waiting on tty->read_wait.

    The story is different with writes. The tty layer dipatches a write via
    the line discpline's write_chan() function. The line discipline is then
    responsible for delivering this data to the driver's write() function.

    If driver->write() returns 0 [device is blocking] (which it can do often
    in the case of hvc_console) the ldisc write_chan() function will
    schedule() and get put on the tty->write_wait queue with the expectation
    that it will be awoken by the driver when the device is available for
    further writes.

    If tty release_dev() is called when write_chan() is blocked awaiting I/O
    with the driver and tty->count == 1 the final call to driver->close()
    will clean up the driver without the ldisc knowing that the driver has
    closed the device.

    Following this release_dev() actually awakens the tty->write_wait, which
    basically tells the ldisc to continue trying to send data to the driver
    for the device it has just told the driver to close!

    Ideally I would want hvc_close() to be able to tell the ldisc to wake up
    the thread blocking on tty->write_wait and discard any unsent data.
    Unless I'm mistaken there is currrently no way to do this.

    I suppose I could make hvc_write() return -EIO if hvc_struct->count ==
    0. I can't think of any races with this but it doesn't seem correct.

    Additionally, there doesn't seem to be anything to prevent hangup being
    called on a tty during the final close operation (this actually happened
    in hvc_console).

    Do you have any wisdom or advice? Am I missing something obvious?

    Ryan S. Arnold
    IBM Linux Technology Center

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