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    Subject[05/35] TTY: ldisc, do not close until there are readers
    2.6.32-longterm review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Jiri Slaby <jslaby@suse.cz>

    commit 92f6fa09bd453ffe3351fa1f1377a1b7cfa911e6 upstream.

    We restored tty_ldisc_wait_idle in 100eeae2c5c (TTY: restore
    tty_ldisc_wait_idle). We used it in the ldisc changing path to fix the
    case where there are tasks in n_tty_read waiting for data and somebody
    tries to change ldisc.

    Similar to the case above, there may be also tasks waiting in
    n_tty_read while hangup is performed. As 65b770468e98 (tty-ldisc: turn
    ldisc user count into a proper refcount) removed the wait-until-idle
    from all paths, hangup path won't wait for them to disappear either
    now. So add it back even to the hangup path.

    There is a difference, we need uninterruptible sleep as there is
    obviously HUP signal pending. So tty_ldisc_wait_idle now sleeps
    without possibility to be interrupted. This is what original
    tty_ldisc_wait_idle did. After the wait idle reintroduction
    (100eeae2c5c), we have had interruptible sleeps for the ldisc changing
    path. But as there is a 5s timeout anyway, we don't allow it to be
    interrupted from now on. It's not worth the added complexity of
    deciding what kind of sleep we want.

    Before 65b770468e98 tty_ldisc_release was called also from
    tty_ldisc_release. It is called from tty_release, so I don't think we
    need to restore that one.

    This is nicely reproducible after constifying the timing when
    drivers/tty/n_tty.c is patched as follows ("TTY: ntty, add one more
    sanity check" patch is needed to actually see it explode):
    %% -1548,6 +1549,7 @@ static int n_tty_open(struct tty_struct *tty)

    /* These are ugly. Currently a malloc failure here can panic */
    if (!tty->read_buf) {
    + msleep(100);
    tty->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
    if (!tty->read_buf)
    return -ENOMEM;
    %% -1785,6 +1788,7 @@ do_it_again:
    break;
    }
    timeout = schedule_timeout(timeout);
    + msleep(20);
    continue;
    }
    __set_current_state(TASK_RUNNING);
    ===== With a process: =====
    while (1) {
    int fd = open(argv[1], O_RDWR);
    read(fd, buf, sizeof(buf));
    close(fd);
    }
    ===== and its child: =====
    setsid();
    while (1) {
    int fd = open(tty, O_RDWR|O_NOCTTY);
    ioctl(fd, TIOCSCTTY, 1);
    vhangup();
    close(fd);
    usleep(100 * (10 + random() % 1000));
    }
    ===== EOF =====

    References: https://bugzilla.novell.com/show_bug.cgi?id=693374
    References: https://bugzilla.novell.com/show_bug.cgi?id=694509
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    drivers/char/tty_ldisc.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    --- a/drivers/char/tty_ldisc.c
    +++ b/drivers/char/tty_ldisc.c
    @@ -539,7 +539,7 @@ static int tty_ldisc_halt(struct tty_str
    static int tty_ldisc_wait_idle(struct tty_struct *tty)
    {
    int ret;
    - ret = wait_event_interruptible_timeout(tty_ldisc_idle,
    + ret = wait_event_timeout(tty_ldisc_idle,
    atomic_read(&tty->ldisc->users) == 1, 5 * HZ);
    if (ret < 0)
    return ret;
    @@ -735,6 +735,8 @@ static int tty_ldisc_reinit(struct tty_s
    if (IS_ERR(ld))
    return -1;

    + WARN_ON_ONCE(tty_ldisc_wait_idle(tty));
    +
    tty_ldisc_close(tty, tty->ldisc);
    tty_ldisc_put(tty->ldisc);
    tty->ldisc = NULL;



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