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    Subject[PATCH] Remove race between con_open and con_close
    I have a laptop (G3 powerbook) which will pretty reliably hit a race
    between con_open and con_close late in the boot process and oops in
    vt_ioctl due to tty->driver_data being NULL.

    What happens is this: process A opens /dev/tty6; it comes into
    con_open() (drivers/char/vt.c) and assign a non-NULL value to
    tty->driver_data. Then process A closes that and concurrently process
    B opens /dev/tty6. Process A gets through con_close() and clears
    tty->driver_data, since tty->count == 1. However, before process A
    can decrement tty->count, we switch to process B (e.g. at the
    down(&tty_sem) call at drivers/char/tty_io.c line 1626).

    So process B gets to run and comes into con_open with tty->count == 2,
    as tty->count is incremented (in init_dev) before con_open is called.
    Because tty->count != 1, we don't set tty->driver_data. Then when the
    process tries to do anything with that fd, it oopses.

    The simple and effective fix for this is to test tty->driver_data
    rather than tty->count in con_open. The testing and setting of
    tty->driver_data is serialized with respect to the clearing of
    tty->driver_data in con_close by the console_sem. We can't get a
    situation where con_open sees tty->driver_data != NULL and then
    con_close on a different fd clears tty->driver_data, because
    tty->count is incremented before con_open is called. Thus this patch
    eliminates the race, and in fact with this patch my laptop doesn't

    Could this go into 2.6.13 please?

    Signed-off-by: Paul Mackerras <>

    diff -urN linux-2.6/drivers/char/vt.c pmac-2.6/drivers/char/vt.c
    --- linux-2.6/drivers/char/vt.c 2005-07-17 10:59:52.000000000 +1000
    +++ pmac-2.6/drivers/char/vt.c 2005-08-27 22:59:36.000000000 +1000
    @@ -2433,7 +2433,7 @@
    int ret = 0;

    - if (tty->count == 1) {
    + if (tty->driver_data == NULL) {
    ret = vc_allocate(currcons);
    if (ret == 0) {
    struct vc_data *vc = vc_cons[currcons].d;
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      Last update: 2005-08-28 01:42    [W:0.019 / U:21.440 seconds]
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