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    SubjectRe: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008

    On Mon, 24 Aug 2009, Linus Torvalds wrote:
    > But I wanted to let people know that the patch is clearly not the "last
    > word" on this. It's a useful thing to try, but we need something better.

    This may be better (this is a replacement for the previous patch).

    Instead of using 'cancel_delayed_work_sync()', it makes tty_ldisc_hangup()
    do a 'flush_scheduled_work()' afterwards, like the other callers already

    And like 'tty_ldisc_release()' already does, it does this all before even
    getting the ldisc_mutex, avoiding the deadlock.

    I'm not 100% happy with this patch either, but my remaining unhappiness is
    more with the tty locking in general that causes this all. I suspect this
    patch in itself is not any worse than the other hacks we have.

    Oh, and in case you didn't guess - this is _STILL_ totally untested. It
    compiles for me, but that's all I'm going to guarantee. I'm just looking
    at the code (and getting pretty fed up with it ;)

    And as already mentioned: I doubt the deadlock on tty->ldisc_mutex is
    anything that would be hit in practice. And even if it can be triggered,
    the previous patch I sent out is still interesting in a "does it make the
    problem go away" sense. Because if it doesn't (with or without a new
    deadlock), then I'm looking at all the wrong places.


    drivers/char/tty_ldisc.c | 10 +++++++---
    1 files changed, 7 insertions(+), 3 deletions(-)

    diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
    index 1733d34..f893d18 100644
    --- a/drivers/char/tty_ldisc.c
    +++ b/drivers/char/tty_ldisc.c
    @@ -508,8 +508,9 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
    * be obtained while the delayed work queue halt ensures that no more
    * data is fed to the ldisc.
    - * In order to wait for any existing references to complete see
    - * tty_ldisc_wait_idle.
    + * You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex
    + * in order to make sure any currently executing ldisc work is also
    + * flushed.

    static int tty_ldisc_halt(struct tty_struct *tty)
    @@ -753,11 +754,14 @@ void tty_ldisc_hangup(struct tty_struct *tty)
    * N_TTY.
    if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
    + /* Make sure the old ldisc is quiescent */
    + tty_ldisc_halt(tty);
    + flush_scheduled_work();
    /* Avoid racing set_ldisc or tty_ldisc_release */
    if (tty->ldisc) { /* Not yet closed */
    /* Switch back to N_TTY */
    - tty_ldisc_halt(tty);
    /* At this point we have a closed ldisc and we want to
    reopen it. We could defer this to the next open but

     \ /
      Last update: 2009-10-18 23:28    [W:0.049 / U:0.928 seconds]
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