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    SubjectRe: BUG: tty_insert_flip_string_fixed_flag, unable to handle kernel NULL pointer dereference at 00000004
    > [ 5350.556089]  [<c027adc4>] pty_write+0x2c/0x4c
    > [ 5350.556089] [<c0277a44>] n_tty_write+0x24e/0x2d6
    > [ 5350.556089] [<c0144a2b>] ? try_to_wake_up+0x18c/0x18c

    Basically the other end hung up just as it went to write.

    It's a known race. In the current codebase it's very hard to fix up nicely
    as we have to drop tty_lock during a pty close or we deadlock. It's very
    hard to hit but your environment sounds rather like my test case for it !

    It's one of a series of related bugs (another in n_gsm) that we can't fix
    elegantly until the tty locking is fixed - which has been pushed from 3.5
    hopefully to 3.6 assuming we can fix a couple of other locking problems
    first.

    What may reduce it a lot is to change pty_write to do


    struct tty_struct *to = tty->link;

    if (tty->stopped)
    return 0;

    if (to == NULL)
    return -ENODEV;
    to = tty_kref_get(to);
    if (tty->link == NULL) {
    tty_kref_put(to);
    return -ENODEV;
    }
    if (c > 0) {
    ...
    }
    tty_kref_put(to);
    return c;
    }

    Thats a pretty ugly bandaid but I'd be interested to know if it helps, as
    that would verify the race you are hitting is the one I think it is.

    Alan


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