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SubjectRe: BUG: tty insert flip string fixed flag, unable to handle kernel NULL pointer dereference at 00000004
On 2012-06-06 02:27, Alan Cox wrote:
>> [ 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.
Thank you a lot, i will try apply this.
This crash are only few times per week, but i will try to see, if it
will help or change anything.

>
> Alan


---
Denys Fedoryshchenko, Network Engineer, Virtual ISP S.A.L.


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