Re: [PATCH] tty: fix ldisc lock inversion trace

[Ming Lei]

On Thu, May 24, 2012 at 11:51 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
> From: Alan Cox <alan@xxxxxxxxxxxxxxx>
>
> This is caused by tty_release using tty_lock_pair to lock both
> sides of the pty/tty pair, and then tty_ldisc_release dropping
> and relocking one side only. We can drop both fine, so drop both
> to avoid any lock ordering concerns.
>
> Signed-off-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
> ---
>
>  drivers/tty/tty_ldisc.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
>
>
> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> index 173a900..833e851 100644
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -912,10 +912,13 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
>         * race with the set_ldisc code path.
>         */
>
> -       tty_unlock(tty);
> +       /* We don't give a monkeys' about the o_tty lock here but we
> +          must do both to avoid lock inversions against another single
> +          locking case */
> +       tty_unlock_pair(tty, o_tty);
>        tty_ldisc_halt(tty);
>        tty_ldisc_flush_works(tty);
> -       tty_lock(tty);
> +       tty_lock_pair(tty, o_tty);

This still may cause lockdep warning by the following line:

             if (o_tty)
                    tty_ldisc_release(o_tty, NULL);

The tty_unlock_pair in 'tty_ldisc_release(o_tty, NULL)'  may
not match the previous tty_lock_pair. If tty > o_tty, then lockdep
warning will be triggered since the lock to be released is not
at the top of current lock stack.

Thanks,
-- 
Ming Lei
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