Re: [PATCHv3 4/6] tty/lockdep: Add ldisc_sem asserts

[Peter Zijlstra]

On Tue, Sep 11, 2018 at 02:48:19AM +0100, Dmitry Safonov wrote:
> Make sure under CONFIG_LOCKDEP that each change to line discipline
> is done with held write semaphor.

But you don't do that. You just assert it is held, not for writing.

> Otherwise potential reader will have a good time dereferencing
> incomplete/uninitialized ldisc.
> 
> Exception here is tty_ldisc_open(), as it's called without ldisc_sem
> locked by tty_init_dev() for the tty->link.

You fail to explain how that is not broken...

> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Slaby <jslaby@xxxxxxxx>
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---
>  drivers/tty/tty_ldisc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> index fc4c97cae01e..202cb645582f 100644
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -471,6 +471,7 @@ static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
>  
>  static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
>  {
> +	lockdep_assert_held(&tty->ldisc_sem);

Did you want:

	lockdep_assert_held_exclusive(&tty->ldisc_sem);

?

>  	WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
>  	clear_bit(TTY_LDISC_OPEN, &tty->flags);
>  	if (ld->ops->close)