linux-kernel - Re: [PATCH printk v5 37/40] tty: serial: kgdboc: synchronize tty_find_polling_driver() and register

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Date:   Wed, 16 Nov 2022 16:59:08 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org
Subject: Re: [PATCH printk v5 37/40] tty: serial: kgdboc: synchronize
 tty_find_polling_driver() and register_console()

Hi,

On Wed, Nov 16, 2022 at 8:22 AM John Ogness <john.ogness@...utronix.de> wrote:
>
> Calling tty_find_polling_driver() can lead to uart_set_options() being
> called (via the poll_init() callback of tty_operations) to configure the
> uart. But uart_set_options() can also be called by register_console()
> (via the setup() callback of console).
>
> Take the console_list_lock to synchronize against register_console() and
> also use it for console list traversal. This also ensures the console list
> cannot change until the polling console has been chosen.
>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> ---
>  drivers/tty/serial/kgdboc.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index 82b4b4d67823..8c2b7ccdfebf 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -189,12 +189,20 @@ static int configure_kgdboc(void)
>         if (kgdboc_register_kbd(&cptr))
>                 goto do_register;
>
> +       /*
> +        * tty_find_polling_driver() can call uart_set_options()
> +        * (via poll_init) to configure the uart. Take the console_list_lock
> +        * in order to synchronize against register_console(), which can also
> +        * configure the uart via uart_set_options(). This also allows safe
> +        * traversal of the console list.
> +        */
> +       console_list_lock();
> +
>         p = tty_find_polling_driver(cptr, &tty_line);
> -       if (!p)
> +       if (!p) {
> +               console_list_unlock();
>                 goto noconfig;
> -
> -       /* For safe traversal of the console list. */
> -       console_list_lock();
> +       }

Seems OK to me, though I guess I would have moved console_lock() up
too just because this isn't a case we need to optimize and then we can
be extra certain that nobody else is messing with console structures
while we're looking at them.

Reviewed-by: Douglas Anderson <dianders@...omium.org>