diff mbox series

[printk,v5,37/40] tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()

Message ID 20221116162152.193147-38-john.ogness@linutronix.de
State New
Headers
Series reduce console_lock scope |

Commit Message

John Ogness Nov. 16, 2022, 4:21 p.m. UTC 
  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@linutronix.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 drivers/tty/serial/kgdboc.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

Comments

Doug Anderson Nov. 17, 2022, 12:59 a.m. UTC | #1 
Hi,

On Wed, Nov 16, 2022 at 8:22 AM John Ogness <john.ogness@linutronix.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@linutronix.de>
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
> Reviewed-by: Petr Mladek <pmladek@suse.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@chromium.org>
John Ogness Nov. 17, 2022, 10 a.m. UTC | #2 
On 2022-11-16, Doug Anderson <dianders@chromium.org> wrote:
> 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.

Actually this series is not about optimization. It is about reducing the
scope of console_lock and removing unnecessary use of it.

If tty_find_polling_driver() needs to be called under the console_lock,
then we need to document exactly why. I could not find any situations
where it is necessary.

Also keep in mind that in the long term we will be completely removing
the console_lock. It is a painful process of identifying and dismantling
its scope and replacing it with multiple clearly scoped locking
mechanisms.

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

Thanks.

John
diff mbox series

Patch

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();
+	}
 
 	/*
 	 * Take console_lock to serialize device() callback with