On Wed 2023-01-11 15:57:10, John Ogness wrote:
> Several mutexes are taken while setting up console serial ports. In
> particular, the tty_port->mutex and @console_mutex are taken:
>
> serial_pnp_probe
> serial8250_register_8250_port
> uart_add_one_port (locks tty_port->mutex)
> uart_configure_port
> register_console (locks @console_mutex)
>
> In order to synchronize kgdb's tty_find_polling_driver() with
> register_console(), commit 6193bc90849a ("tty: serial: kgdboc:
> synchronize tty_find_polling_driver() and register_console()") takes
> the @console_mutex. However, this leads to the following call chain
> (with locking):
>
> platform_probe
> kgdboc_probe
> configure_kgdboc (locks @console_mutex)
> tty_find_polling_driver
> uart_poll_init (locks tty_port->mutex)
> uart_set_options
>
> This is clearly deadlock potential due to the reverse lock ordering.
Great catch!
> Since uart_set_options() requires holding @console_mutex in order to
> serialize early initialization of the serial-console lock, take the
> @console_mutex in uart_poll_init() instead of configure_kgdboc().
>
> Since configure_kgdboc() was using @console_mutex for safe traversal
> of the console list, change it to use the SRCU iterator instead.
>
> Add comments to uart_set_options() kerneldoc mentioning that it
> requires holding @console_mutex (aka the console_list_lock).
>
> Fixes: 6193bc90849a ("tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()")
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -189,15 +190,6 @@ 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) {
> console_list_unlock();
This console_list_unlock() should be removed here as well.
Best Regards,
Petr
PS: I still have to double check all the dependencies. I think that I need
to do it with a fresh head tomorrow.
