Re: [PATCH] serial: 8250: drop lockdep annotation from serial8250_clear_IER()

Jiri Slaby
2023-08-15T10:10:58Z

On 15. 08. 23, 11:27, Petr Mladek wrote:
> On Mon 2023-08-14 12:21:23, Jiri Slaby wrote:
>> On 14. 08. 23, 12:00, Petr Mladek wrote:
>>> I personally vote to keep it as is unless people see this warning
>>> on daily basis. After all, the lockdep splat is correct. The serial
>>> console might not work correctly in panic() when there is the race.
>>
>> Sorry, but no, the warning is not correct at all. The code path deliberately
>> does NOT take the lock and calls a function which is currently annotated
>> that the lock is _always_ taken. Therefore, the warning is clearly a false
>> positive and I see no reason in keeping it.
>
> There might be a misunderstanding. I only want to keep panic()
> implementation as it is for now. I mean to keep calling
> debug_locks_off() right before console_flush_on_panic().
> The lockdep should stay on before to report potential problems
> in non-printk code, like kexec, panic notifiers.
>
> But I am fine with disabling the particular lockdep_assert_held_once()
> during panic().
>
> It should stay during the normal system state to catch not
> yet discovered races. John is working hard on preventing any
> races which might blow up after introducing the printk kthreads.
>
> I mean something like:
>
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index ecfdc4534123..9533c1eedfb1 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -704,7 +704,8 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
> static void serial8250_clear_IER(struct uart_8250_port *up)
> {
> /* Port locked to synchronize UART_IER access against the console. */
> - lockdep_assert_held_once(&up->port.lock);
> + if (!oops_in_progress)
> + lockdep_assert_held_once(&up->port.lock);

Yes, this is one of my suggestions ;). (Which I thought are not worth
it, but I am not opposing either.)

thanks,
--
js
suse labs