Re: [RESEND RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

[Ilpo Järvinen]

On Tue, 16 Jan 2024, Leonardo Bras wrote:

> With PREEMPT_RT enabled, a spin_lock_irqsave() becomes a possibly sleeping
> spin_lock(), without preempt_disable() or irq_disable().
> 
> This allows a task T1 to get preempted or interrupted while holding the
> port->lock. If the preempting task T2 need the lock, spin_lock() code
> will schedule T1 back until it finishes using the lock, and then go back to
> T2.
> 
> There is an issue if a T1 holding port->lock is interrupted by an
> IRQ, and this IRQ handler needs to get port->lock for writting (printk):
> spin_lock() code will try to reschedule the interrupt handler, which is in
> atomic context, causing a BUG() for trying to reschedule/sleep in atomic
> context.

I thought that the printk side was supposed to be become aware when it's 
not safe to write to serial side so the printing can be deferred... Has 
that plan changed?

-- 
 i.

> So for the case (PREEMPT_RT && in_atomic()) try to get the lock, and if it
> fails proceed anyway, just like it's done in oops_in_progress case.
> 
> Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
> ---
>  drivers/tty/serial/8250/8250_port.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 8ca061d3bbb92..8480832846319 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -3397,7 +3397,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
>  
>  	touch_nmi_watchdog();
>  
> -	if (oops_in_progress)
> +	if (oops_in_progress || (IS_ENABLED(CONFIG_PREEMPT_RT) && in_atomic())
>  		locked = uart_port_trylock_irqsave(port, &flags);
>  	else
>  		uart_port_lock_irqsave(port, &flags);
>