Re: [PATCH v9 4/4] serial: 8250_dw: assert port->lock is held in dw8250_force_idle()

[John Ogness]

On 2025-06-10, Yunhui Cui <cuiyunhui@xxxxxxxxxxxxx> wrote:
> Reading UART_RX and checking whether UART_LSR_DR is set should be
> atomic. Ensure the caller of dw8250_force_idle() holds port->lock.
>
> Signed-off-by: Yunhui Cui <cuiyunhui@xxxxxxxxxxxxx>
> ---
>  drivers/tty/serial/8250/8250_dw.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index 082b7fcf251db..686f9117a3339 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -13,6 +13,7 @@
>  #include <linux/delay.h>
>  #include <linux/device.h>
>  #include <linux/io.h>
> +#include <linux/lockdep.h>
>  #include <linux/mod_devicetable.h>
>  #include <linux/module.h>
>  #include <linux/notifier.h>
> @@ -117,6 +118,9 @@ static void dw8250_force_idle(struct uart_port *p)
>  	struct uart_8250_port *up = up_to_u8250p(p);
>  	unsigned int lsr;
>  
> +	/* Reading UART_LSR and UART_RX should be atomic. */
> +	lockdep_assert_held_once(&p->lock);
> +

It may be possible that during panic the port lock might not be held for
console printing:

serial8250_console_write()
  oops_in_progress and failed trylock
  serial8250_console_restore()
    serial_port_out(..., UART_LCR, ...)
      dw8250_serial_out*()
        dw8250_check_lcr()
          dw8250_force_idle()

A similar incident was discussed before [0]. In that case the result was
that the lockdep assertion was removed.

John Ogness

[0] https://lore.kernel.org/r/20230811064340.13400-1-jirislaby@xxxxxxxxxx