Re: [PATCH v2] serial: 8250: Clear CON_PRINTBUFFER on port re-registration

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, May 15, 2026 at 03:44:02PM +0800, Fushuai Wang wrote:
> >> From: Fushuai Wang <wangfushuai@baidu.com>
> >> 
> >> When two PnP devices map to the same physical port, the serial8250 driver
> >> removes and re-registers the console structure for the same port.
> >> 
> >> During re-registration, the console structure still has CON_PRINTBUFFER set
> >> from the initial registration, which causes console_init_seq() to set
> >> console->seq to syslog_seq. This results in re-printing the entire
> >> system log buffer, which may lead to RCU stall on slow serial consoles.
> >> 
> >> Clear CON_PRINTBUFFER when re-registering a port to prevent duplicate
> >> log printing.
> >> 
> >> Fixes: 835d844d1a28 ("8250_pnp: do pnp probe before legacy probe")
> >> Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
> >> ---
> >> V1->V2: Add Fixes tag
> >> previous discussion: https://lore.kernel.org/all/20260416092917.27301-1-fushuai.wang@linux.dev/T/#u
> >> 
> >> Please ignore previous email if you received it before. There is something wrong with my email client.
> >> 
> >>  drivers/tty/serial/8250/8250_core.c | 9 ++++++++-
> >>  1 file changed, 8 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> >> index a428e88938eb..01b14392d9f7 100644
> >> --- a/drivers/tty/serial/8250/8250_core.c
> >> +++ b/drivers/tty/serial/8250/8250_core.c
> >> @@ -694,6 +694,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
> >>  {
> >>  	struct uart_8250_port *uart;
> >>  	int ret;
> >> +	bool was_removed = false;
> >>  
> >>  	if (up->port.uartclk == 0)
> >>  		return -EINVAL;
> >> @@ -716,8 +717,10 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
> >>  	if (uart->port.type == PORT_8250_CIR)
> >>  		return -ENODEV;
> >>  
> >> -	if (uart->port.dev)
> >> +	if (uart->port.dev) {
> >>  		uart_remove_one_port(&serial8250_reg, &uart->port);
> >> +		was_removed = true;
> >> +	}
> >>  
> >>  	uart->port.ctrl_id	= up->port.ctrl_id;
> >>  	uart->port.port_id	= up->port.port_id;
> >> @@ -819,6 +822,10 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
> >>  					&uart->capabilities);
> >>  
> >>  		serial8250_apply_quirks(uart);
> >> +
> >> +		if (was_removed && uart_console(&uart->port))
> >> +			uart->port.cons->flags &= ~CON_PRINTBUFFER;
> > 
> > Why not set the flag up above when you remove the port?  Why down here?
> > 
> > thanks,
> > 
> > greg k-h
> 
> Hi, Greg
> 
> I just felt it's cleaner to clear it only when re-registration happens.
> Do you think there is any problem with doing it right after the removal?

If you do that, no flag is needed.