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

[Fushuai Wang <fushuai.wang@linux.dev>]

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:
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

 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;
+
 		ret = uart_add_one_port(&serial8250_reg,
 					&uart->port);
 		if (ret)
-- 
2.36.1