[PATCH v5 3/4] serial: 8250_dw: warning on entering dw8250_force_idle unlocked

Yunhui Cui
2025-05-06T11:23:42Z

Read UART_RX and check UART_LSR_DR in critical section. Unsure if
caller of dw8250_force_idle() holds port->lock. Don't acquire it
directly to avoid deadlock. Use lockdep_assert_held_once for warning.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
---
drivers/tty/serial/8250/8250_dw.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index af24ec25d976..f41c4a9ed58b 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>
@@ -112,6 +113,13 @@ static void dw8250_force_idle(struct uart_port *p)
struct uart_8250_port *up = up_to_u8250p(p);
unsigned int lsr;

+ /*
+ * The serial_in(p, UART_RX) should be under port->lock, but we can't add
+ * it to avoid AA deadlock as we're unsure if serial_out*(...UART_LCR)
+ * is under port->lock.
+ */
+ lockdep_assert_held_once(&p->lock);
+
serial8250_clear_and_reinit_fifos(up);

/*
--
2.39.2