[PATCH v4 3/4] serial: 8250: warning on entering dw8250_force_idle unlocked

[Yunhui Cui]

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@xxxxxxxxxxxxx>
---
 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..07f9be074b4b 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;
 
+	/*
+	 * 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