linux-kernel - [PATCH v4 3/4] serial: 8250: warning on entering dw8250_force

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Message-Id: <20250425062425.68761-3-cuiyunhui@bytedance.com>
Date: Fri, 25 Apr 2025 14:24:24 +0800
From: Yunhui Cui <cuiyunhui@...edance.com>
To: arnd@...db.de,
	andriy.shevchenko@...ux.intel.com,
	benjamin.larsson@...exis.eu,
	cuiyunhui@...edance.com,
	gregkh@...uxfoundation.org,
	heikki.krogerus@...ux.intel.com,
	ilpo.jarvinen@...ux.intel.com,
	jirislaby@...nel.org,
	jkeeping@...usicbrands.com,
	john.ogness@...utronix.de,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	markus.mayer@...aro.org,
	matt.porter@...aro.org,
	namcao@...utronix.de,
	paulmck@...nel.org,
	pmladek@...e.com,
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	sunilvl@...tanamicro.com,
	tim.kryger@...aro.org
Subject: [PATCH v4 3/4] serial: 8250: warning on entering dw8250_force_idle unlocked

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@...edance.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..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