linux-kernel - [PATCH V2] drivers: tty: serial: Fix deadlock in sa1100_set

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Message-Id: <20220417111626.7802-1-duoming@zju.edu.cn>
Date:   Sun, 17 Apr 2022 19:16:26 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-kernel@...r.kernel.org
Cc:     linux-serial@...r.kernel.org, jirislaby@...nel.org,
        gregkh@...uxfoundation.org, Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH V2] drivers: tty: serial: Fix deadlock in sa1100_set_termios()

There is a deadlock in sa1100_set_termios(), which is shown
below:

   (Thread 1)              |      (Thread 2)
                           | sa1100_enable_ms()
sa1100_set_termios()       |  mod_timer()
 spin_lock_irqsave() //(1) |  (wait a time)
 ...                       | sa1100_timeout()
 del_timer_sync()          |  spin_lock_irqsave() //(2)
 (wait timer to stop)      |  ...

We hold sport->port.lock in position (1) of thread 1 and
use del_timer_sync() to wait timer to stop, but timer handler
also need sport->port.lock in position (2) of thread 2. As a result,
sa1100_set_termios() will block forever.

This patch moves del_timer_sync() before spin_lock_irqsave()
in order to prevent the deadlock.

Signed-off-by: Duoming Zhou <duoming@....edu.cn>
---
Changes in V2:
  - Move del_timer_sync() before spin_lock_irqsave().

 drivers/tty/serial/sa1100.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/sa1100.c b/drivers/tty/serial/sa1100.c
index 5fe6cccfc1a..e64e42a19d1 100644
--- a/drivers/tty/serial/sa1100.c
+++ b/drivers/tty/serial/sa1100.c
@@ -446,6 +446,8 @@ sa1100_set_termios(struct uart_port *port, struct ktermios *termios,
 	baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk/16); 
 	quot = uart_get_divisor(port, baud);
 
+	del_timer_sync(&sport->timer);
+
 	spin_lock_irqsave(&sport->port.lock, flags);
 
 	sport->port.read_status_mask &= UTSR0_TO_SM(UTSR0_TFS);
@@ -476,8 +478,6 @@ sa1100_set_termios(struct uart_port *port, struct ktermios *termios,
 				UTSR1_TO_SM(UTSR1_ROR);
 	}
 
-	del_timer_sync(&sport->timer);
-
 	/*
 	 * Update the per-port timeout.
 	 */
-- 
2.17.1