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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Duoming Zhou <duoming@zju.edu.cn>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.14 187/218] drivers: tty: serial: Fix deadlock in sa1100_set_termios()
Date:   Mon, 13 Jun 2022 12:10:45 +0200
Message-Id: <20220613094926.286902618@linuxfoundation.org>
In-Reply-To: <20220613094908.257446132@linuxfoundation.org>
References: <20220613094908.257446132@linuxfoundation.org>
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From: Duoming Zhou <duoming@zju.edu.cn>

[ Upstream commit 62b2caef400c1738b6d22f636c628d9f85cd4c4c ]

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@zju.edu.cn>
Link: https://lore.kernel.org/r/20220417111626.7802-1-duoming@zju.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 fd3d1329d48c..68eb1c9faa29 100644
--- a/drivers/tty/serial/sa1100.c
+++ b/drivers/tty/serial/sa1100.c
@@ -452,6 +452,8 @@ 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);
@@ -482,8 +484,6 @@ 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.
 	 */
-- 
2.35.1