netdev - [PATCH 01/11] drivers: tty: serial: Fix deadlock in sa1100_set

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Date:   Thu,  7 Apr 2022 14:33:17 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-kernel@...r.kernel.org
Cc:     chris@...kel.net, jcmvbkbc@...il.com, mustafa.ismail@...el.com,
        shiraz.saleem@...el.com, jgg@...pe.ca, wg@...ndegger.com,
        mkl@...gutronix.de, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, jes@...ined-monkey.org,
        gregkh@...uxfoundation.org, jirislaby@...nel.org,
        alexander.deucher@....com, linux-xtensa@...ux-xtensa.org,
        linux-rdma@...r.kernel.org, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-hippi@...site.dk,
        linux-staging@...ts.linux.dev, linux-serial@...r.kernel.org,
        linux-usb@...r.kernel.org, Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH 01/11] 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 extracts del_timer_sync() from the protection of
spin_lock_irqsave(), which could let timer handler to obtain
the needed lock.

Signed-off-by: Duoming Zhou <duoming@....edu.cn>
---
 drivers/tty/serial/sa1100.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/serial/sa1100.c b/drivers/tty/serial/sa1100.c
index 5fe6cccfc1a..3a5f12ced0b 100644
--- a/drivers/tty/serial/sa1100.c
+++ b/drivers/tty/serial/sa1100.c
@@ -476,7 +476,9 @@ sa1100_set_termios(struct uart_port *port, struct ktermios *termios,
 				UTSR1_TO_SM(UTSR1_ROR);
 	}
 
+	spin_unlock_irqrestore(&sport->port.lock, flags);
 	del_timer_sync(&sport->timer);
+	spin_lock_irqsave(&sport->port.lock, flags);
 
 	/*
 	 * Update the per-port timeout.
-- 
2.17.1