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@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>

diff --git a/drivers/tty/serial/sa1100.c b/drivers/tty/serial/sa1100.c
index 5fe6cccfc1aebc043a801ff5006b380916f18789..e64e42a19d1a042e679e4d864339b646364a2a6b 100644 (file)
--- 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.
         */