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[PATCH 4/8] serial: core: Take port mutex for send_xchar() tty method

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Prepare for separate methods of preventing unsafe uart port access
when sending xchar; claim port mutex for the tty ops method (which
might sleep) and rcu for the throttle/unthrottle uses.

The implied limitation is that uart drivers which support
AUTOXOFF flow control cannot define a send_xchar() method that sleeps.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
 drivers/tty/serial/serial_core.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 4430518..806eba81 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -610,23 +610,31 @@ static void uart_flush_buffer(struct tty_struct *tty)
  * This function is used to send a high-priority XON/XOFF character to
  * the device
  */
-static void uart_send_xchar(struct tty_struct *tty, char ch)
+static void __uart_send_xchar(struct uart_port *uport, char ch)
 {
-	struct uart_state *state = tty->driver_data;
-	struct uart_port *port = state->uart_port;
 	unsigned long flags;
 
-	if (port->ops->send_xchar)
-		port->ops->send_xchar(port, ch);
+	if (uport->ops->send_xchar)
+		uport->ops->send_xchar(uport, ch);
 	else {
-		spin_lock_irqsave(&port->lock, flags);
-		port->x_char = ch;
+		spin_lock_irqsave(&uport->lock, flags);
+		uport->x_char = ch;
 		if (ch)
-			port->ops->start_tx(port);
-		spin_unlock_irqrestore(&port->lock, flags);
+			uport->ops->start_tx(port);
+		spin_unlock_irqrestore(&uport->lock, flags);
 	}
 }
 
+static void uart_send_xchar(struct tty_struct *tty, char ch)
+{
+	struct uart_state *state = tty->driver_data;
+	struct uart_port *uport = state->uart_port;
+
+	mutex_lock(&state->port.mutex);
+	__uart_send_xchar(uport, ch);
+	mutex_unlock(&state->port.mutex);
+}
+
 static void uart_throttle(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
@@ -644,7 +652,7 @@ static void uart_throttle(struct tty_struct *tty)
 	}
 
 	if (mask & UPSTAT_AUTOXOFF)
-		uart_send_xchar(tty, STOP_CHAR(tty));
+		__uart_send_xchar(port, STOP_CHAR(tty));
 
 	if (mask & UPSTAT_AUTORTS)
 		uart_clear_mctrl(port, TIOCM_RTS);
@@ -667,7 +675,7 @@ static void uart_unthrottle(struct tty_struct *tty)
 	}
 
 	if (mask & UPSTAT_AUTOXOFF)
-		uart_send_xchar(tty, START_CHAR(tty));
+		__uart_send_xchar(port, START_CHAR(tty));
 
 	if (mask & UPSTAT_AUTORTS)
 		uart_set_mctrl(port, TIOCM_RTS);
-- 
2.6.3

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[PATCH 0/8] Fix unsafe uart port access Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 4/8] serial: core: Take port mutex for send_xchar() tty method Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 3/8] serial: core: Use tty->index for port # in debug messages Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 1/8] serial: core: Fold __uart_put_char() into caller Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 5/8] serial: core: Expand port mutex section in uart_poll_init() Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 6/8] serial: core: Prevent unsafe uart port access, part 1 Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 2/8] serial: core: Fold do_uart_get_info() into caller Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100
  [PATCH 7/8] serial: core: Prevent unsafe uart port access, part 2 Peter Hurley <peter@hurleysoftware.com> - 2015-12-13 01:40 +0100

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