tty: Fix close races in USB serial

USB serial has always had races where the tty port usage count can hit zero
during a receive event. The internal locking is a mutex so we can't use
that in the IRQ handlers.

With krefs we can tackle this differently but we still need to be careful.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 5 deletions

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index 794b5ffe4397..aafa684a900f 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -269,15 +269,19 @@ static void serial_close(struct tty_struct *tty, struct file *filp)
                 return;
         }
 
-        --port->port.count;
-        if (port->port.count == 0)
+        if (port->port.count == 1)
                 /* only call the device specific close if this
-                 * port is being closed by the last owner */
+                 * port is being closed by the last owner. Ensure we do
+                 * this before we drop the port count. The call is protected
+                 * by the port mutex
+                 */
                 port->serial->type->close(tty, port, filp);
 
-        if (port->port.count == (port->console? 1 : 0)) {
+        if (port->port.count == (port->console ? 2 : 1)) {
                 struct tty_struct *tty = tty_port_tty_get(&port->port);
                 if (tty) {
+                        /* We must do this before we drop the port count to
+                           zero. */
                         if (tty->driver_data)
                                 tty->driver_data = NULL;
                         tty_port_tty_set(&port->port, NULL);
@@ -285,13 +289,14 @@ static void serial_close(struct tty_struct *tty, struct file *filp)
                 }
         }
 
-        if (port->port.count == 0) {
+        if (port->port.count == 1) {
                 mutex_lock(&port->serial->disc_mutex);
                 if (!port->serial->disconnected)
                         usb_autopm_put_interface(port->serial->interface);
                 mutex_unlock(&port->serial->disc_mutex);
                 module_put(port->serial->type->driver.owner);
         }
+        --port->port.count;
 
         mutex_unlock(&port->mutex);
         usb_serial_put(port->serial);