USB: serial: fix stalled writes

As David VomLehn points out, it was possible to receive an interrupt
before clearing the free-urb flag which could lead to the urb being
incorrectly marked as busy.

For the same reason, move tx_bytes accounting so that it will never be
negative.

Note that the free-flags set and clear operations do not need any
additional locking as they are manipulated while USB_SERIAL_WRITE_BUSY
is set.

Reported-by: David VomLehn <dvomlehn@cisco.com>
Tested-by: David VomLehn <dvomlehn@cisco.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 10 insertions, 5 deletions

diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
index a817ced82835..ca92f67747cc 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -208,18 +208,23 @@ retry:
         urb->transfer_buffer_length = count;
         usb_serial_debug_data(debug, &port->dev, __func__, count,
                                                 urb->transfer_buffer);
+        spin_lock_irqsave(&port->lock, flags);
+        port->tx_bytes += count;
+        spin_unlock_irqrestore(&port->lock, flags);
+
+        clear_bit(i, &port->write_urbs_free);
         result = usb_submit_urb(urb, GFP_ATOMIC);
         if (result) {
                 dev_err(&port->dev, "%s - error submitting urb: %d\n",
                                                 __func__, result);
+                set_bit(i, &port->write_urbs_free);
+                spin_lock_irqsave(&port->lock, flags);
+                port->tx_bytes -= count;
+                spin_unlock_irqrestore(&port->lock, flags);
+
                 clear_bit_unlock(USB_SERIAL_WRITE_BUSY, &port->flags);
                 return result;
         }
-        clear_bit(i, &port->write_urbs_free);
-
-        spin_lock_irqsave(&port->lock, flags);
-        port->tx_bytes += count;
-        spin_unlock_irqrestore(&port->lock, flags);
 
         /* Try sending off another urb, unless in irq context (in which case
          * there will be no free urb). */