cdc-acm: fix race between callback and unthrottle

Abn URB may be may marked free only after the buffer has been
processed or there is a small window during which it could
be submitted on another CPU and overwrite an unprocessed buffer
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -417,25 +417,33 @@ static void acm_read_bulk_callback(struct urb *urb)
 	struct acm_rb *rb = urb->context;
 	struct acm *acm = rb->instance;
 	unsigned long flags;
+	int status = urb->status;
 
 	dev_vdbg(&acm->data->dev, "%s - urb %d, len %d\n", __func__,
 					rb->index, urb->actual_length);
-	set_bit(rb->index, &acm->read_urbs_free);
 
 	if (!acm->dev) {
+		set_bit(rb->index, &acm->read_urbs_free);
 		dev_dbg(&acm->data->dev, "%s - disconnected\n", __func__);
 		return;
 	}
 
 	if (urb->status) {
+		set_bit(rb->index, &acm->read_urbs_free);
 		dev_dbg(&acm->data->dev, "%s - non-zero urb status: %d\n",
-							__func__, urb->status);
+							__func__, status);
 		return;
 	}
 
 	usb_mark_last_busy(acm->dev);
 
 	acm_process_read_urb(acm, urb);
+	/*
+	 * Unthrottle may run on another CPU which needs to see events
+	 * in the same order. Submission has an implict barrier
+	 */
+	smp_mb__before_atomic();
+	set_bit(rb->index, &acm->read_urbs_free);
 
 	/* throttle device if requested by tty */
 	spin_lock_irqsave(&acm->read_lock, flags);