- * If privdata != NULL the function will try to put more jobs in the queue
- * of IO jobs to process as more room is made. */
+ * If privdata == NULL the function will try to put more jobs in the queue
+ * of IO jobs to process as more room is made. privdata is equal to NULL
+ * when the function is called from the event loop, so we want to push
+ * more IO jobs in the queue. Instead when the function is called by
+ * other functions that want to create a write-barrier to avoid race
+ * conditions we don't push new jobs in the queue. */