A prepared transaction holds an IX lock on its collection. An index build is about to run its second drain phase and enqueues a collection S lock (behind the prepared transaction) after explicitly unlocking the RSTL. The RecoverRefreshThread runs, taking the RSTL in IX and (uninterruptiblty) enqueuing a collection IX lock. (Since the previously enqueued index build collection lock is mode S, this IX lock will queue behind it despite the lock granted to the prepared transaction also being IX.) Then a step down occurs, enqueueing the RSTL in X mode behind RecoverRefreshThread. The RecoverRefreshThread is explicitly marked as being killable by step down, but the lock acquisition is uninterruptible. Thus a four-way deadlock has been reached.