> So how exactly did they bring about the impossible? They put an await call inside the critical section. The part of the code base that is not allowed to be subject to arbitrary delays. Massive facepalm.

I'm not sure where you got the impression that the example code was where we found the problem. That's a minimal reproducer trying to explain the problem from first principles because most people look at that code and think "that shouldn't deadlock". It uses a Mutex because people are familiar with Mutexes and `sleep` just to control the interleaving of execution. The RFD shows the problem in other examples without Mutexes. Here's a reproducer that futurelocks even though nobody uses `await` with the lock held: https://play.rust-lang.org/?version=stable&mode=debug&editio...

> I was honestly wondering how you could possibly cause this in any sane code base.

The actual issue is linked at the very top of the RFD. In our cases, we had a bounded mpsc channel used to send messages to an actor running in a separate task. That actor was working fine. But the channel did become briefly saturated (i.e., at capacity) at a point where someone tried to send on it via a `tokio::select!` similar to the one in the example.