Comment by scottlamb

5 hours ago

> futurelock [0] was special specifically because of this aspect where even a future which seemingly acquires and releases a lock in a single poll triggers a deadlock.

Their description at the top doesn't seem to match that:

RFD> This RFD describes futurelock: a type of deadlock where a resource owned by Future A is required for another Future B to proceed, while the Task responsible for both Futures is no longer polling A. Futurelock is a particularly subtle risk in writing asynchronous Rust.

...and further on they describe lock acquisition as an example of the resource:

RFD> future F1 is blocked on future F2 in some way (e.g., acquiring a shared Mutex)

...so I think they meant it to be more general.

> what you describe is just a standard async deadlock. much easier to spot when debugging. and one can reason about those deadlocks in pretty much the same way one would reason about deadlocks between threads.

I think the not-being-polled aspect of it is a bit more subtle than between threads. More like thread vs signal/interrupt handler actually, except it's not as well-known that "branch taken after a `select!`" or "place where two futures exist and `join!`/`spawn` isn't being used" is such a special case for scheduling.

...and anyway, with a mutex that has an actual reason to be async, how can you have only a acquire bug but not also have a potential mid-holding bug? You can say the latter is a different class of bug so you've solved futurelock, but you still have a bug any time you would have had futurelock, so semantics 1 working program 0.

1 comment

scottlamb

skavi 3 hours ago

If do_async_thing was implemented as below, i can’t imagine the futurelock post getting anywhere near this much attention. As for why you might use an async lock in a future which acquires and unlocks in the same poll, there still may be other actors which hold the lock across multiple polls. (there is one in the RFD’s minimized example).

    async fn do_async_thing(label: &str, lock: Arc<Mutex<()>>) {
        println!("{label}: started");
        let _guard = lock.lock().await;
        println!("{label}: acquired lock");
        sleep(Duration::from_secs(5)).await;
        println!("{label}: done");
    }