Comment by scottlamb

8 hours ago

> The issues is that they are not "fair" in that they don't poll the future holding the lock.

How would you change that? A section of code doesn't have access to its own future to call into.

Best I can think is that you can't just call `let guard = mut.lock().await` but instead have to do `mut.do_locked(fut).await`, so that other `do_locked` calls can poll `fut`. I think that would work, but it seems quite awkward. Then again, imho async mutexes are something that should be used quite sparingly, so maybe that's okay.

4 comments

scottlamb

elchananHaas 7 hours ago

Disclaimer - I'm not a Tokio dev so what I say may be very wrong. Some definitions:

    Future = a structure with a method poll(self: Pin<&mut Self>, ...) -> Poll<Self::Output>; Futures are often composed of other futures and need to poll them. 


    Tokio task = A top-level future that is driven by the Tokio runtime. These are the only futures that will be run even if not polled.

My understanding is that Tokio async locks have a queue of tasks waiting on lock. When a lock is unlocked, the runtime polls the task at the front of the queue. Futurelock happens when the task locks the lock, then attempts to lock it a second time. This can happen when a sub-future of the top level task already has the lock, then it polls a different future which tries to take the lock.

This situation should be detectable because Tokio tracks which task is holding an async lock. One improvement could be to panic when this deadlock is spotted. This would at least make the issue easier to debug.

But yes, I think you are right in that the async mutex would need to take the future by value if it has the capability of polling it.

scottlamb 6 hours ago
> This situation should be detectable because Tokio tracks which task is holding an async lock. One improvement could be to panic when this deadlock is spotted. This would at least make the issue easier to debug.
That'd be a nice improvement! It could give a clear error message instead of hanging.
...but if they actually are polling both futures correctly via `tokio::join!` or similar, wouldn't it also cause an error where otherwise it'd actually work?
- elchananHaas 6 hours ago
  
  Oof, I think that you are right. The issue with Futurelock is a failure of liveness, where the Future holding the lock doesn't get polled. tokio::join! would keep it alive and therefore my suggestion would mistakenly panic.
  Yeah, the true fix is probably some form of the fabled Linear types/Structured concurrency where you can guarantee liveness properties.
  
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