tokio: Runtime gets stuck

Version

current master branch (27e5b41067d01c0c9fac230c5addb58034201a63)

Platform

Windows 10

Description

The following program gets stuck in a release build. After some amount of time, no more printouts will happen. This indicates that a wakeup got lost. I played a bit around, and discovered the following:

• The biggest difference lies within the value of NR_ITERATIONS - the smaller it is the faster it gets stuck. Maybe an issue with tasks joining “too early”?
• If the task::spawn is removed, everything looks fine
• Caught the issue in the debugger, but it only tells me one thread is waiting on thread::park, and others are waiting on IO completion ports
• The amount of runtime threads does not seem to make a difference. I just adjusted it to 1 to have less noise in the debugger.
• It does not seem to happen with the current_thread runtime

use tokio::sync::Mutex;
use tokio::sync::oneshot::channel;
use std::sync::Arc;

async fn do_iteration() {
    // Changing this to a higher value will prevent it from getting stuck
    const NR_ITERATIONS: usize = 5;
    let m = Arc::new(Mutex::new(()));//IntrusiveMutex::new((), true));
    let (sender, receiver) = channel();

    let m = m.clone();
    tokio::spawn(async move {
        for _ in 0..NR_ITERATIONS {
            let _ = m.lock().await;
        }
        sender.send(());
    });

    receiver.await;
}

fn main() {
    const NR_ITERATIONS: usize = 10000;
    const BLOCK_ON_PER_RT: usize = 3000;

    for i in 0 .. NR_ITERATIONS {
        {
            println!("Starting runtime {}", i);
            let mut builder = tokio::runtime::Builder::new();
            builder.thread_pool().num_threads(1);
            let mut rt = builder.build().unwrap();

            for _ in 0 .. BLOCK_ON_PER_RT {
                rt.block_on(async {
                    do_iteration().await;
                });
            }

            println!("Stopping runtime");
        }
        println!("Stopped runtime");
    }
}